3.7 BSS Packet Flow Context Definition
A BSS Packet Flow Context contains the aggregate BSS QoS profile that is identical or similar for one or more activated PDP contexts. A BSS Packet Flow Context can be shared between several mobiles; several BSS Packet Flow Contexts can be defined. A BSS Packet FLow Context may be created, modified, or deleted every time a PDP context is activated, modified, or deleted.
A packet flow identifier (PFI), assigned by the SGSN, is used to identify each BSS Packet Flow Context. The PFI is assigned to the BSS at the creation of the BSS Packet Flow Context. It is assigned to the mobile when accepting the activation of the PDP context.
Three PFI values are reserved for best-effort service, SMS, and signaling.
Whenever the BSS receives an LLC PDU to transmit either in the direction of the SGSN or in the direction of the mobile, it deduces the QoS profile to use for the transmission from the PFI associated to the PDU.
The BSS Packet Flow contect concept has been introduced in the GPRS Release 99 recommendations.
Gb Interface
The Gb interface connects the BSS and the SGSN. It allows for the exchange of signaling information and user data. Many users are multiplexed on the same physical resource. Resources are allocated to the user only during activity periods; after these periods, resources are immediately released and reallocated to other users. This is in contrast to the GSM A interface where one user has the sole use of a dedicated physical resource during the lifetime of a call.
No dedicated physical resources are required to be allocated for signaling purposes. Signaling and user data are sent in the same transmission plane. Figure 3.28 shows the transmission plane on the Gb interface.
Figure 3.28: Transmission plane on Gb interface.
Transmission over the Gb interface is based on frame relay. Point-to-point (PTP) physical lines or an intermediate frame relay network can be used to connect the SGSN and the BSS.
3.8.1 NS Layer
The NS layer provides a frame-based, multiplexed link layer transport mechanism across the Gb interface that relies on the frame relay protocol.
The NS layer has been split into two sublayers, subnetwork service (SNS) and network service control (NSC) in order to make one sublayer independent of the intermediate transmission network. SNS is based on frame relay but NSC is independent of the transmission network. Later, it will be possible to change the transmission network (e.g., with an IP network) without changing the NSC sublayer.
Peer-to-peer communication across the Gb interface between the two remote NS entities in the BSS and the SGSN is performed over virtual connections. The NS layer is responsible for the management of the virtual connections between the BSS and the SGSN (verification of the availability of the virtual connections, initialization, and restoring of a virtual connection). It provides information on the status and the availability of the virtual connections to the BSSGP layer. It ensures the distribution of upper-layer PDUs between the different possible virtual connections (load-sharing function).
SNS provides access to the intermediate transmission network (i.e., the frame relay network). NSC is responsible for upper-layer data (BSSGP PDUs) transmission, load sharing, and virtual connection management.
3.8.2 BSSGP Principle
The BSSGP layer ensures the transmission of upper-layer data (LLC PDUs) from the BSS to the SGSN or from the SGSN to the BSS. It ensures the transmission of GMM signaling and NM signaling.
The peer-to-peer communication across the Gb interface between the two remote BSSGP entities in the BSS and the SGSN is performed over virtual connections. There is one virtual connection per cell at BSSGP layer. Each virtual connection can be supported by several layer 2 links between the SGSN and the BSS.
The BSSGP layer is responsible for the management of the virtual connections between the SGSN and the BSS (verification of the availability of the virtual connections, initialization and restoring of a virtual connection). The BSSGP layer also ensures the data flow control between the SGSN and the BSS.
There is a one-to-one relationship between the BSSGP in the SGSN and in the BSS. That means if one SGSN handles several BSSs, the SGSN must have one BSSGP protocol machine for each BSS. Figure 3.29 shows the position of the BSSGP layer within the BSS and the SGSN.
3.9 GPRS Backbone Network Architecture
Figure 3.30 shows the architecture of the GPRS backbone network (see gray boxes) made up of GSNs.
Figure 3.30: Architecture of GPRS backbone network. (From— [1].)
All PDUs conveyed in the GPRS backbone network across the Gn/Gp interface are encapsulated by GTP. GTP allows IP PDUs to be tunneled through the GPRS backbone network and allows signaling exchange to be performed between GSNs. UDP/IP are backbone network protocols used for user data routing and control signaling.
3.9.1 Tunneling
A GTP tunnel is a two-way PTP path between two GSNs used to deliver packets between an external PDN and an MS. A GTP tunnel is created during a PDP context activation procedure. A GTP tunnel is identified in each GSN node by a tunnel endpoint identifier (TEID), a GSN IP address, and a UDP port number. These identifiers are contained in IP and GTP PDU headers. There are two types of GTP tunnels:
GTP-U tunnel (user plane), defined for each PDP context in the GSNs;
GTP-C tunnel (control plane), defined for all PDP contexts with the same PDP address and access point network (APN).
The IP datagram tunneled in a GTP tunnel is called a T-PDU. With the tunneling mechanism, T-PDUs are multiplexed and demultiplexed by GTP between two GSNs by using the TEID field present in the GTP headers, which indicates the tunnel of a particular T-PDU. A GTP header is added to the T-PDU to constitute a G-PDU (or GTP-U PDU), which is sent in an UDP/IP path, a connectionless path between two endpoints.
Figure 3.31 illustrates a tunneling mechanism for IP packet sending toward MS.
Figure 3.31: Tunneling mechanism for IP packet sending toward MS.
All signaling procedures (path management, tunnel management, location management, mobility management) between GSNs are tunneled in a GTP tunnel in the control plane. A GTP header is added to the GTP signaling message to constitute a GTP-C PDU, which is sent in a UDP/IP path.
3.9.2 Path Protocols
The UDP/IP path protocol is used to convey GTP signaling messages between GSNs or T-PDU in connectionless mode. Each UDP/IP path may multiplex several GTP tunnels. An endpoint of the UDP/IP path is defined by an IP address and a UDP port number. For UDP/IP path, the IP source address is the IP address of the source GSN, while the IP destination source is the IP address of the destination GSN. Note that the IP addresses of GSNs within the GPRS backbone network are private. This means that GSNs are not accessible from the public Internet.
Tuesday, December 11, 2007
Mobility
3.5 Mobility
3.5.1 RA
A PLMN network supporting GPRS is divided into RAs. Each RA is defined by the operator of the PLMN network and may contain one or several cells. A LA is a group of one or several RAs. The RA defines a paging area for GPRS, while the LA defines a paging area for incoming circuit-switched calls. Actually, when the network receives an incoming call for a mobile not localized at cell level but localized at RA level, it broadcasts a paging on every cell belonging to this RA. The RA concept is illustrated in Figure 3.19.
Figure 3.19: RA concept.
If the MS moves to a new LA, it also moves to a new RA. Each RA is identified by a routing area identifier (RAI). This is made up of a location area identifier (LAI) and a routing area code (RAC). Figure 3.20 gives the structure of the RAI.
Figure 3.20: Structure of RAI.
The LAI identifies the LA, with the mobile country code (MCC) indicating the PLMN country, the mobile network code (MNC) identifying the PLMN network in this country, and the location area code (LAC) identifying the LA.
The RAI of each RA is broadcast on all cells belonging to this RA. This way, the MS is able to detect a new RA by comparing the RAI it had previously saved with the one broadcast in the new cell, and then to signal to the network its RA change.
The MS may also signal to the network the RA in which it is located upon expiry of a periodic timer. This procedure allows the network to check that the MS is within coverage of its RA. Owing to this procedure, the network knows if it may continue to route incoming calls for this MS toward this RA.
When an MS attached for circuit and packet services detects a new LA on the serving cell after having changed the cell, it will signal to the network its LA and RA change.
3.5.2 GMM States
Three global states are defined for GPRS mobility at the GMM layer level. These global states, GMM IDLE, STANDBY, and READY allow for characterization of the GMM activity of a GPRS mobile. They are managed in the MS and in the SGSN for each MS, and the transitions between states are slightly different on the MS and SGSN sides. A GPRS mobile is in GMM IDLE state when it is not attached for GPRS service. In this state, there is no GPRS mobility context established between the MS and the SGSN; this means that no information related to the MS is stored at SGSN level. In GMM STANDBY and READY states, a GPRS mobility context is established between the MS and the SGSN. A GPRS mobile is in GMM STANDBY state when it is attached for GPRS services and its location is known by the network at the RA level. A GPRS mobile is in GMM READY state when it is attached for GPRS services and its location is known by the network at the cell level.
A GPRS mobile goes to GMM IDLE state when it has just detached from GPRS. The SGSN goes to GMM IDLE state for a given MS upon receipt of the GPRS detach message, upon implicit detach when no MS activity is detected, or upon receipt of cancel location from HLR for operator purposes.
A GPRS mobile goes to GMM READY state when it has just sent a packet to the network. For each packet sent to the network, the MS reinitializes a READY timer. The SGSN goes to GMM READY state for a given MS when it receives an LLC PDU from it. For each LLC PDU received from the MS, the SGSN reinitializes a READY timer related to the MS.
A GPRS mobile goes to GMM STANDBY state from GMM READY state either upon expiry of the READY timer, or upon the receipt of an explicit request from the SGSN to force the GMM STANDBY state. The SGSN goes to GMM STANDBY state for one given MS either upon expiry of the READY timer, or upon explicit request from the network to force the GMM STANDBY state, or on an irrecoverable disruption of a radio transmission found at RLC layer level.
Note The network may force the GMM STANDBY state in order to reduce the signaling load in the network. In fact, as explained below, the MS has to perform several procedures in GMM READY state that are not authorized in GMM STANDBY state, such as notification of cell change. Furthermore, the GMM STANDBY state enables optimization of MS autonomy, since the MS need not send as much signaling over the air interface as compared with the GMM READY state.
Figure 3.21 shows the transitions between the three GMM states.
Figure 3.21: Global states of GPRS mobility. (From— [1].)
Table 3.4 summarizes the list of authorized procedures relative to GMM states.
Table 3.4: Authorized Procedures Relative to GMM States IDLE
STANDBY
READY
PLMN selection
Yes
Yes
Yes
GPRS cell (re)selection
No
Yes
Yes
GPRS packet transfer
No
Yes
Yes
Paging
No
Yes
No
Routing area update procedure
No
Yes
Yes
Notification of cell change
No
No
Yes
Radio link measurement reporting
No
No
Yes
The three global states lead to different behaviors of the MS at the radio interface level. They are therefore sent to the RR management layer of the MS.
3.5.3 Overview of GMM Procedures
3.5.3.1 Paging
The network may page an MS for circuit-switched and packet-switched services. These two services are managed in the backbone network by two different nodes: the MSC for routing of circuit-switched calls and the SGSN for routing of packet-switched calls. If there is no paging coordination between the circuit-switched and packet-switched services, the paging for circuit-switched and packet-switched services will not necessarily arrive at the MS on the same logical channel over the radio interface. This implies that the MS has to simultaneously monitor several logical channels for paging detection, a difficult task for MS receivers. In order to ease the MS behavior with respect to paging detection, paging coordination between circuit-switched and packet-switched services may be implemented in the network by adding a new interface, called the Gs interface, between the MSC and SGSN. This interface enables an incoming circuit-switched call to be routed from the MSC to the SGSN; this will allow the mobile to detect the circuit-switched and packet-switched services in the same logical channel.
Paging modes are defined by the recommendations to allow different paging implementations in the network. These paging modes take into account parameters such as the paging coordination method between circuit-switched and packet-switched services and the presence or absence of PCCCH paging channels. The paging mode is broadcast by the network on each GPRS cell.
Three network modes of operation (NMOs) are defined for paging:
Mode I. Circuit-switched paging messages are sent on the same PCHs as packet-switched paging, since paging coordination is supported (i.e., on PCCCH paging channels if allocated in the cell, or otherwise on CCCH paging channels).
Mode II. Packet-switched paging messages are sent on CCCH paging channels.
Mode III. Circuit-switched paging messages are sent on CCCH paging channels and packet-switched paging messages are sent on PCCCH paging channels if they exist in the cell, or on CCCH paging channels otherwise. Mode III is thus equivalent to mode II if the PCCCH paging channels are not present in the cell.
Mode I
The network operates in mode I when the Gs interface is present between the MSC and the SGSN. In this mode, the network sends the paging messages on the same logical channels for circuit-switched and packet-switched services. When an MS both IMSI- and GPRS-attached is in idle mode, it monitors for any kind of incoming calls (circuit-switched and GPRS) the PCCCH channels if they are present, and the CCCH channels otherwise. Figure 3.22 illustrates a paging in idle mode when the network operates in mode I.
Figure 3.22: Paging in idle mode with CCCH/PCCCH for network operation mode I.
In mode I, when an MS both IMSI- and GPRS-attached is in packet transfer mode, the circuit-switched paging occurs on the PACCH of the TBF in progress. Figure 3.23 illustrates a paging in packet transfer mode when the network operates in mode I.
Figure 3.23: CS paging in packet transfer mode for network operation mode I.
Mode II
In network operation mode II, the Gs interface is not present between the MSC and the SGSN, and the PPCHs are not present in the cell. In this mode the network sends paging messages for circuit-switched and packet-switched services on CCCH paging channels.
In the case of a packet transfer in progress, the MS may receive a circuit-switched paging on the CCCH paging channels. The MS then has the choice either to regularly suspend its packet transfer for listening to the CCCH paging channel or ignore the circuit-switched paging. The network may also send the circuit-switched paging on the PACCH; in this case the MS need not suspend its packet transfer for the paging decoding. Figure 3.24 illustrates a paging in idle mode when the network operates in mode II.
Figure 3.24: Paging in idle mode for network operation mode II.
Mode III
In network operation mode III, the network sends circuit-switched paging messages on CCCH channels and packet-switched paging messages on PCCCH channels if they are present, or otherwise on CCCH channels. The Gs interface is not present.
In the case of a packet transfer in progress, the MS may receive a circuit-switched paging either on the CCCH paging channels or on PACCH. The MS behavior is the same as that defined in mode II during a packet transfer.
In the case of a circuit-switched call in progress, the MS is not able to monitor PCCCH paging channels if they are present, or otherwise CCCH paging channels for a packet-switched paging. The MS is not authorized to suspend its circuit-switched call for listening PCHs. Figure 3.25 illustrates a paging in idle mode when the network operates in mode III.
Figure 3.25: Paging in idle mode for network operation mode III.
3.5.3.2 GPRS Attach
In order to access GPRS services, an MS performs an IMSI attach for GPRS services to signal its presence to the network. During the attach procedure, the MS provides its identity, either a temporary identifier or packet temporary mobile station identity (P-TMSI) previously allocated by the SGSN, or an IMSI identifier when P-TMSI is not valid. When the MS is GPRS-attached, an MM context is established between the MS and the SGSN. This means that information related to this MS (i.e., IMSI, P-TMSI, cell identity, and RA) is stored in the SGSN. A GPRS-attached mobile is localized by the network at least at RA level and may be paged at any moment in GMM STANDBY state.
In network operation modes II or III, the MS both IMSI- and GPRS-attached is obliged to initiate separate attach procedures for circuit-switched and packet-switched services with MSC/VLR and SGSN entities.
In network operation mode I, there is a combined IMSI and GPRS attach procedure for MSs wishing to be configured in class A or in class B. In this case, the MS initiates an attach procedure for circuit-switched and packet-switched services with the SGSN. As the Gs interface is present between the MSC/VLR and SGSN, the latter is able to forward the IMSI attach request to the MSC/VLR.
3.5.3.3 GPRS Detach
A GPRS MS can no longer access GPRS service when it is GPRS-detached. The detach may be explicit or implicit. It is explicit when signaling is exchanged between the MS and the network. The detach is implicit when the network detaches the MS without any notification, and it may occur when the network does not detect any activity related to the MS for a certain amount of time.
An MS both IMSI- and GPRS-attached (MS configured in class A or class B) in a network that operates in network operation modes II or III is obliged to initiate separate procedures for IMSI detach and GPRS detach with MSC/VLR and SGSN entities.
An MS both IMSI- and GPRS-attached (MS configured in class A or class B) in a network that operates in network operation mode I may perform an IMSI and GPRS combined detach procedure. As Gs interface is present in mode I between the MSC/VLR and SGSN entities, the request of IMSI detach included in the request of combined detach is forwarded to the MSC/VLR entity by the SGSN entity.
3.5.3.4 Security Aspects
Principles of Authentication and Kc Key Establishment
The authentication procedure is equivalent to that existing in GSM, with the difference being that it is handled by the SGSN entity. This procedure allows the LLC layer between the MS and the SGSN to be protected against unauthorized GPRS calls. The GPRS authentication uses a nonpredictable number provided by HLR/AUC. From this random number, the MS and the HLR/AUC calculate a number called the SRES by using an algorithm A3 and a key Ki specific to the GPRS subscriber.
The establishment of ciphering key Kc for a GPRS subscriber is also performed during authentication procedure from the random number provided by HLR/AUC. The ciphering key Kc is also computed from the random number by using an algorithm A8 and a key Ki belonging to the GPRS subscriber. The principle of GPRS authentication is illustrated in Figure 3.26.
Figure 3.26: Principle of GPRS authentication.
User Identity Confidentiality
The network guarantees user identity confidentiality while accessing GPRS radio resources. The user identity confidentiality is ensured by identifiers such as the P-TMSI and temporary logical link identity (TLLI). These identifiers are solely known between the MS and the SGSN, and are used on the radio interface to identify the called or calling MS.
P-TMSI is locally allocated by the SGSN. As this attribute is handled locally at the SGSN level, the change of P-TMSI may take place at every SGSN change. The P-TMSI reallocation can be performed during the attach procedure, during the RA update procedure, or during the P-TMSI reallocation procedure requested by the network.
TLLI allows for identification of a GPRS subscriber. This identifier is deduced from the P-TMSI structure. The relation between TLLI and IMSI is only known by the MS and the SGSN. This identifier is calculated by the GMM entity and is sent to the RLC/MAC entity in the MS and the BSS. The SGSN may send a P-TMSI signature to the MS during the GPRS attach procedure or during the RA update procedure.
In this case, the MS must include this signature in the next attach procedure or in the next RA update procedure. The P-TMSI signature conveys the proof that the P-TMSI returned by the MS is the one allocated by the SGSN. The SGSN checks the signature sent by the MS and the one sent previously to the MS. If these values do not match, then the SGSN initiates a MS authentication procedure.
Identity Verification
The SGSN may ask for MS identity if the Gf interface is present between the SGSN and the EIR entity. That way, the SGSN may compare the IMEI identifier returned by the MS and the one saved in the EIR database.
Call Ciphering
The network provides the call confidentiality by ciphering it. The data ciphering for GPRS is performed at the LLC frame level between the MS and the SGSN, unlike in GSM, where the ciphering is performed on the radio interface between the MS and the BTS.
During GPRS ciphering, an XOR operation is performed between an LLC frame and a mask. This latter has the same length as the LLC frame. During the GPRS deciphering, an XOR operation is performed between a ciphered LLC frame and the same mask used for ciphering. The mask for the ciphering and deciphering procedures is calculated from ciphering algorithm A5 with the following input parameters:
Key Kc, determined during authentication procedure;
Direction, indicating the transmission direction (uplink or downlink);
Input, which depends on the LLC frame type (acknowledged mode—incremented value for each new LLC frame; unacknowledged mode—value derived from the LLC header).
The ciphering and deciphering procedure is illustrated in Figure 3.27.
Figure 3.27: Ciphering and deciphering procedure.
3.5.3.5 Location Updating Procedures
When a GPRS MS camps on a new cell, it reads the cell identifier (CI), the RAI, and the LAI. If at least one of these identifiers has changed, the MS may initiate one of the GPRS location procedures:
Notification of cell update;
Normal RA update;
Combined RA and LA update.
If a GPRS-attached MS detects a new cell within its current RA, it performs a cell update procedure when it is in GMM READY state.
If a GPRS-attached MS detects a new RA, it performs an RA update procedure. This procedure may also occur at the expiry of a periodic timer or at the end of a circuit-switched connection. In fact, during a circuit-switched connection, if the MS changes RA then the SGSN is not notified of this change.
If an MS that is both IMSI- and GPRS-attached has detected a new LA in a network that operates in network operation mode I, then it performs a combined RA and LA update procedure.
If an MS both IMSI- and GPRS-attached has detected a new LA in a network that operates in network operation modes II or III, then it first performs an LA update procedure, and then an RA update procedure. The LA update is performed by the MM entity and requires the establishment of a dedicated connection.
If a GPRS MS detects a new cell in a new RA, then it performs an RA update procedure. This procedure may also occur at expiry of a periodic timer or at the end of a dedicated connection.
3.6 PDP Context
A PDP context specifies access to an external packet-switching network. The data associated with the PDP context contains information such as the type of packet-switching network, the MS PDP address that is the IP address, the reference of GGSN, and the requested QoS. A PDP context is handled by the MS, SGSN, and GGSN and is identified by a mobile's PDP address within these entities. Several PDP contexts can be activated at the same time within a given MS.
A PDP context activation procedure is used to create a PDP context. This procedure may be initiated either by the MS or by the network. The MS is always GPRS-attached before PDP context negotiation. The PDP context activation may be performed:
Automatically, if it is generated during a given procedure to perform a GPRS data transfer;
Manually, if it is generated by user intervention.
A PDP context deactivation procedure is used to remove a PDP context. This procedure may be initiated either by the MS or by the network (SGSN or GGSN). The PDP context deactivation may be generated either during an application deactivation or during the GPRS detach or delete subscriber data procedures.
A PDP context modification procedure is used to modify the PDP context. This procedure may be initiated either by the MS or by the network in order to change QoS parameters or traffic flow template (TFT) parameters.
All PDP context procedures are handled by the SM protocol between the MS and the SGSN and by the GTP between the SGSN and the GGSN.
3.5.1 RA
A PLMN network supporting GPRS is divided into RAs. Each RA is defined by the operator of the PLMN network and may contain one or several cells. A LA is a group of one or several RAs. The RA defines a paging area for GPRS, while the LA defines a paging area for incoming circuit-switched calls. Actually, when the network receives an incoming call for a mobile not localized at cell level but localized at RA level, it broadcasts a paging on every cell belonging to this RA. The RA concept is illustrated in Figure 3.19.
Figure 3.19: RA concept.
If the MS moves to a new LA, it also moves to a new RA. Each RA is identified by a routing area identifier (RAI). This is made up of a location area identifier (LAI) and a routing area code (RAC). Figure 3.20 gives the structure of the RAI.
Figure 3.20: Structure of RAI.
The LAI identifies the LA, with the mobile country code (MCC) indicating the PLMN country, the mobile network code (MNC) identifying the PLMN network in this country, and the location area code (LAC) identifying the LA.
The RAI of each RA is broadcast on all cells belonging to this RA. This way, the MS is able to detect a new RA by comparing the RAI it had previously saved with the one broadcast in the new cell, and then to signal to the network its RA change.
The MS may also signal to the network the RA in which it is located upon expiry of a periodic timer. This procedure allows the network to check that the MS is within coverage of its RA. Owing to this procedure, the network knows if it may continue to route incoming calls for this MS toward this RA.
When an MS attached for circuit and packet services detects a new LA on the serving cell after having changed the cell, it will signal to the network its LA and RA change.
3.5.2 GMM States
Three global states are defined for GPRS mobility at the GMM layer level. These global states, GMM IDLE, STANDBY, and READY allow for characterization of the GMM activity of a GPRS mobile. They are managed in the MS and in the SGSN for each MS, and the transitions between states are slightly different on the MS and SGSN sides. A GPRS mobile is in GMM IDLE state when it is not attached for GPRS service. In this state, there is no GPRS mobility context established between the MS and the SGSN; this means that no information related to the MS is stored at SGSN level. In GMM STANDBY and READY states, a GPRS mobility context is established between the MS and the SGSN. A GPRS mobile is in GMM STANDBY state when it is attached for GPRS services and its location is known by the network at the RA level. A GPRS mobile is in GMM READY state when it is attached for GPRS services and its location is known by the network at the cell level.
A GPRS mobile goes to GMM IDLE state when it has just detached from GPRS. The SGSN goes to GMM IDLE state for a given MS upon receipt of the GPRS detach message, upon implicit detach when no MS activity is detected, or upon receipt of cancel location from HLR for operator purposes.
A GPRS mobile goes to GMM READY state when it has just sent a packet to the network. For each packet sent to the network, the MS reinitializes a READY timer. The SGSN goes to GMM READY state for a given MS when it receives an LLC PDU from it. For each LLC PDU received from the MS, the SGSN reinitializes a READY timer related to the MS.
A GPRS mobile goes to GMM STANDBY state from GMM READY state either upon expiry of the READY timer, or upon the receipt of an explicit request from the SGSN to force the GMM STANDBY state. The SGSN goes to GMM STANDBY state for one given MS either upon expiry of the READY timer, or upon explicit request from the network to force the GMM STANDBY state, or on an irrecoverable disruption of a radio transmission found at RLC layer level.
Note The network may force the GMM STANDBY state in order to reduce the signaling load in the network. In fact, as explained below, the MS has to perform several procedures in GMM READY state that are not authorized in GMM STANDBY state, such as notification of cell change. Furthermore, the GMM STANDBY state enables optimization of MS autonomy, since the MS need not send as much signaling over the air interface as compared with the GMM READY state.
Figure 3.21 shows the transitions between the three GMM states.
Figure 3.21: Global states of GPRS mobility. (From— [1].)
Table 3.4 summarizes the list of authorized procedures relative to GMM states.
Table 3.4: Authorized Procedures Relative to GMM States IDLE
STANDBY
READY
PLMN selection
Yes
Yes
Yes
GPRS cell (re)selection
No
Yes
Yes
GPRS packet transfer
No
Yes
Yes
Paging
No
Yes
No
Routing area update procedure
No
Yes
Yes
Notification of cell change
No
No
Yes
Radio link measurement reporting
No
No
Yes
The three global states lead to different behaviors of the MS at the radio interface level. They are therefore sent to the RR management layer of the MS.
3.5.3 Overview of GMM Procedures
3.5.3.1 Paging
The network may page an MS for circuit-switched and packet-switched services. These two services are managed in the backbone network by two different nodes: the MSC for routing of circuit-switched calls and the SGSN for routing of packet-switched calls. If there is no paging coordination between the circuit-switched and packet-switched services, the paging for circuit-switched and packet-switched services will not necessarily arrive at the MS on the same logical channel over the radio interface. This implies that the MS has to simultaneously monitor several logical channels for paging detection, a difficult task for MS receivers. In order to ease the MS behavior with respect to paging detection, paging coordination between circuit-switched and packet-switched services may be implemented in the network by adding a new interface, called the Gs interface, between the MSC and SGSN. This interface enables an incoming circuit-switched call to be routed from the MSC to the SGSN; this will allow the mobile to detect the circuit-switched and packet-switched services in the same logical channel.
Paging modes are defined by the recommendations to allow different paging implementations in the network. These paging modes take into account parameters such as the paging coordination method between circuit-switched and packet-switched services and the presence or absence of PCCCH paging channels. The paging mode is broadcast by the network on each GPRS cell.
Three network modes of operation (NMOs) are defined for paging:
Mode I. Circuit-switched paging messages are sent on the same PCHs as packet-switched paging, since paging coordination is supported (i.e., on PCCCH paging channels if allocated in the cell, or otherwise on CCCH paging channels).
Mode II. Packet-switched paging messages are sent on CCCH paging channels.
Mode III. Circuit-switched paging messages are sent on CCCH paging channels and packet-switched paging messages are sent on PCCCH paging channels if they exist in the cell, or on CCCH paging channels otherwise. Mode III is thus equivalent to mode II if the PCCCH paging channels are not present in the cell.
Mode I
The network operates in mode I when the Gs interface is present between the MSC and the SGSN. In this mode, the network sends the paging messages on the same logical channels for circuit-switched and packet-switched services. When an MS both IMSI- and GPRS-attached is in idle mode, it monitors for any kind of incoming calls (circuit-switched and GPRS) the PCCCH channels if they are present, and the CCCH channels otherwise. Figure 3.22 illustrates a paging in idle mode when the network operates in mode I.
Figure 3.22: Paging in idle mode with CCCH/PCCCH for network operation mode I.
In mode I, when an MS both IMSI- and GPRS-attached is in packet transfer mode, the circuit-switched paging occurs on the PACCH of the TBF in progress. Figure 3.23 illustrates a paging in packet transfer mode when the network operates in mode I.
Figure 3.23: CS paging in packet transfer mode for network operation mode I.
Mode II
In network operation mode II, the Gs interface is not present between the MSC and the SGSN, and the PPCHs are not present in the cell. In this mode the network sends paging messages for circuit-switched and packet-switched services on CCCH paging channels.
In the case of a packet transfer in progress, the MS may receive a circuit-switched paging on the CCCH paging channels. The MS then has the choice either to regularly suspend its packet transfer for listening to the CCCH paging channel or ignore the circuit-switched paging. The network may also send the circuit-switched paging on the PACCH; in this case the MS need not suspend its packet transfer for the paging decoding. Figure 3.24 illustrates a paging in idle mode when the network operates in mode II.
Figure 3.24: Paging in idle mode for network operation mode II.
Mode III
In network operation mode III, the network sends circuit-switched paging messages on CCCH channels and packet-switched paging messages on PCCCH channels if they are present, or otherwise on CCCH channels. The Gs interface is not present.
In the case of a packet transfer in progress, the MS may receive a circuit-switched paging either on the CCCH paging channels or on PACCH. The MS behavior is the same as that defined in mode II during a packet transfer.
In the case of a circuit-switched call in progress, the MS is not able to monitor PCCCH paging channels if they are present, or otherwise CCCH paging channels for a packet-switched paging. The MS is not authorized to suspend its circuit-switched call for listening PCHs. Figure 3.25 illustrates a paging in idle mode when the network operates in mode III.
Figure 3.25: Paging in idle mode for network operation mode III.
3.5.3.2 GPRS Attach
In order to access GPRS services, an MS performs an IMSI attach for GPRS services to signal its presence to the network. During the attach procedure, the MS provides its identity, either a temporary identifier or packet temporary mobile station identity (P-TMSI) previously allocated by the SGSN, or an IMSI identifier when P-TMSI is not valid. When the MS is GPRS-attached, an MM context is established between the MS and the SGSN. This means that information related to this MS (i.e., IMSI, P-TMSI, cell identity, and RA) is stored in the SGSN. A GPRS-attached mobile is localized by the network at least at RA level and may be paged at any moment in GMM STANDBY state.
In network operation modes II or III, the MS both IMSI- and GPRS-attached is obliged to initiate separate attach procedures for circuit-switched and packet-switched services with MSC/VLR and SGSN entities.
In network operation mode I, there is a combined IMSI and GPRS attach procedure for MSs wishing to be configured in class A or in class B. In this case, the MS initiates an attach procedure for circuit-switched and packet-switched services with the SGSN. As the Gs interface is present between the MSC/VLR and SGSN, the latter is able to forward the IMSI attach request to the MSC/VLR.
3.5.3.3 GPRS Detach
A GPRS MS can no longer access GPRS service when it is GPRS-detached. The detach may be explicit or implicit. It is explicit when signaling is exchanged between the MS and the network. The detach is implicit when the network detaches the MS without any notification, and it may occur when the network does not detect any activity related to the MS for a certain amount of time.
An MS both IMSI- and GPRS-attached (MS configured in class A or class B) in a network that operates in network operation modes II or III is obliged to initiate separate procedures for IMSI detach and GPRS detach with MSC/VLR and SGSN entities.
An MS both IMSI- and GPRS-attached (MS configured in class A or class B) in a network that operates in network operation mode I may perform an IMSI and GPRS combined detach procedure. As Gs interface is present in mode I between the MSC/VLR and SGSN entities, the request of IMSI detach included in the request of combined detach is forwarded to the MSC/VLR entity by the SGSN entity.
3.5.3.4 Security Aspects
Principles of Authentication and Kc Key Establishment
The authentication procedure is equivalent to that existing in GSM, with the difference being that it is handled by the SGSN entity. This procedure allows the LLC layer between the MS and the SGSN to be protected against unauthorized GPRS calls. The GPRS authentication uses a nonpredictable number provided by HLR/AUC. From this random number, the MS and the HLR/AUC calculate a number called the SRES by using an algorithm A3 and a key Ki specific to the GPRS subscriber.
The establishment of ciphering key Kc for a GPRS subscriber is also performed during authentication procedure from the random number provided by HLR/AUC. The ciphering key Kc is also computed from the random number by using an algorithm A8 and a key Ki belonging to the GPRS subscriber. The principle of GPRS authentication is illustrated in Figure 3.26.
Figure 3.26: Principle of GPRS authentication.
User Identity Confidentiality
The network guarantees user identity confidentiality while accessing GPRS radio resources. The user identity confidentiality is ensured by identifiers such as the P-TMSI and temporary logical link identity (TLLI). These identifiers are solely known between the MS and the SGSN, and are used on the radio interface to identify the called or calling MS.
P-TMSI is locally allocated by the SGSN. As this attribute is handled locally at the SGSN level, the change of P-TMSI may take place at every SGSN change. The P-TMSI reallocation can be performed during the attach procedure, during the RA update procedure, or during the P-TMSI reallocation procedure requested by the network.
TLLI allows for identification of a GPRS subscriber. This identifier is deduced from the P-TMSI structure. The relation between TLLI and IMSI is only known by the MS and the SGSN. This identifier is calculated by the GMM entity and is sent to the RLC/MAC entity in the MS and the BSS. The SGSN may send a P-TMSI signature to the MS during the GPRS attach procedure or during the RA update procedure.
In this case, the MS must include this signature in the next attach procedure or in the next RA update procedure. The P-TMSI signature conveys the proof that the P-TMSI returned by the MS is the one allocated by the SGSN. The SGSN checks the signature sent by the MS and the one sent previously to the MS. If these values do not match, then the SGSN initiates a MS authentication procedure.
Identity Verification
The SGSN may ask for MS identity if the Gf interface is present between the SGSN and the EIR entity. That way, the SGSN may compare the IMEI identifier returned by the MS and the one saved in the EIR database.
Call Ciphering
The network provides the call confidentiality by ciphering it. The data ciphering for GPRS is performed at the LLC frame level between the MS and the SGSN, unlike in GSM, where the ciphering is performed on the radio interface between the MS and the BTS.
During GPRS ciphering, an XOR operation is performed between an LLC frame and a mask. This latter has the same length as the LLC frame. During the GPRS deciphering, an XOR operation is performed between a ciphered LLC frame and the same mask used for ciphering. The mask for the ciphering and deciphering procedures is calculated from ciphering algorithm A5 with the following input parameters:
Key Kc, determined during authentication procedure;
Direction, indicating the transmission direction (uplink or downlink);
Input, which depends on the LLC frame type (acknowledged mode—incremented value for each new LLC frame; unacknowledged mode—value derived from the LLC header).
The ciphering and deciphering procedure is illustrated in Figure 3.27.
Figure 3.27: Ciphering and deciphering procedure.
3.5.3.5 Location Updating Procedures
When a GPRS MS camps on a new cell, it reads the cell identifier (CI), the RAI, and the LAI. If at least one of these identifiers has changed, the MS may initiate one of the GPRS location procedures:
Notification of cell update;
Normal RA update;
Combined RA and LA update.
If a GPRS-attached MS detects a new cell within its current RA, it performs a cell update procedure when it is in GMM READY state.
If a GPRS-attached MS detects a new RA, it performs an RA update procedure. This procedure may also occur at the expiry of a periodic timer or at the end of a circuit-switched connection. In fact, during a circuit-switched connection, if the MS changes RA then the SGSN is not notified of this change.
If an MS that is both IMSI- and GPRS-attached has detected a new LA in a network that operates in network operation mode I, then it performs a combined RA and LA update procedure.
If an MS both IMSI- and GPRS-attached has detected a new LA in a network that operates in network operation modes II or III, then it first performs an LA update procedure, and then an RA update procedure. The LA update is performed by the MM entity and requires the establishment of a dedicated connection.
If a GPRS MS detects a new cell in a new RA, then it performs an RA update procedure. This procedure may also occur at expiry of a periodic timer or at the end of a dedicated connection.
3.6 PDP Context
A PDP context specifies access to an external packet-switching network. The data associated with the PDP context contains information such as the type of packet-switching network, the MS PDP address that is the IP address, the reference of GGSN, and the requested QoS. A PDP context is handled by the MS, SGSN, and GGSN and is identified by a mobile's PDP address within these entities. Several PDP contexts can be activated at the same time within a given MS.
A PDP context activation procedure is used to create a PDP context. This procedure may be initiated either by the MS or by the network. The MS is always GPRS-attached before PDP context negotiation. The PDP context activation may be performed:
Automatically, if it is generated during a given procedure to perform a GPRS data transfer;
Manually, if it is generated by user intervention.
A PDP context deactivation procedure is used to remove a PDP context. This procedure may be initiated either by the MS or by the network (SGSN or GGSN). The PDP context deactivation may be generated either during an application deactivation or during the GPRS detach or delete subscriber data procedures.
A PDP context modification procedure is used to modify the PDP context. This procedure may be initiated either by the MS or by the network in order to change QoS parameters or traffic flow template (TFT) parameters.
All PDP context procedures are handled by the SM protocol between the MS and the SGSN and by the GTP between the SGSN and the GGSN.
Overview of GPRS
Overview
GPRS represents an evolution of the GSM standard, allowing data transmission in packet mode and providing higher throughputs as compared with the circuit-switched mode. This evolution is usually presented under the designation of 2.5G to point out that it is a transition technology between 2G and 3G.
The GPRS network architecture reuses the GSM network nodes such as MSC/VLR, HLR, and BSS. New network nodes have been introduced for the transport of packet data. These nodes are the gateway GPRS support nodes (GGSN) and serving GPRS support nodes (SGSN). The subnetwork formed by the SGSNs and the GGSNs is called the GPRS core network. In order to reuse the GSM nodes, new interfaces have been defined between the GSM network nodes and the different elements of the GPRS core network. The GPRS logical architecture is described in Section 3.1.
The protocol layer has been split into two planes. On one side there is the transmission plane, which is mainly used for the transfer of user data. The signaling plane is used for the control and support of the transmission plane functions. Section 3.2 deals with the transmission and signaling planes.
GPRS has kept such main principles of the GSM radio interface as the notions of time slot, frame, multiframe, and hyperframe structures. It was indeed chosen by the operators and manufacturers involved in the system design to provide high-data-rate packet-switched services with minimized impacts on the GSM standard. The principles of the physical layer are given in Section 3.3.1. The details related to the physical layer are presented in Chapter 4.
One of the main GPRS characteristics is that a physical connection is established in uplink only when the MS needs to send continuous data to the network, and in downlink only when the network needs to send continuous data to the MS. This physical connection is released in one direction as soon as the sending entity has no more data to send. Different allocation schemes for radio resource (RR) management have been defined in order to multiplex several MSs on the same physical channel. An overview of the principles related to RR management is presented in Section 3.3.2. A complete description of RR management can be found in Chapter 5.
A logical entity called PCU has been introduced within the BSS to manage the GPRS functions over the radio interface. Section 3.4 deals with the BSS architecture and discusses the several possible locations of the PCU.
In a GPRS network, an RA identifies one or several cells. As soon as an MS enters a new RA, the network must be notified of this change in order to update its location. The SGSN is in charge of GPRS mobility management (GMM). An overview of GPRS mobility is proposed in Section 3.5. The details related to GMM are given in Chapter 7.
If data transmission in packet mode does not require the establishment of an end-to-end connection, it is necessary to establish a context between the mobile and the network in order to exchange packets. This context allows the network to identify the IP address of the MS, identify the access point with the external network, and define the QoS associated with data transmission in packet mode. The concept of PDP context is explained in Section 3.6. The details related to PDP context management are given in Chapter 7.
The BSS and the GPRS backbone network are connected via the Gb interface in order to exchange user data and signaling information. The principles of the Gb interface are given in Section 3.8. The details related to the Gb interface are given in Chapter 6.
When a context is established between the MS and the network, IP packet exchange may start at any time between the mobile and the network without establishing a connection beforehand. The packets are conveyed in the GPRS backbone network. An overview of the general architecture of the GPRS backbone network is presented in Section 3.9. A complete description of the user plane between the MS and external data packet network is given in Chapter 8.
3.1 GPRS Logical Architecture
Figure 3.1 shows the elements that are part of a GPRS network and their associated interfaces. A GPRS network is composed of the following network nodes:
SGSN. The SGSN is the node that is serving the MS; it is responsible for GMM. It delivers packets to the MSs and communicates with the HLR to obtain the GPRS subscriber profile. It manages the registration of the new mobile subscribers in order to keep a record of their LA for routing purposes. The SGSN can be connected to one or several BSSs.
GGSN. The GGSN provides interworking with external packet data networks (PDNs). It may be linked to one or several data networks. It is connected with SGSNs via an IP-based GPRS backbone network. The GGSN is a router that forwards incoming packets from the external PDN to the SGSN of the addressed MS. It also forwards outgoing packets to the external PDN. The PDN is the external fixed data network to which is connected the GPRS network. An example of a PDN is the Internet network.
HLR. The HLR is a database that contains, among other things, packet domain subscription data and routing information.
Mobile switching center/visitor location register (MSC/VLR). The MSC coordinates the setting up of calls to and from GSM users and manages GSM mobility. The MSC is not directly involved in the GPRS network. It forwards circuit-switched paging for the GPRS-attached MSs to the SGSN when the Gs interface is present.
BSS. The BSS ensures the radio connection between the mobile and the network. It is responsible for radio access management. The BSS is composed of two elements: the BTS and the BSC. The BTS integrates all the radio transmission and radio reception boards. The BSC is responsible for the management of the radio channels. The BSC has switching capabilities that are used for circuit-switched calls and can also be used for GPRS traffic.
EIR. The EIR is a database that contains terminal identities.
3.2 Transmission and Signaling Planes
3.2.1 Transmission Plane
The transmission plane consists of a layered protocol structure providing user data transfer, along with associated procedures that control the information transfer such as flow control, error detection, and error correction. Figure 3.2 illustrates the layered protocol structure between the MS and the GGSN.
Figure 3.2: Transmission plane MS-GGSN.
3.2.1.1 Air Interface
The air interface is located between the MS and the BSS. The protocols used on the air interface are as follows:
Radio link control/medium access control (RLC/MAC). RLC provides a reliable radio link between the mobile and the BSS. MAC controls the access signaling procedures to the GPRS radio channel, and the multiplexing of signaling and RLC blocks from different users onto the GSM physical channel.
GSM-RF layer. It is the radio subsystem that supports a certain number of logical channels. This layer is split into two sublayers: the radio frequency layer (RFL), which handles the radio and baseband part (physical channel management, modulation, demodulation, and transmission and reception of radio blocks), and the physical link layer (PLL), which manages control of the RFL (power control, synchronization, measurements, and channel coding/decoding).
A relay function is implemented in the BSS to relay the LLC PDUs between the air interface and the Gb interface.
3.2.1.2 Gb Interface
The Gb interface is located between the SGSN and the BSS. It supports data transfer in the transmission plane. The Gb interface supports the following protocols:
BSS GPRS protocol (BSSGP). This layer conveys routing and QoS-related information between the BSS and SGSN.
Network service (NS). It transports BSSGP PDUs and is based on a frame relay connection between the BSS and SGSN.
A relay function is implemented in the SGSN to relay the packet data protocol (PDP) PDUs between the Gb and Gn interfaces (IP PDUs in Figure 3.2).
3.2.1.3 Gn/Gp Interface
The Gn interface is located between two GSNs (SGSN or GGSN) within the same PLMN, while the Gp interface is between two GSNs in different PLMNs. The Gn/Gp interface is used for the transfer of packets between the SGSN and the GGSN in the transmission plane.
The Gn/Gp interface supports the following protocols:
GPRS tunnelling protocol (GTP). This protocol tunnels user data between the SGSN and GGSN in the GPRS backbone network. GTP operates on top of UDP over IP. The layers L1 and L2 of the Gn interfaces are not specified in the GSM/GPRS standard.
User datagram protocol (UDP). It carries GTP packet data units (PDUs) in the GPRS Core Network for protocols that do not need a reliable data link (e.g., IP).
Internet protocol (IP). This is the protocol used for routing user data and control signaling within the GPRS backbone network.
3.2.1.4 Interface Between MS and SGSN
This interface supports the following protocols:
Subnetwork-dependent convergence protocol (SNDCP). This protocol maps the IP protocol to the underlying network. SNDCP also provides other functions such as compression, segmentation, and multiplexing of network layer messages.
Logical link control (LLC). This layer provides a highly reliable logical link that is independent of the underlying radio interface protocols. LLC is also responsible for the GPRS ciphering.
3.2.2 Signaling Plane
The signaling plane consists of protocols for control and support of the transmission plane functions. It controls both the access connections to the GPRS network (e.g., GPRS attach and GPRS detach) and the attributes of an established network access connection (e.g., activation of a PDP address), manages the routing of information for a dedicated network connection in order to support user mobility, adapts network resources depending on the QoS parameters, and provides supplementary services.
3.2.2.1 Between MS and SGSN
Figure 3.3 shows the signaling plane between the MS and the SGSN. This plane is made up of the following protocols:
GMM. The GMM protocol supports mobility management functionalities such as GPRS attach, GPRS detach, security, RA update, and location update (see Section 3.5).
Session management (SM). The SM protocol supports functionalities such as PDP context activation, PDP context modification, and PDP context deactivation (see Section 3.6).
Figure 3.3: Signaling plane MS-SGSN.
3.2.2.2 Between Two GSNs
In the signaling plane, Gn/Gp interfaces are used for the transfer of signaling between the GSNs in the GPRS backbone network. Figure 3.4 shows the signaling plane between two GSNs.
Figure 3.4: Signaling plane GSN-GSN.
The signaling plane between two GSNs is made up of the following protocols:
GTP for the control plane (GTP-C). This protocol tunnels signaling messages between SGSNs and GGSNs, and between SGSNs, in the GPRS core network (see Section 3.9).
UDP. This protocol transfers signaling messages between GSNs.
3.2.2.3 Interface with Signaling System No. 7
The various GSNs of the GPRS backbone network use a Signaling System No. 7 (SS7) network to exchange information with GSM SS7 network nodes such as HLR, MSC/VLR, EIR, and SMS-GMSC. The SS7 network provides facilities to quickly exchange messages between GPRS backbone network nodes irrespective of data transmission through the GPRS PLMN network.
In the GSM/GPRS backbone network, we found the following protocols for SS7 signaling:
Message transfer part (MTP). The three MTP layers allow signaling messages to be exchanged through the SS7 network.
Signaling connection control part (SCCP). The SCCP protocol layer allows the service to be used in connected mode and messages to be exchanged between different PLMNs by using an international gateway for SS7 address translation between an SS7 global address (based on the E.164 numbering plan) and an SS7 local address.
Transaction capabilities application part (TCAP). The TCAP protocol layer allows dialogs to be structured in an independent manner from any application.
Mobile application part (MAP). The MAP protocol layer allows mobile mobility to be managed within different equipment nodes of the NSS across SS7 networks.
As new equipment nodes have been introduced in GSM networks to support the GPRS feature, new interfaces were defined with the HLR, MSC/VLR, EIR, and SMS-GMSC. Table 3.1 lists the new interfaces with SS7 network.
Table 3.1: New Interfaces with the SS7 Network Interface Name
Localization
Mandatory/Optional
Gr
SGSN-HLR
Mandatory
Gc
GGSN-HLR
Optional
Gf
SGSN-EIR
Optional
Gd
SGSN-SMS GMSC or SGSN-SMS IWMSC
Optional
Gs
SGSN-MSC/VLR
Optional
Gr Interface
The Gr interface is defined between the SGSN and HLR. It allows the SGSN to retrieve or update GPRS subscription and GPRS location information in the HLR during location-management or authentication procedures. The MAP protocol has been modified to take into account this interface. Figure 3.5 shows the signaling plane on the Gr interface.
Figure 3.5: Signaling plane on the Gr interface.
Gc Interface
The Gc interface is defined between the GGSN and the HLR. The GGSN contacts the HLR in order to determine the SGSN address where the MS is located and if the MS is reachable. The MAP protocol has been modified to take into account this interface. Figure 3.6 shows the signaling plane on the Gc interface. (Note: If a GGSN does not have a SS7 MAP interface, it will interface to a GSN performing a GTP-MAP protocol-conversion in order to retrieve the needed information from the HLR via the Gc interface.)
Figure 3.6: Signaling plane on the Gc interface.
Gf Interface
The Gf interface is defined between the SGSN and EIR. It is used by the SGSN to contact the EIR database during the identity check procedure. It allows the SGSN to check the IMEI against the EIR. The MAP protocol has been modified to take into account this interface. Figure 3.7 shows the signaling plane on the Gf interface.
Figure 3.7: Signaling plane on the Gf interface.
Gs Interface
The Gs interface is defined between the MSC/VLR and the SGSN. It allows for the coordination of circuit-switched and packet-switched paging in the SGSN as well as location information of any MS attached to both circuit and packet services. This interface is only present in a network that operates in network operation mode I (see definition in Section 3.5.3.1). The BSS application part+ (BSSAP+) allows mobility functionality to be managed on the Gs interface. Figure 3.8 shows the signaling plane on the Gs interface.
Figure 3.8: Signaling plane on the Gs interface.
Gd Interface
The Gd interface is defined between an SGSN and an SMS-GMSC or an SMS-IWMSC. The progress of a short message intended for delivery to an MS requires in circuit and packet modes a gateway function Short Message Service-gateway MSC (SMS-GMSC) between the mobile network and the network that provides access to the SMS center. An SMS to be delivered to an MS is routed from the SMS-GMSC toward the SGSN on the Gd interface if this SMS is to be sent over GPRS.
The progress of a short message originated by the MSs requires in circuit and packet modes a PLMN interworking server SMS-IWMSC (inter-working MSC) that provides access to the SMS center. An SMS originated by an MS is routed from the SGSN toward the SMS-IWMSC on the Gd interface if the SMS is to be sent over GPRS.
The MAP protocol has been updated as a consequence of the signaling exchange between the SGSN and the SMS-GMSC or the SMS-IWMSC. Figure 3.9 shows the signaling plane on the Gd interface.
Figure 3.9: Signaling plane on the Gd interface.
3.3 Radio Interface
3.3.1 Physical Layer Principles
3.3.1.1 Packet Data Channel
The GPRS physical layer is based on that of the GSM (see Chapter 1). The access scheme is TDMA, with eight basic physical channels per carrier (TS 0 to 7).
A physical channel uses a combination of frequency- and time-division multiplexing and is defined as a radio frequency channel and time slot pair. The physical channel that is used for packet logical channels is called a packet data channel (PDCH). PDCHs are dynamically allocated in the cell by the network. The PDCH is mapped on a 52-multiframe, as shown in Figure 3.10. The 52-multiframe consists of 12 radio blocks (BO to B11) of 4 consecutive TDMA frames and 4 idle frames (frames 12, 25, 38, and 51), amounting to a total of 52 frames.
Figure 3.10: Time slots and TDMA frames.
3.3.1.2 Packet Data Logical Channel
GPRS, like GSM, uses the concept of logical channels mapped on top of the physical channels. Two types of logical channels have been introduced, namely traffic channels and control channels. Three subtypes of control channels have been defined for GPRS: broadcast, common control, and associated. In addition, the GSM common control channels (BCCH, CCCH, and RACH) may be used to access the network and establish packet transfer.
The different packet data logical channels are:
Packet broadcast control channel (PBCCH). The presence of PBCCH in the cell is optional. The PBCCH broadcasts information relative to the cell in which the mobile camps and information on the neighbor cells. This information is used by the mobile in order to access the network. When there is no PBCCH in the cell, the information needed by the mobile to access the network for a packet transfer is broadcast on BCCH.
Packet common control channel (PCCCH). The PCCCH is present in the cell only if the PBCCH is present in the cell. When it is not present in the cell, the common control signaling for GPRS is handled on the GSM common control channels (CCCH). PCCCH is composed of packet random access channel (PRACH), used for random access, packet paging channel (PPCH), used for paging, and packet access grant channel (PAGCH), used for access grant. The PRACH is used by the MS to initiate uplink access to the network. The PPCH is used by the network to page the mobile in order to establish a downlink packet transfer. The PAGCH is used by the network to assign radio resources to the mobile for a packet transfer.
Packet data traffic channel (PDTCH). The PDTCH is used to transfer user data during uplink or downlink packet transfer. The PDTCH is a unidirectional channel, either uplink (PDTCH/U) for a mobile-originated packet transfer or downlink (PDTCH/D) for a mobile-terminated packet transfer. A PDTCH is a resource allocated on one physical channel by the network for user data transmission.
Packet associated control channel (PACCH). The PACCH is a unidirectional channel that is used to carry signaling during uplink or downlink packet data transfer. The uplink PACCH carries signaling from the MS to the network and the downlink PACCH carries signaling from the network to the mobile. The PACCH is dynamically allocated on a block basis.
Packet timing advance control channel (PTCCH). The PTCCH is a bidirectional channel that is used for TA update. The PTCCH is an optional channel. The PTCCH when present is mapped on frames number 12 and 38 of the 52—multiframe.
Table 3.2 lists the various GPRS logical channels.
Table 3.2: GPRS Logical Channels Logical Channel
Abbreviation
Uplink/ Downlink
Task
Packet broadcast control channel
PBCCH
DL
Packet system broadcast information
Packet paging channel
PPCH
DL
MS paging for downlink transfer establishment
Packet random access channel
PRACH
UL
MS random access for uplink transferestablishment
Packet access grant channel
PAGCH
DL
Radio resources assignment
Packet timing advance control channel
PTCCH
UL/DL
Timing advance update
Packet associated control channel
PACCH
UL/DL
Signaling associated with data transfer
Packet data traffic channel
PDTCH
UL/DL
Data channel
3.3.1.3 Multislot Classes Definition
In order to provide higher throughputs, a GPRS MS may transmit or receive in several time slots of the TDMA frame. The multislot capability is indicated by the multislot class of the GPRS MS. This multislot class is defined by several parameters such as the maximum number of time slots supported by the MS per TDMA frame in uplink and in downlink. The multislot class of the MS is sent to the network during the GPRS attach procedure. A detail of multislot classes is given in Section 4.2.1.
3.3.1.4 Cell Reselection
In GPRS as in GSM, the mobile performs cell reselection. However, there are some differences compared with GSM.
In GPRS the mobile performs cell reselection when it is in idle mode but also during packet transfer. The cell reselection is either performed by the mobile autonomously or optionally controlled by the network. Unlike GSM, there is no handover in GPRS but only cell reselections. So when there is a reselection during a packet transfer, this latter is interrupted and it has to be started again in the new cell. There is an interruption of the packet transfer during the reselection phase.
Although the GPRS cell reselection algorithms used by the mobile are based on the same principles as those used in GSM, they have been slightly modified in order to provide more flexibility. These algorithms are described in Section 5.3.
3.3.1.5 Radio Environment Monitoring
The MS performs different types of radio measurements that are reported to the network and used by it for RLC. These estimations are also used by the mobile itself to compute its transmission power (open-loop power control; see Section 4.1.3.1), for cell selection and cell reselection.
The mobile performs the following types of measurement:
Received signal level (RXLEV) measurements. The RXLEV measurements are performed on both serving cell and neighbor cells for the purpose of cell reselection. During packet-transfer mode (see Section 3.3.2.1), the serving cell RXLEV measurement can also be used for downlink coding scheme adaptation (see Section 4.1.2.1), network-controlled cell reselection, and downlink and uplink power control.
Quality (RXQUAL) measurements. The RXQUAL is computed from the average BER before channel decoding. During packet-transfer mode the mobile estimates the quality of the downlink blocks it receives. In packet idle mode, no quality measurements are performed. The RXQUAL can be used by the network for network-controlled cell reselection, dynamic coding scheme adaptation, and downlink power control. The RXQUAL is the current GSM quality indicator.
Interference measurements. These measurements have been introduced for GPRS. They correspond to a received signal level measurement performed on a frequency that is different from a beacon frequency. The interest is in having an estimation of the interference level on the PDTCH. It can be used by the network to optimize the mobile RR allocation, to select a more appropriate coding scheme, to trigger a network-controlled cell reselection, and for power control or for network statistics.
The measurements are either used by the MS for its own purposes or by the network. The RXLEV and RXQUAL measurements are also performed at the BSS side for each MS. They are used for network-controlled cell reselection, uplink power control (closed loop, see Section 4.1.3.1), and dynamic coding scheme adaptation.
3.3.1.6 Principles of Power Control
Power control can be used in order to improve the spectrum efficiency while maintaining radio link quality and reducing the power consumption in the MS. Power control in GPRS is more complicated than for a circuit-switched connection, since there is not necessarily a continuous two-way connection.
Note that power control can be performed in both uplink and downlink directions. Uplink power control and downlink power control are described in Sections 4.1.3.1 and 4.1.3.2, respectively.
3.3.1.7 Channel Coding
The GPRS user data is sent on radio blocks encoded with one of four channel coding schemes (CS1, CS2, CS3, CS4). The GPRS signaling is sent with the CS1 channel coding scheme. This scheme provides the highest protection level against error transmission, while the CS4 channel coding scheme provides the lowest protection level. The more the channel coding scheme provides an efficient protection level against error transmission, the more the useful data throughput decreases due to redundant information added to source data. The channel coding scheme used on uplink and downlink depends on the radio quality between the network and the MS. Table 3.3 gives the throughputs associated with each coding scheme.
Table 3.3: Throughput Associated with Coding Scheme Coding Scheme
Throughput (Kbps)
CS-1
9.05
CS-2
13.4
CS-3
15.6
CS-4
21.4
3.3.2 RR Management Principles
3.3.2.1 RR Operating Modes
At the RR level, the MS behavior is dependent on two operating RR states. These states, packet idle mode and packet transfer mode, allow the RR activity of the MS to be characterized.
Packet Idle Mode
When the MS is in packet idle mode, no radio resources are allocated. Leaving packet idle mode occurs when upper layers request the transfer of uplink data requiring the assignment of uplink resources from the network. It also occurs at the reception of a downlink resource assignment command from the network for a downlink transfer.
In case of downlink transfer, the mobile switches from packet idle mode to packet transfer mode when it receives the downlink assignment command from the network. In the case of uplink transfer, the mobile leaves packet idle mode when it requests the assignment of uplink resources to the network. However, switching to packet transfer mode is not instantaneous. The mobile switches to packet transfer mode only when it has been uniquely identified at the network side; this will be explained in more detail in Chapter 5. Thus there is a period between packet idle mode and packet transfer mode during which the mobile is in a transitory state.
During packet idle mode, the MS listens to its PCH and the CBCH. This last one is the PBCCH when present in the cell; otherwise it is the BCCH.
Packet Transfer Mode
When the MS is in packet transfer mode, it is clearly identified at the network side and uplink or/and downlink radio resources are allocated.
Switching from packet transfer mode to packet idle mode occurs when the network releases all downlink and uplink resources. This transition can also occur in the case of an abnormal condition during packet transfer mode (e.g., radio link failure) or when the mobile decides on a cell reselection toward a new cell.
During packet transfer mode, the mobile transmits and receives data. Figure 3.11 summarizes the transition between the different RR states.
Figure 3.11: Transition between RR operating modes.
3.3.2.2 Temporary Block Flow
A temporary block flow (TBF) is a physical connection between the RR entity in the MS and the RR entity at the network side to support the unidirectional transfer of LLC protocol data units over PDCH. A TBF is characterized by one or several PDCHs allocated by the network to an MS for the duration of the data transfer. Once the data transfer is finished, the TBF is released.
When the mobile must send continuous data to the network, it requests the establishment of an uplink TBF by sending signaling information over CCCH or PCCCH. When the network wants to send data to the mobile, it assigns a downlink TBF between the two RR entities.
A downlink TBF supports the transfer of data from the network to the mobile, while an uplink TBF supports the transfer of data from the mobile to the network. One uplink and one downlink TBF can be supported at the same time between the two RR entities. These two TBFs are defined as concurrent TBFs.
The number of TBFs per mobile and per direction is limited to one. However TBFs belonging to different mobiles can share the same PDCH.
Note As a packet transfer session is composed of a lot of requests, responses, and acknowledgments, many consecutive uplink and downlink TBFs are established for the same session (e.g., Web browsing).
Each TBF is identified by a temporary flow identifier (TFI) assigned by the network. So in case of concurrent TBFs, one TFI identifies the uplink TBF and another one the downlink TBF. The TFI is used to differentiate TBFs sharing the same PDCHs in one direction.
Figure 3.12 gives an example of TBF mapping for different mobiles onto PDCHs. A downlink TBF, identified by a TFI equal to 2, has assigned resources on the PDCH numbers 2, 3, and 4, while another downlink TBF identified by a TFI equal to 1 has assigned resources on the PDCH numbers 4, 5, and 6. An uplink TBF identified by a TFI equal to 4 has assigned resources on the PDCH numbers 3 and 4.
Figure 3.12: Example of mapping of TBFs with their respective TFI onto PDCHs.
3.3.2.3 Allocation Modes on the Uplink
Several MSs may be multiplexed on the same PDCH. In order to share the uplink bandwidth between several mobiles mapped on the same PDCH, different allocation schemes have been defined to allocate an uplink radio block instance to a particular mobile.
Three allocation schemes exist for medium access control:
Dynamic allocation;
Extended dynamic allocation;
Fixed allocation.
The second scheme is optional for the mobile while the others are mandatory. On the network side, either fixed allocation or dynamic allocation must be implemented. Extended dynamic allocation is optional for the network.
Dynamic Allocation
In principle, dynamic allocation allows uplink transmission to mobiles sharing the same PDCH, on a block-by-block basis. During the uplink TBF establishment, an uplink state flag (USF) is given to the MS for each allocated uplink PDCH. The USF is used as a token given by the network to allow transmission of one uplink block.
Whenever the network wants to allocate one radio block occurrence on one uplink PDCH, it includes, on the associated downlink PDCH, the USF in the radio block immediately preceding the allocated block occurrence. When the mobile decodes its assigned USF value in a radio block sent on a downlink PDCH associated with an allocated uplink one, it transmits an uplink radio block in the next uplink radio block occurrence, that is, the B(x) radio block if the USF was detected in the B(x- 1) radio block.
The principle of dynamic allocation is illustrated in Figure 3.13.
Figure 3.13: Principle of dynamic allocation.
The USF is included in the header of each downlink RLC/MAC block. The USF coding (3 bits) enables eight mobiles to be multiplexed on the same uplink PDCH.
Dynamic allocation implies the constant monitoring (radio block decoding) of the downlink PDCHs associated with the allocated uplink PDCHs.
As explained previously, the USF allows the sending of one block in the next uplink occurrence. However, dynamic allocation can also be used in such a way that the decoding of one USF value allows the mobile to send four consecutive uplink blocks on the same PDCH. The choice between one block or four blocks is indicated during the TBF establishment by the network to the mobile.
The concept of USF granularity has been introduced in order to indicate the number of uplink radio blocks to be sent upon detection of the assigned USF by the MS. The USF granularity is conveyed to the MS during the uplink TBF establishment. Thus it involves the MS transmitting either a single radio block or a sequence of four consecutive radio blocks starting on the B(x) radio block if the USF was detected in the B(x - 1) radio block.
Extended Dynamic Allocation
The extended dynamic allocation scheme offers an improvement over the dynamic allocation scheme. Some RR configurations are not compliant with all MS multislot classes in the dynamic allocation scheme. In the dynamic allocation scheme, the MS must decode all USF values on all downlink PDCHs associated with the allocated uplink PDCHs.
The mobile monitors its assigned PDCHs starting from the lowest numbered one (the one that is mapped on the first allocated time slot in the TDMA frame), then it monitors the next lowest numbered time slot, and so on. Whenever the MS detects its assigned USF value on a PDCH, it transmits one radio block or a sequence of four radio blocks on the same PDCH and all higher-numbered assigned PDCHs. The mobile does not need to monitor the USF on these higher PDCHs. This is of particular interest in some RR configurations that are not compliant with all MS multislot classes in the dynamic allocation scheme.
Let us take the example of a class 12 MS, which is defined by:
A maximum number of four receive time slots per TDMA frame;
A maximum number of four transmit time slots per TDMA frame;
A total number of transmit and receive time slots per TDMA frame less than or equal to five.
The network cannot allocate four uplink PDCHs to a MS multislot class 12 with the dynamic allocation. Indeed, the MS must decode the USF fields on the four associated downlink PDCHs. This means that the MS would have to receive on four time slots to be able to transmit on four time slots. That gives a total number of eight received and transmit time slots, which is not compliant with a multislot class 12 MS. In the case of extended dynamic allocation, the network can allocate four uplink PDCHs without exceeding a total number of five receive and transmit time slots.
Fixed Allocation
Fixed allocation enables a given MS to be signaled predetermined uplink block occurrences on which it is allowed to transmit. The network assigns to each mobile a fixed uplink resource allocation of radio blocks onto one or several PDCHs.
The network allocates uplink radio blocks using bitmaps (series of zeros and ones). A 0 indicates that the mobile is not allowed to transmit, and a 1 indicates a transmission occurrence. The bitmaps are sent during the establishment of the uplink TBF. If more uplink resources are required during the uplink TBF, the network sends a bitmap in downlink on the PACCH.
A fixed allocation TBF operates as an open-ended TBF when an arbitrary number of octets are transferred during the uplink TBF. When the allocated bitmap ends, the MS requests a new bitmap if it wishes to continue the TBF.
A fixed allocation TBF operates as a close-ended TBF when the MS specifies the number of octets to be transferred during the uplink TBF establishment.
Comparison of Allocation Schemes
Fixed allocation allows for an efficient usage of the MS multislot capability, as the downlink monitoring is limited to one time slot (listening of the PACCH). The advantage of dynamic allocation is that the management of uplink resources is much more flexible.
From an implementation point of view, dynamic allocation is easier to implement on the network side. An efficient management of bitmap in fixed allocation is not so easy compared with USF handling. In fact, the management of resource allocation with anticipation (as needed in fixed allocation) is made more complex by the bursty nature of packet transfer. Most of the time the duration of the uplink TBF is completely unknown at the network side at the beginning of the TBF. This is why it is very difficult to manage radio block allocation at the beginning of the TBF. If the bitmap allocation is too short, the reaction time of the network for reallocation of new bitmaps will increase the duration of the TBF. If the bitmap allocation is too large compared with the TBF duration, block bitmap will be reallocated at the network side in order to avoid waste of uplink resources. Dynamic allocation is very easy to manage, since the USF allocation needs to anticipate only a few block periods.
3.4 BSS Architecture
3.4.1 PCU
In order to introduce GPRS within the BSS, the PCU concept has been defined. The PCU stands for a logical entity that manages packet toward the radio interface. The PCU communicates with the channel codec unit (CCU), positioned in the BTS.
The PCU is in charge of RLC/MAC functions such as segmentation and reassembly of LLC frames, transfer of RLC blocks in acknowledged or unacknowledged mode, radio resource assignment, and radio channel management. The CCU handles GSM layer 1 functions such as channel decoding, channel encoding, equalization, and radio channel measurements.
As shown in Figure 3.14, the PCU can be located either at the BTS site, the BSC site, or the SGSN site. When the PCU is located at the BSC or SGSN site, it is referred to as being a remote PCU.
Figure 3.14: Remote PCU position. (From- [1].)
If the PCU is located at the BSC, it could be implemented as an adjunct unit to the BSC. When the PCU is located at the SGSN side, the BSC is transparent for frames transmitted between the PCU and the CCU. This PCU location implies the implementation of a signaling protocol between the BSC and the PCU (e.g., for time slot management, access on CCCH management). A protocol is also needed when the PCU is located at the BTS side.
In the case of remote PCU, GPRS traffic between the PCU and the CCU are transferred through the Abis interface. The Abis interface in GSM is based on TRAU frames carrying speech data and having a fixed length of 320 bits (every 20 ms). This corresponds to a throughput of 16 Kbps per Abis channel.
As the PCU is supporting functions such as RLC block handling (retransmission, segmentation, and so forth) and access control, it needs to know the GSM radio interface timing. This implies in the case of a remote PCU the design of a synchronous interface between the PCU and the CCU. The PCU must be able to determine in which radio frames is sent an RLC/ MAC block. In case of a PCU located at the BTS side, the PCU easily knows the radio interface time.
The remote PCU solution requires the sending of in-band information between the PCU and the CCU (for transmission power indication, channel coding indication, and synchronization between the CCU and the PCU).
The advantages and drawbacks of each solution are listed next.
PCU at the BTS Side
Advantages:
There is an internal interface between the PCU and the CCU.
There is a low round-trip delay (the round-trip delay is the time between the transmission of a block and the reception of the answer).
There is no waste of Abis bandwidth due to retransmission of RLC blocks.
Drawbacks:
There is a likely impact on the existing BTS hardware (when migrating from a circuit-switched network to a GPRS one). This is an important drawback considering the number of BTSs in the field.
PCU at the BSC Side
Advantages:
There is an internal interface between the PCU and the BSC.
Drawbacks:
There are likely hardware impacts on the current BSC (however, the number of BSCs is lower compared with the number of BTSs).
There is a greater round-trip delay.
A synchronous protocol is needed between the PCU and the CCU.
Abis bandwidth is wasted in case of RLC blocks retransmission.
PCU at the SGSN Side
Advantages:
There is no hardware impact on the current GSM network (BTS, BSC).
There is a smooth introduction of GPRS in the network.
Drawbacks:
There is a greater round-trip delay (longer TBF establishment).
A synchronous protocol is needed between the PCU and the CCU.
Abis bandwidth is wasted in case of RLC blocks retransmission.
A protocol is needed between the BSC and the PCU.
3.4.2 Transmission Plane
When the PCU function is not implemented in the BTS, a new protocol (L1/L2) between the PCU and the BTS is introduced. This protocol ensures transmission of RLC/MAC blocks from the PCU to the CCU, and allows in-band signaling for control of the CCU from the PCU and synchronization between both entities. Figure 3.15 shows the BSS transmission plane for a remote PCU.
Figure 3.15: BSS transmission plane for a remote PCU.
When the PCU is located in the BTS, a protocol (L1/L2/L3) is needed between the BSC and the BTS for the transmission of the LLC frames. The BSS transmission plane is shown in Figure 3.16.
Figure 3.16: BSS transmission plane when the PCU is in the BTS.
3.4.3 Signaling Plane
Depending on the PCU location, the signaling plane within the BSS will not be the same.
3.4.3.1 PCU in the BTS
The RR layer is at the BTS side. The (L1/L2/L3) protocol is used for the transmission of signaling between the BSC and the BTS. Figure 3.17 shows the signaling plane when the PCU is located in the BTS.
Figure 3.17: Signaling plane when PCU is in the BTS.
3.4.3.2 PCU at BSC or SGSN Side
The signaling plane implies the usage of the (L1/L2) protocol for RR signaling transmission. Figure 3.18 shows the signaling plane when the PCU is located at BSC side or SGSN side.
Figure 3.18: Signaling plane when PCU is at BSC or SGSN side.
GPRS represents an evolution of the GSM standard, allowing data transmission in packet mode and providing higher throughputs as compared with the circuit-switched mode. This evolution is usually presented under the designation of 2.5G to point out that it is a transition technology between 2G and 3G.
The GPRS network architecture reuses the GSM network nodes such as MSC/VLR, HLR, and BSS. New network nodes have been introduced for the transport of packet data. These nodes are the gateway GPRS support nodes (GGSN) and serving GPRS support nodes (SGSN). The subnetwork formed by the SGSNs and the GGSNs is called the GPRS core network. In order to reuse the GSM nodes, new interfaces have been defined between the GSM network nodes and the different elements of the GPRS core network. The GPRS logical architecture is described in Section 3.1.
The protocol layer has been split into two planes. On one side there is the transmission plane, which is mainly used for the transfer of user data. The signaling plane is used for the control and support of the transmission plane functions. Section 3.2 deals with the transmission and signaling planes.
GPRS has kept such main principles of the GSM radio interface as the notions of time slot, frame, multiframe, and hyperframe structures. It was indeed chosen by the operators and manufacturers involved in the system design to provide high-data-rate packet-switched services with minimized impacts on the GSM standard. The principles of the physical layer are given in Section 3.3.1. The details related to the physical layer are presented in Chapter 4.
One of the main GPRS characteristics is that a physical connection is established in uplink only when the MS needs to send continuous data to the network, and in downlink only when the network needs to send continuous data to the MS. This physical connection is released in one direction as soon as the sending entity has no more data to send. Different allocation schemes for radio resource (RR) management have been defined in order to multiplex several MSs on the same physical channel. An overview of the principles related to RR management is presented in Section 3.3.2. A complete description of RR management can be found in Chapter 5.
A logical entity called PCU has been introduced within the BSS to manage the GPRS functions over the radio interface. Section 3.4 deals with the BSS architecture and discusses the several possible locations of the PCU.
In a GPRS network, an RA identifies one or several cells. As soon as an MS enters a new RA, the network must be notified of this change in order to update its location. The SGSN is in charge of GPRS mobility management (GMM). An overview of GPRS mobility is proposed in Section 3.5. The details related to GMM are given in Chapter 7.
If data transmission in packet mode does not require the establishment of an end-to-end connection, it is necessary to establish a context between the mobile and the network in order to exchange packets. This context allows the network to identify the IP address of the MS, identify the access point with the external network, and define the QoS associated with data transmission in packet mode. The concept of PDP context is explained in Section 3.6. The details related to PDP context management are given in Chapter 7.
The BSS and the GPRS backbone network are connected via the Gb interface in order to exchange user data and signaling information. The principles of the Gb interface are given in Section 3.8. The details related to the Gb interface are given in Chapter 6.
When a context is established between the MS and the network, IP packet exchange may start at any time between the mobile and the network without establishing a connection beforehand. The packets are conveyed in the GPRS backbone network. An overview of the general architecture of the GPRS backbone network is presented in Section 3.9. A complete description of the user plane between the MS and external data packet network is given in Chapter 8.
3.1 GPRS Logical Architecture
Figure 3.1 shows the elements that are part of a GPRS network and their associated interfaces. A GPRS network is composed of the following network nodes:
SGSN. The SGSN is the node that is serving the MS; it is responsible for GMM. It delivers packets to the MSs and communicates with the HLR to obtain the GPRS subscriber profile. It manages the registration of the new mobile subscribers in order to keep a record of their LA for routing purposes. The SGSN can be connected to one or several BSSs.
GGSN. The GGSN provides interworking with external packet data networks (PDNs). It may be linked to one or several data networks. It is connected with SGSNs via an IP-based GPRS backbone network. The GGSN is a router that forwards incoming packets from the external PDN to the SGSN of the addressed MS. It also forwards outgoing packets to the external PDN. The PDN is the external fixed data network to which is connected the GPRS network. An example of a PDN is the Internet network.
HLR. The HLR is a database that contains, among other things, packet domain subscription data and routing information.
Mobile switching center/visitor location register (MSC/VLR). The MSC coordinates the setting up of calls to and from GSM users and manages GSM mobility. The MSC is not directly involved in the GPRS network. It forwards circuit-switched paging for the GPRS-attached MSs to the SGSN when the Gs interface is present.
BSS. The BSS ensures the radio connection between the mobile and the network. It is responsible for radio access management. The BSS is composed of two elements: the BTS and the BSC. The BTS integrates all the radio transmission and radio reception boards. The BSC is responsible for the management of the radio channels. The BSC has switching capabilities that are used for circuit-switched calls and can also be used for GPRS traffic.
EIR. The EIR is a database that contains terminal identities.
3.2 Transmission and Signaling Planes
3.2.1 Transmission Plane
The transmission plane consists of a layered protocol structure providing user data transfer, along with associated procedures that control the information transfer such as flow control, error detection, and error correction. Figure 3.2 illustrates the layered protocol structure between the MS and the GGSN.
Figure 3.2: Transmission plane MS-GGSN.
3.2.1.1 Air Interface
The air interface is located between the MS and the BSS. The protocols used on the air interface are as follows:
Radio link control/medium access control (RLC/MAC). RLC provides a reliable radio link between the mobile and the BSS. MAC controls the access signaling procedures to the GPRS radio channel, and the multiplexing of signaling and RLC blocks from different users onto the GSM physical channel.
GSM-RF layer. It is the radio subsystem that supports a certain number of logical channels. This layer is split into two sublayers: the radio frequency layer (RFL), which handles the radio and baseband part (physical channel management, modulation, demodulation, and transmission and reception of radio blocks), and the physical link layer (PLL), which manages control of the RFL (power control, synchronization, measurements, and channel coding/decoding).
A relay function is implemented in the BSS to relay the LLC PDUs between the air interface and the Gb interface.
3.2.1.2 Gb Interface
The Gb interface is located between the SGSN and the BSS. It supports data transfer in the transmission plane. The Gb interface supports the following protocols:
BSS GPRS protocol (BSSGP). This layer conveys routing and QoS-related information between the BSS and SGSN.
Network service (NS). It transports BSSGP PDUs and is based on a frame relay connection between the BSS and SGSN.
A relay function is implemented in the SGSN to relay the packet data protocol (PDP) PDUs between the Gb and Gn interfaces (IP PDUs in Figure 3.2).
3.2.1.3 Gn/Gp Interface
The Gn interface is located between two GSNs (SGSN or GGSN) within the same PLMN, while the Gp interface is between two GSNs in different PLMNs. The Gn/Gp interface is used for the transfer of packets between the SGSN and the GGSN in the transmission plane.
The Gn/Gp interface supports the following protocols:
GPRS tunnelling protocol (GTP). This protocol tunnels user data between the SGSN and GGSN in the GPRS backbone network. GTP operates on top of UDP over IP. The layers L1 and L2 of the Gn interfaces are not specified in the GSM/GPRS standard.
User datagram protocol (UDP). It carries GTP packet data units (PDUs) in the GPRS Core Network for protocols that do not need a reliable data link (e.g., IP).
Internet protocol (IP). This is the protocol used for routing user data and control signaling within the GPRS backbone network.
3.2.1.4 Interface Between MS and SGSN
This interface supports the following protocols:
Subnetwork-dependent convergence protocol (SNDCP). This protocol maps the IP protocol to the underlying network. SNDCP also provides other functions such as compression, segmentation, and multiplexing of network layer messages.
Logical link control (LLC). This layer provides a highly reliable logical link that is independent of the underlying radio interface protocols. LLC is also responsible for the GPRS ciphering.
3.2.2 Signaling Plane
The signaling plane consists of protocols for control and support of the transmission plane functions. It controls both the access connections to the GPRS network (e.g., GPRS attach and GPRS detach) and the attributes of an established network access connection (e.g., activation of a PDP address), manages the routing of information for a dedicated network connection in order to support user mobility, adapts network resources depending on the QoS parameters, and provides supplementary services.
3.2.2.1 Between MS and SGSN
Figure 3.3 shows the signaling plane between the MS and the SGSN. This plane is made up of the following protocols:
GMM. The GMM protocol supports mobility management functionalities such as GPRS attach, GPRS detach, security, RA update, and location update (see Section 3.5).
Session management (SM). The SM protocol supports functionalities such as PDP context activation, PDP context modification, and PDP context deactivation (see Section 3.6).
Figure 3.3: Signaling plane MS-SGSN.
3.2.2.2 Between Two GSNs
In the signaling plane, Gn/Gp interfaces are used for the transfer of signaling between the GSNs in the GPRS backbone network. Figure 3.4 shows the signaling plane between two GSNs.
Figure 3.4: Signaling plane GSN-GSN.
The signaling plane between two GSNs is made up of the following protocols:
GTP for the control plane (GTP-C). This protocol tunnels signaling messages between SGSNs and GGSNs, and between SGSNs, in the GPRS core network (see Section 3.9).
UDP. This protocol transfers signaling messages between GSNs.
3.2.2.3 Interface with Signaling System No. 7
The various GSNs of the GPRS backbone network use a Signaling System No. 7 (SS7) network to exchange information with GSM SS7 network nodes such as HLR, MSC/VLR, EIR, and SMS-GMSC. The SS7 network provides facilities to quickly exchange messages between GPRS backbone network nodes irrespective of data transmission through the GPRS PLMN network.
In the GSM/GPRS backbone network, we found the following protocols for SS7 signaling:
Message transfer part (MTP). The three MTP layers allow signaling messages to be exchanged through the SS7 network.
Signaling connection control part (SCCP). The SCCP protocol layer allows the service to be used in connected mode and messages to be exchanged between different PLMNs by using an international gateway for SS7 address translation between an SS7 global address (based on the E.164 numbering plan) and an SS7 local address.
Transaction capabilities application part (TCAP). The TCAP protocol layer allows dialogs to be structured in an independent manner from any application.
Mobile application part (MAP). The MAP protocol layer allows mobile mobility to be managed within different equipment nodes of the NSS across SS7 networks.
As new equipment nodes have been introduced in GSM networks to support the GPRS feature, new interfaces were defined with the HLR, MSC/VLR, EIR, and SMS-GMSC. Table 3.1 lists the new interfaces with SS7 network.
Table 3.1: New Interfaces with the SS7 Network Interface Name
Localization
Mandatory/Optional
Gr
SGSN-HLR
Mandatory
Gc
GGSN-HLR
Optional
Gf
SGSN-EIR
Optional
Gd
SGSN-SMS GMSC or SGSN-SMS IWMSC
Optional
Gs
SGSN-MSC/VLR
Optional
Gr Interface
The Gr interface is defined between the SGSN and HLR. It allows the SGSN to retrieve or update GPRS subscription and GPRS location information in the HLR during location-management or authentication procedures. The MAP protocol has been modified to take into account this interface. Figure 3.5 shows the signaling plane on the Gr interface.
Figure 3.5: Signaling plane on the Gr interface.
Gc Interface
The Gc interface is defined between the GGSN and the HLR. The GGSN contacts the HLR in order to determine the SGSN address where the MS is located and if the MS is reachable. The MAP protocol has been modified to take into account this interface. Figure 3.6 shows the signaling plane on the Gc interface. (Note: If a GGSN does not have a SS7 MAP interface, it will interface to a GSN performing a GTP-MAP protocol-conversion in order to retrieve the needed information from the HLR via the Gc interface.)
Figure 3.6: Signaling plane on the Gc interface.
Gf Interface
The Gf interface is defined between the SGSN and EIR. It is used by the SGSN to contact the EIR database during the identity check procedure. It allows the SGSN to check the IMEI against the EIR. The MAP protocol has been modified to take into account this interface. Figure 3.7 shows the signaling plane on the Gf interface.
Figure 3.7: Signaling plane on the Gf interface.
Gs Interface
The Gs interface is defined between the MSC/VLR and the SGSN. It allows for the coordination of circuit-switched and packet-switched paging in the SGSN as well as location information of any MS attached to both circuit and packet services. This interface is only present in a network that operates in network operation mode I (see definition in Section 3.5.3.1). The BSS application part+ (BSSAP+) allows mobility functionality to be managed on the Gs interface. Figure 3.8 shows the signaling plane on the Gs interface.
Figure 3.8: Signaling plane on the Gs interface.
Gd Interface
The Gd interface is defined between an SGSN and an SMS-GMSC or an SMS-IWMSC. The progress of a short message intended for delivery to an MS requires in circuit and packet modes a gateway function Short Message Service-gateway MSC (SMS-GMSC) between the mobile network and the network that provides access to the SMS center. An SMS to be delivered to an MS is routed from the SMS-GMSC toward the SGSN on the Gd interface if this SMS is to be sent over GPRS.
The progress of a short message originated by the MSs requires in circuit and packet modes a PLMN interworking server SMS-IWMSC (inter-working MSC) that provides access to the SMS center. An SMS originated by an MS is routed from the SGSN toward the SMS-IWMSC on the Gd interface if the SMS is to be sent over GPRS.
The MAP protocol has been updated as a consequence of the signaling exchange between the SGSN and the SMS-GMSC or the SMS-IWMSC. Figure 3.9 shows the signaling plane on the Gd interface.
Figure 3.9: Signaling plane on the Gd interface.
3.3 Radio Interface
3.3.1 Physical Layer Principles
3.3.1.1 Packet Data Channel
The GPRS physical layer is based on that of the GSM (see Chapter 1). The access scheme is TDMA, with eight basic physical channels per carrier (TS 0 to 7).
A physical channel uses a combination of frequency- and time-division multiplexing and is defined as a radio frequency channel and time slot pair. The physical channel that is used for packet logical channels is called a packet data channel (PDCH). PDCHs are dynamically allocated in the cell by the network. The PDCH is mapped on a 52-multiframe, as shown in Figure 3.10. The 52-multiframe consists of 12 radio blocks (BO to B11) of 4 consecutive TDMA frames and 4 idle frames (frames 12, 25, 38, and 51), amounting to a total of 52 frames.
Figure 3.10: Time slots and TDMA frames.
3.3.1.2 Packet Data Logical Channel
GPRS, like GSM, uses the concept of logical channels mapped on top of the physical channels. Two types of logical channels have been introduced, namely traffic channels and control channels. Three subtypes of control channels have been defined for GPRS: broadcast, common control, and associated. In addition, the GSM common control channels (BCCH, CCCH, and RACH) may be used to access the network and establish packet transfer.
The different packet data logical channels are:
Packet broadcast control channel (PBCCH). The presence of PBCCH in the cell is optional. The PBCCH broadcasts information relative to the cell in which the mobile camps and information on the neighbor cells. This information is used by the mobile in order to access the network. When there is no PBCCH in the cell, the information needed by the mobile to access the network for a packet transfer is broadcast on BCCH.
Packet common control channel (PCCCH). The PCCCH is present in the cell only if the PBCCH is present in the cell. When it is not present in the cell, the common control signaling for GPRS is handled on the GSM common control channels (CCCH). PCCCH is composed of packet random access channel (PRACH), used for random access, packet paging channel (PPCH), used for paging, and packet access grant channel (PAGCH), used for access grant. The PRACH is used by the MS to initiate uplink access to the network. The PPCH is used by the network to page the mobile in order to establish a downlink packet transfer. The PAGCH is used by the network to assign radio resources to the mobile for a packet transfer.
Packet data traffic channel (PDTCH). The PDTCH is used to transfer user data during uplink or downlink packet transfer. The PDTCH is a unidirectional channel, either uplink (PDTCH/U) for a mobile-originated packet transfer or downlink (PDTCH/D) for a mobile-terminated packet transfer. A PDTCH is a resource allocated on one physical channel by the network for user data transmission.
Packet associated control channel (PACCH). The PACCH is a unidirectional channel that is used to carry signaling during uplink or downlink packet data transfer. The uplink PACCH carries signaling from the MS to the network and the downlink PACCH carries signaling from the network to the mobile. The PACCH is dynamically allocated on a block basis.
Packet timing advance control channel (PTCCH). The PTCCH is a bidirectional channel that is used for TA update. The PTCCH is an optional channel. The PTCCH when present is mapped on frames number 12 and 38 of the 52—multiframe.
Table 3.2 lists the various GPRS logical channels.
Table 3.2: GPRS Logical Channels Logical Channel
Abbreviation
Uplink/ Downlink
Task
Packet broadcast control channel
PBCCH
DL
Packet system broadcast information
Packet paging channel
PPCH
DL
MS paging for downlink transfer establishment
Packet random access channel
PRACH
UL
MS random access for uplink transferestablishment
Packet access grant channel
PAGCH
DL
Radio resources assignment
Packet timing advance control channel
PTCCH
UL/DL
Timing advance update
Packet associated control channel
PACCH
UL/DL
Signaling associated with data transfer
Packet data traffic channel
PDTCH
UL/DL
Data channel
3.3.1.3 Multislot Classes Definition
In order to provide higher throughputs, a GPRS MS may transmit or receive in several time slots of the TDMA frame. The multislot capability is indicated by the multislot class of the GPRS MS. This multislot class is defined by several parameters such as the maximum number of time slots supported by the MS per TDMA frame in uplink and in downlink. The multislot class of the MS is sent to the network during the GPRS attach procedure. A detail of multislot classes is given in Section 4.2.1.
3.3.1.4 Cell Reselection
In GPRS as in GSM, the mobile performs cell reselection. However, there are some differences compared with GSM.
In GPRS the mobile performs cell reselection when it is in idle mode but also during packet transfer. The cell reselection is either performed by the mobile autonomously or optionally controlled by the network. Unlike GSM, there is no handover in GPRS but only cell reselections. So when there is a reselection during a packet transfer, this latter is interrupted and it has to be started again in the new cell. There is an interruption of the packet transfer during the reselection phase.
Although the GPRS cell reselection algorithms used by the mobile are based on the same principles as those used in GSM, they have been slightly modified in order to provide more flexibility. These algorithms are described in Section 5.3.
3.3.1.5 Radio Environment Monitoring
The MS performs different types of radio measurements that are reported to the network and used by it for RLC. These estimations are also used by the mobile itself to compute its transmission power (open-loop power control; see Section 4.1.3.1), for cell selection and cell reselection.
The mobile performs the following types of measurement:
Received signal level (RXLEV) measurements. The RXLEV measurements are performed on both serving cell and neighbor cells for the purpose of cell reselection. During packet-transfer mode (see Section 3.3.2.1), the serving cell RXLEV measurement can also be used for downlink coding scheme adaptation (see Section 4.1.2.1), network-controlled cell reselection, and downlink and uplink power control.
Quality (RXQUAL) measurements. The RXQUAL is computed from the average BER before channel decoding. During packet-transfer mode the mobile estimates the quality of the downlink blocks it receives. In packet idle mode, no quality measurements are performed. The RXQUAL can be used by the network for network-controlled cell reselection, dynamic coding scheme adaptation, and downlink power control. The RXQUAL is the current GSM quality indicator.
Interference measurements. These measurements have been introduced for GPRS. They correspond to a received signal level measurement performed on a frequency that is different from a beacon frequency. The interest is in having an estimation of the interference level on the PDTCH. It can be used by the network to optimize the mobile RR allocation, to select a more appropriate coding scheme, to trigger a network-controlled cell reselection, and for power control or for network statistics.
The measurements are either used by the MS for its own purposes or by the network. The RXLEV and RXQUAL measurements are also performed at the BSS side for each MS. They are used for network-controlled cell reselection, uplink power control (closed loop, see Section 4.1.3.1), and dynamic coding scheme adaptation.
3.3.1.6 Principles of Power Control
Power control can be used in order to improve the spectrum efficiency while maintaining radio link quality and reducing the power consumption in the MS. Power control in GPRS is more complicated than for a circuit-switched connection, since there is not necessarily a continuous two-way connection.
Note that power control can be performed in both uplink and downlink directions. Uplink power control and downlink power control are described in Sections 4.1.3.1 and 4.1.3.2, respectively.
3.3.1.7 Channel Coding
The GPRS user data is sent on radio blocks encoded with one of four channel coding schemes (CS1, CS2, CS3, CS4). The GPRS signaling is sent with the CS1 channel coding scheme. This scheme provides the highest protection level against error transmission, while the CS4 channel coding scheme provides the lowest protection level. The more the channel coding scheme provides an efficient protection level against error transmission, the more the useful data throughput decreases due to redundant information added to source data. The channel coding scheme used on uplink and downlink depends on the radio quality between the network and the MS. Table 3.3 gives the throughputs associated with each coding scheme.
Table 3.3: Throughput Associated with Coding Scheme Coding Scheme
Throughput (Kbps)
CS-1
9.05
CS-2
13.4
CS-3
15.6
CS-4
21.4
3.3.2 RR Management Principles
3.3.2.1 RR Operating Modes
At the RR level, the MS behavior is dependent on two operating RR states. These states, packet idle mode and packet transfer mode, allow the RR activity of the MS to be characterized.
Packet Idle Mode
When the MS is in packet idle mode, no radio resources are allocated. Leaving packet idle mode occurs when upper layers request the transfer of uplink data requiring the assignment of uplink resources from the network. It also occurs at the reception of a downlink resource assignment command from the network for a downlink transfer.
In case of downlink transfer, the mobile switches from packet idle mode to packet transfer mode when it receives the downlink assignment command from the network. In the case of uplink transfer, the mobile leaves packet idle mode when it requests the assignment of uplink resources to the network. However, switching to packet transfer mode is not instantaneous. The mobile switches to packet transfer mode only when it has been uniquely identified at the network side; this will be explained in more detail in Chapter 5. Thus there is a period between packet idle mode and packet transfer mode during which the mobile is in a transitory state.
During packet idle mode, the MS listens to its PCH and the CBCH. This last one is the PBCCH when present in the cell; otherwise it is the BCCH.
Packet Transfer Mode
When the MS is in packet transfer mode, it is clearly identified at the network side and uplink or/and downlink radio resources are allocated.
Switching from packet transfer mode to packet idle mode occurs when the network releases all downlink and uplink resources. This transition can also occur in the case of an abnormal condition during packet transfer mode (e.g., radio link failure) or when the mobile decides on a cell reselection toward a new cell.
During packet transfer mode, the mobile transmits and receives data. Figure 3.11 summarizes the transition between the different RR states.
Figure 3.11: Transition between RR operating modes.
3.3.2.2 Temporary Block Flow
A temporary block flow (TBF) is a physical connection between the RR entity in the MS and the RR entity at the network side to support the unidirectional transfer of LLC protocol data units over PDCH. A TBF is characterized by one or several PDCHs allocated by the network to an MS for the duration of the data transfer. Once the data transfer is finished, the TBF is released.
When the mobile must send continuous data to the network, it requests the establishment of an uplink TBF by sending signaling information over CCCH or PCCCH. When the network wants to send data to the mobile, it assigns a downlink TBF between the two RR entities.
A downlink TBF supports the transfer of data from the network to the mobile, while an uplink TBF supports the transfer of data from the mobile to the network. One uplink and one downlink TBF can be supported at the same time between the two RR entities. These two TBFs are defined as concurrent TBFs.
The number of TBFs per mobile and per direction is limited to one. However TBFs belonging to different mobiles can share the same PDCH.
Note As a packet transfer session is composed of a lot of requests, responses, and acknowledgments, many consecutive uplink and downlink TBFs are established for the same session (e.g., Web browsing).
Each TBF is identified by a temporary flow identifier (TFI) assigned by the network. So in case of concurrent TBFs, one TFI identifies the uplink TBF and another one the downlink TBF. The TFI is used to differentiate TBFs sharing the same PDCHs in one direction.
Figure 3.12 gives an example of TBF mapping for different mobiles onto PDCHs. A downlink TBF, identified by a TFI equal to 2, has assigned resources on the PDCH numbers 2, 3, and 4, while another downlink TBF identified by a TFI equal to 1 has assigned resources on the PDCH numbers 4, 5, and 6. An uplink TBF identified by a TFI equal to 4 has assigned resources on the PDCH numbers 3 and 4.
Figure 3.12: Example of mapping of TBFs with their respective TFI onto PDCHs.
3.3.2.3 Allocation Modes on the Uplink
Several MSs may be multiplexed on the same PDCH. In order to share the uplink bandwidth between several mobiles mapped on the same PDCH, different allocation schemes have been defined to allocate an uplink radio block instance to a particular mobile.
Three allocation schemes exist for medium access control:
Dynamic allocation;
Extended dynamic allocation;
Fixed allocation.
The second scheme is optional for the mobile while the others are mandatory. On the network side, either fixed allocation or dynamic allocation must be implemented. Extended dynamic allocation is optional for the network.
Dynamic Allocation
In principle, dynamic allocation allows uplink transmission to mobiles sharing the same PDCH, on a block-by-block basis. During the uplink TBF establishment, an uplink state flag (USF) is given to the MS for each allocated uplink PDCH. The USF is used as a token given by the network to allow transmission of one uplink block.
Whenever the network wants to allocate one radio block occurrence on one uplink PDCH, it includes, on the associated downlink PDCH, the USF in the radio block immediately preceding the allocated block occurrence. When the mobile decodes its assigned USF value in a radio block sent on a downlink PDCH associated with an allocated uplink one, it transmits an uplink radio block in the next uplink radio block occurrence, that is, the B(x) radio block if the USF was detected in the B(x- 1) radio block.
The principle of dynamic allocation is illustrated in Figure 3.13.
Figure 3.13: Principle of dynamic allocation.
The USF is included in the header of each downlink RLC/MAC block. The USF coding (3 bits) enables eight mobiles to be multiplexed on the same uplink PDCH.
Dynamic allocation implies the constant monitoring (radio block decoding) of the downlink PDCHs associated with the allocated uplink PDCHs.
As explained previously, the USF allows the sending of one block in the next uplink occurrence. However, dynamic allocation can also be used in such a way that the decoding of one USF value allows the mobile to send four consecutive uplink blocks on the same PDCH. The choice between one block or four blocks is indicated during the TBF establishment by the network to the mobile.
The concept of USF granularity has been introduced in order to indicate the number of uplink radio blocks to be sent upon detection of the assigned USF by the MS. The USF granularity is conveyed to the MS during the uplink TBF establishment. Thus it involves the MS transmitting either a single radio block or a sequence of four consecutive radio blocks starting on the B(x) radio block if the USF was detected in the B(x - 1) radio block.
Extended Dynamic Allocation
The extended dynamic allocation scheme offers an improvement over the dynamic allocation scheme. Some RR configurations are not compliant with all MS multislot classes in the dynamic allocation scheme. In the dynamic allocation scheme, the MS must decode all USF values on all downlink PDCHs associated with the allocated uplink PDCHs.
The mobile monitors its assigned PDCHs starting from the lowest numbered one (the one that is mapped on the first allocated time slot in the TDMA frame), then it monitors the next lowest numbered time slot, and so on. Whenever the MS detects its assigned USF value on a PDCH, it transmits one radio block or a sequence of four radio blocks on the same PDCH and all higher-numbered assigned PDCHs. The mobile does not need to monitor the USF on these higher PDCHs. This is of particular interest in some RR configurations that are not compliant with all MS multislot classes in the dynamic allocation scheme.
Let us take the example of a class 12 MS, which is defined by:
A maximum number of four receive time slots per TDMA frame;
A maximum number of four transmit time slots per TDMA frame;
A total number of transmit and receive time slots per TDMA frame less than or equal to five.
The network cannot allocate four uplink PDCHs to a MS multislot class 12 with the dynamic allocation. Indeed, the MS must decode the USF fields on the four associated downlink PDCHs. This means that the MS would have to receive on four time slots to be able to transmit on four time slots. That gives a total number of eight received and transmit time slots, which is not compliant with a multislot class 12 MS. In the case of extended dynamic allocation, the network can allocate four uplink PDCHs without exceeding a total number of five receive and transmit time slots.
Fixed Allocation
Fixed allocation enables a given MS to be signaled predetermined uplink block occurrences on which it is allowed to transmit. The network assigns to each mobile a fixed uplink resource allocation of radio blocks onto one or several PDCHs.
The network allocates uplink radio blocks using bitmaps (series of zeros and ones). A 0 indicates that the mobile is not allowed to transmit, and a 1 indicates a transmission occurrence. The bitmaps are sent during the establishment of the uplink TBF. If more uplink resources are required during the uplink TBF, the network sends a bitmap in downlink on the PACCH.
A fixed allocation TBF operates as an open-ended TBF when an arbitrary number of octets are transferred during the uplink TBF. When the allocated bitmap ends, the MS requests a new bitmap if it wishes to continue the TBF.
A fixed allocation TBF operates as a close-ended TBF when the MS specifies the number of octets to be transferred during the uplink TBF establishment.
Comparison of Allocation Schemes
Fixed allocation allows for an efficient usage of the MS multislot capability, as the downlink monitoring is limited to one time slot (listening of the PACCH). The advantage of dynamic allocation is that the management of uplink resources is much more flexible.
From an implementation point of view, dynamic allocation is easier to implement on the network side. An efficient management of bitmap in fixed allocation is not so easy compared with USF handling. In fact, the management of resource allocation with anticipation (as needed in fixed allocation) is made more complex by the bursty nature of packet transfer. Most of the time the duration of the uplink TBF is completely unknown at the network side at the beginning of the TBF. This is why it is very difficult to manage radio block allocation at the beginning of the TBF. If the bitmap allocation is too short, the reaction time of the network for reallocation of new bitmaps will increase the duration of the TBF. If the bitmap allocation is too large compared with the TBF duration, block bitmap will be reallocated at the network side in order to avoid waste of uplink resources. Dynamic allocation is very easy to manage, since the USF allocation needs to anticipate only a few block periods.
3.4 BSS Architecture
3.4.1 PCU
In order to introduce GPRS within the BSS, the PCU concept has been defined. The PCU stands for a logical entity that manages packet toward the radio interface. The PCU communicates with the channel codec unit (CCU), positioned in the BTS.
The PCU is in charge of RLC/MAC functions such as segmentation and reassembly of LLC frames, transfer of RLC blocks in acknowledged or unacknowledged mode, radio resource assignment, and radio channel management. The CCU handles GSM layer 1 functions such as channel decoding, channel encoding, equalization, and radio channel measurements.
As shown in Figure 3.14, the PCU can be located either at the BTS site, the BSC site, or the SGSN site. When the PCU is located at the BSC or SGSN site, it is referred to as being a remote PCU.
Figure 3.14: Remote PCU position. (From- [1].)
If the PCU is located at the BSC, it could be implemented as an adjunct unit to the BSC. When the PCU is located at the SGSN side, the BSC is transparent for frames transmitted between the PCU and the CCU. This PCU location implies the implementation of a signaling protocol between the BSC and the PCU (e.g., for time slot management, access on CCCH management). A protocol is also needed when the PCU is located at the BTS side.
In the case of remote PCU, GPRS traffic between the PCU and the CCU are transferred through the Abis interface. The Abis interface in GSM is based on TRAU frames carrying speech data and having a fixed length of 320 bits (every 20 ms). This corresponds to a throughput of 16 Kbps per Abis channel.
As the PCU is supporting functions such as RLC block handling (retransmission, segmentation, and so forth) and access control, it needs to know the GSM radio interface timing. This implies in the case of a remote PCU the design of a synchronous interface between the PCU and the CCU. The PCU must be able to determine in which radio frames is sent an RLC/ MAC block. In case of a PCU located at the BTS side, the PCU easily knows the radio interface time.
The remote PCU solution requires the sending of in-band information between the PCU and the CCU (for transmission power indication, channel coding indication, and synchronization between the CCU and the PCU).
The advantages and drawbacks of each solution are listed next.
PCU at the BTS Side
Advantages:
There is an internal interface between the PCU and the CCU.
There is a low round-trip delay (the round-trip delay is the time between the transmission of a block and the reception of the answer).
There is no waste of Abis bandwidth due to retransmission of RLC blocks.
Drawbacks:
There is a likely impact on the existing BTS hardware (when migrating from a circuit-switched network to a GPRS one). This is an important drawback considering the number of BTSs in the field.
PCU at the BSC Side
Advantages:
There is an internal interface between the PCU and the BSC.
Drawbacks:
There are likely hardware impacts on the current BSC (however, the number of BSCs is lower compared with the number of BTSs).
There is a greater round-trip delay.
A synchronous protocol is needed between the PCU and the CCU.
Abis bandwidth is wasted in case of RLC blocks retransmission.
PCU at the SGSN Side
Advantages:
There is no hardware impact on the current GSM network (BTS, BSC).
There is a smooth introduction of GPRS in the network.
Drawbacks:
There is a greater round-trip delay (longer TBF establishment).
A synchronous protocol is needed between the PCU and the CCU.
Abis bandwidth is wasted in case of RLC blocks retransmission.
A protocol is needed between the BSC and the PCU.
3.4.2 Transmission Plane
When the PCU function is not implemented in the BTS, a new protocol (L1/L2) between the PCU and the BTS is introduced. This protocol ensures transmission of RLC/MAC blocks from the PCU to the CCU, and allows in-band signaling for control of the CCU from the PCU and synchronization between both entities. Figure 3.15 shows the BSS transmission plane for a remote PCU.
Figure 3.15: BSS transmission plane for a remote PCU.
When the PCU is located in the BTS, a protocol (L1/L2/L3) is needed between the BSC and the BTS for the transmission of the LLC frames. The BSS transmission plane is shown in Figure 3.16.
Figure 3.16: BSS transmission plane when the PCU is in the BTS.
3.4.3 Signaling Plane
Depending on the PCU location, the signaling plane within the BSS will not be the same.
3.4.3.1 PCU in the BTS
The RR layer is at the BTS side. The (L1/L2/L3) protocol is used for the transmission of signaling between the BSC and the BTS. Figure 3.17 shows the signaling plane when the PCU is located in the BTS.
Figure 3.17: Signaling plane when PCU is in the BTS.
3.4.3.2 PCU at BSC or SGSN Side
The signaling plane implies the usage of the (L1/L2) protocol for RR signaling transmission. Figure 3.18 shows the signaling plane when the PCU is located at BSC side or SGSN side.
Figure 3.18: Signaling plane when PCU is at BSC or SGSN side.
GPRS Services
2.1 Use of GPRS
The GPRS provides a set of GSM services for data transmission in packet mode within a PLMN. In packet-switched mode, no permanent connection is established between the mobile and the external network during data transfer. Instead, in circuit-switched mode, a connection is established during the transfer duration between the calling entity and the called entity. In packet-switched mode, data is transferred in data blocks, called packets. When the transmission of packets is needed, a channel is allocated, but it is released immediately after. This method increases the network capacity. Indeed, several users can share a given channel, since it is not allocated to a single user during an entire call period.
One of the main purposes of GPRS is to facilitate the interconnection between a mobile and the other packet-switched networks, which opens the doors to the world of the Internet. With the introduction of packet mode, mobile telephony and Internet converge to become mobile Internet technology. This technology introduced in mobile phones allows users to have access to new value-added services, including:
Client-server services, which enable access to data stored in databases. The most famous example of this is access to the World Wide Web (WWW) through a browser.
Messaging services, intended for user-to-user communication between individual users via storage servers for message handling. Multimedia Messaging Service (MMS) is an example of a well-known messaging application.
Real-time conversational services, which provide bidirectional communication in real-time. A number of Internet and multimedia applications require this scheme such as voice over IP and video conferencing.
Tele-action services, which are characterized by short transactions and are required for services such as SMS, electronic monitoring, surveillance systems, and lottery transactions.
GPRS allows for radio resource optimization by using packet switching for data applications that may present the following transmission characteristics:
Infrequent data transmission, as when the time between two transmissions exceeds the average transfer delay (e.g., messaging services);
Frequent transmission of small data blocks, in processes of several transactions of less than 500 octets per minute (e.g., downloading of several HTML pages from a browsing application);
Infrequent transmission of larger data blocks, in processes of several transactions per hour (e.g., access of information stored in database centers);
Asymmetrical throughput between uplink and downlink, such as for data retrieval in a server where the uplink is used to send signaling commands and the downlink is used to receive data as a response of the request (e.g., WEB/WAP browser).
As the GPRS operator optimizes radio resources by sharing them between several users, he is able to propose more attractive fees for data transmission in GPRS mode than in circuit-switched mode. Indeed, the invoicing in circuit-switched mode takes into account the connection time between the calling user and the called user. Studies on data transmission show that data are exchanged from end to end during 20% of a circuit-switched connection time. For example, a user browses the WWW, downloads an HTML page identified by a uniform resource locator (URL), reads the content of the HTML page, then downloads a new HTML page to read. In this example no data is exchanged from end to end between the two HTML page downloads. For this type of application, a more appropriate invoicing would take into account the volume of data exchanged instead of the circuit-switched connection time. In packet mode, the GPRS user may be invoiced according to the requested service type, the volume of data exchanged.
2.2 GPRS MS Classes
Three GPRS classes have been defined: class A, class B, and class C.
The class A mobile can support simultaneously a communication in circuit-switched mode and another one in packet-switched mode. It is also capable of detecting in idle mode an incoming call in circuit or packet-switched mode.
The class B mobile can detect an incoming call in circuit-switched mode or in packet-switched mode during the idle mode but cannot support them simultaneously. The circuit and packet calls are performed sequentially. In some configurations desired by the user, a GPRS communication may be suspended in order to perform a communication in circuit-switched mode and then may be resumed after the communication release in circuit-switched mode.
The class C mobile supports either a communication in circuit-switched mode or in packet-switched mode but is not capable of simultaneously supporting communications in both modes. It is not capable of simultaneously detecting the incoming calls in circuit-switched and packet-switched mode during idle mode. Thus a class C mobile is configured either in circuit-switched mode or in packet-switched mode. The mode configuration is selected either manually by the user or automatically by an application.
A mobile defined in class A or class B is IMSI attached for GPRS services, and non-GPRS services while a mobile defined in class C is IMSI attached if it operates in circuit-switched mode or IMSI attached for GPRS services if it operates in packet-switched mode. (Note: An MS that is IMSI attached means that it is attached to the GSM network.)
2.3 Client-Server Relation
The GPRS packet-transmission mode relies on the "client/server" principle from the computer world, rather than the "calling/called" principle in use in the telephony domain. The client sends a request to the server, which processes the request and sends the result to the client. Thus the mobile may be configured according to the application either in client mode or in server mode.
The mobile may be configured in client mode to have access to the Internet or an intranet or database by initiating a GPRS communication. Usually, the GPRS mobile is configured as a client. Figure 2.1 shows a GPRS MS configured in client mode.
Figure 2.1: GPRS mobile configured as a client.
The mobile may also be configured in server mode for vertical application to telemetry monitoring. In this type of application, the mobile may be connected to different pieces of equipment, such as a camera for monitoring or a captor for measurements. The mobile may configure a piece of equipment in order to process the request and then send back the result to the client. In order to interpret a request from a client, the mobile must be able to route information from the network toward the recipient application. In server mode, the MS must be IMSI attached for GPRS services in order to receive the requests from a client.
2.4 Quality of Service
The network associates a certain quality of service (QoS) with each data transmission in GPRS packet mode. The appropriate QoS is characterized according to a number of attributes negotiated between the MS and the network. Figure 2.2 characterizes the application in terms of error tolerance and delay requirements.
Figure 2.2: Applications in terms of QoS requirements. (From- [1].)
A first list of attributes is defined in Release 97/98 of the 3GPP recommendations. It was replaced in the release 99 by new attributes.
2.4.1 Attributes in Release 97/98
In Release 97/98 of the 3GPP recommendations, QoS is defined according to the following attributes:
Precedence class. This indicates the packet transfer priority under abnormal conditions, as for example during a network congestion load.
Reliability class. This indicates the transmission characteristics; it defines the probability of data loss, data delivered out of sequence, duplicate data delivery, and corrupted data. This parameter enables the configuration of layer 2 protocols in acknowledged or unacknowledged modes.
Peak throughput class. This indicates the expected maximum data transfer rate across the network for a specific access to an external packet switching network (from 8 to 2,048 Kbps).
Mean throughput class. This indicates the average data transfer rate across the network during the remaining lifetime of a specific access to an external packet switching network (best effort, from 0.22 bps to 111 Kbps).
Delay class. This defines the end-to-end transfer delay for the transmission of service data units (SDUs) through the GPRS network. The SDU represents the data unit accepted by the upper layer of GPRS and conveyed through the GPRS network. Table 2.1 shows the delay classes.
Table 2.1: Delay Classes Delay (Maximum Values)
SDU size: 128 octets
SDU size: 1,024 octets
Delay Class
Mean Transfer Delay (s)
95 Percentile Delay (s)
Mean Transfer Delay (s)
95 Percentile Delay (s)
1. (Predictive)
< 0.5
< 1.5
< 2
< 7
2. (Predictive)
< 5
< 25
< 15
< 75
3. (Predictive)
< 50
< 250
< 75
< 375
4. (Best Effort)
Unspecified
From: [2].
The delay class for data transfer gives some information about the number of resources that have to be allocated for a given service. Predictive value in delay class means that the network is able to ensure an end-to-end delay time for the transmission of SDUs; best effort means that the network is not able to ensure a value for an end-to-end transfer delay; in this case transmission of SDUs depends on network load.
2.4.2 Attributes in Release 99
The attributes of GPRS QoS were modified in Release 99 of the 3GPP recommendations in order to be identical to the ones defined for UMTS. The attributes described below apply to both GPRS and UMTS standards. Table 2.2 gives the characteristics of the different classes.
Table 2.2: Traffic Classes Traffic Class
Real-Time Conversational
Real-Time Streaming
Interactive Best Effort
Background Best Effort
Fundamental Characteristics
No transfer delay variation between the sender and the receiver; stringent and low delay transfer
No transfer delay variation between the sender and the receiver
Request response pattern; preserve pattern content
No time constraint; preserve pattern content
Example of Applications
Conversational voice and videophone
One-way video, audio streaming, still image, and bulk data
Web browsing, voice messaging and dictation, server access, and e-commerce
E-mail, SMS, and fax
Four classes of traffic have been defined for QoS:
Conversational class. These services are dedicated to bidirectional communication in real time (e.g., voice over IP and videoconferencing).
Streaming class. These services are dedicated to unidirectional data transfer in real time (e.g., audio streaming, one-way video).
Interactive class. These services are dedicated to the transport of human or machine interaction with remote equipment (e.g., Web browsing, access to a server, access to a database).
Background class. These services are dedicated to machine-to-machine communication that is not delay sensitive (e.g., e-mail and SMS).
Table 2.3 lists the expected performance for conversational services.
Table 2.3: End User Performance Expectations-Conversational/Real-Time Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
End-to-End One-Way Delay
Delay Variation Within a Call
Information Loss
Audio
Conversational voice
Two-way
4-25 Kbps
<150 ms preferred
<400 ms limit Note 1
< 1 ms
< 3% of frame error rate
Video
Videophone
Two-way
32-384 Kbps
< 150 ms preferred
< 400 ms limit
Lip-synch: <100 ms
< 1% of frame error rate
Data
Telemetry - two-way control
Two-way
<28.8 Kbps
< 250 ms
N/A
Zero
Data
Interactive games
Two-way
< 1 KB
< 250 ms
N/A
Zero
Data
Telnet
Two-way (asymmetric)
< 1 KB
< 250 ms
N/A
Zero
From: [1].
Table 2.4 lists the expected performance for streaming services.
Table 2.4: End User Performance Expectations-Streaming Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
One-Way Delay
Delay Variation
Information Loss
Audio
High-quality streaming audio
Primarily oneway
32-128 Kbps
< 10 s
< 1 ms
<1% FER
Video
One-way
One-way
32-384 Kbps
< 10 s
<1% FER
Data
Bulk data transfer/ retrieval
Primarily oneway
< 10 s
N/A
Zero
Data
Still image
One-way
< 10 s
N/A
Zero
Data
Telemetry-monitoring
One-way
<28.8 Kbps
< 10 s
N/A
Zero
From: [1].
Table 2.5 lists the expected performance for interactive services.
Table 2.5: End User Performance Expectations-Interactive Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
One-Way Delay
Delay Variation
Information Loss
Audio
Voice messaging
Primarily no way
4-13 Kbps
< 1 sec for playback
< 2 sec for record
< 1 ms
< 3% FER
Data
Web browsing-HTML
Primarily oneway
< 4 sec/page
N/A
Zero
Data
Transaction services - high priority (e.g., e-commerce and ATM)
Two-way
< 4 sec
N/A
Zero
Data
E-mail (server access)
Primarily oneway
< 4 sec
N/A
Zero
From: [1].
The Release 99 of 3GPP recommendations defines attributes for QoS such as traffic class, delivery order, SDU format information, SDU error ratio, maximum SDU size, maximum bit rate for uplink, maximum bit rate for downlink, residual bit error ratio, transfer delay, traffic-handling priority, allocation/retention priority, and guaranteed bit rate for uplink and guaranteed bit rate for downlink.
Traffic class indicates the application type (conversational, streaming, interactive, background).
Delivery order indicates if there is in-sequence SDU delivery or not.
Delivery of erroneous SDUs indicates if erroneous SDUs are delivered or discarded.
SDU format information indicates the possible exact sizes of SDUs.
SDU error ratio indicates the maximum allowed fraction of SDUs lost or detected as erroneous.
Maximum SDU size indicates the maximum allowed SDU size (from 10 octets to 1,520 octets).
Maximum bit rate for uplink indicates the maximum number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
Maximum bit rate for downlink indicates the maximum number of bits delivered by the network within a period of time (from 0 to 8,640 Kbps).
Residual bit error ratio indicates the undetected bit error ratio for each subflow in the delivered SDUs.
Transfer delay indicates the maximum time of SDU transfer for 95th percentile of the distribution of delay for all delivered SDUs.
Traffic-handling priority indicates the relative importance of all SDUs belonging to a specific GPRS bearer compared with all SDUs of other GPRS bearers.
Allocation/retention priority indicates the relative importance of resource allocation and resource retention for the data flow related to a specific GPRS bearer compared with the data flows of other GPRS bearers (useful when resources are scarce).
Guaranteed bit rate for uplink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
Guaranteed bit rate for downlink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
2.5 Third-Generation Partnership Project
The GPRS recommendations belong to the GSM recommendations. The maintenance of all GSM recommendations is now handled within the Third Generation Partnership Project (3GPP) organization, the partners of which are:
ETSI, the European standardization entity;
Association of Radio Industries and Businesses (ARIB) and Telecommunication Technology Committee (TTC), the Japanese standardization entities;
Telecommunication Technology Association (TTA), the Korean standardization entity;
T1, the American standardization entity;
China Wireless Telecommunication Standard (CWTS) group, the Chinese standardization entity.
These standardization bodies have decided to collaborate within the 3GPP organization in order to produce specifications for a third-generation mobile system. At the beginning, the 3GPP organization was in charge of all specifications related to the third-generation mobile system for radio access technologies called Universal Terrestrial Radio Access (UTRA) and for evolution of GSM core networks. Since August 2000, specifications related to GSM radio access are also the responsibility of the 3GPP.
The 3GPP takes the place of the former Special Mobile Group (SMG) GSM organization. The 3GPP is organized around technical specification groups (TSGs) that deal with the following subjects:
TSG SA (Service Architecture), dealing with service, architecture, security, and speech coding aspects;
TSG RAN (Radio Access Network), focusing on UTRA radio access technologies;
TSG CN (Core Network), dealing with core network specifications;
TSG T (Terminal), covering applications, tests for 3G mobiles, and the USIM card;
TSG GERAN (GSM EDGE Radio Access Network), focusing on GSM radio interface, A and Gb interfaces.
Thus GSM evolutions as GPRS are treated in all TSGs except TSG RAN, which deals exclusively with UTRAN access technologies such as frequency division duplex (FDD), time-division duplex (TDD), and CDMA 2000. The TSG GERAN deals exclusively with the GSM radio interface evolutions and with A and Gb interfaces.
The 3GPP recommendations are ranked according to a version reference. Each new version of 3GPP recommendations contains a list of new features or a list of improvements on existing features. Initially, the GSM recommendations versions were referenced in the following order: Phase 1; Phase 2; Release 96; Release 97; Release 98; Release 99. As the reference year for the new version of phase 2++ recommendations no longer matched the release year of these specifications, it was decided that the versions following Release 99 will be referenced according to a version number, Release 4 being the first new version reference.
The GSM recommendations are organized up to Release 99 in the following series
01 series: General;
02 series: Service Aspects;
03 series: Network Aspects;
04 series: MS-BS Interface and Protocols;
05 series: Physical Layer on the Radio Path;
06 series: Speech Coding Specification;
07 series: Terminal Adaptors for MSs;
08 series: BS-MSC Interface;
09 series: Network Interworking;
11 series: Equipment and Type Approval Specification;
12 series: Operation and Maintenance.
Each of the series contains a list of specifications identified by numbers. A given specification is therefore defined by its series number, followed by a recommendation number. For example, the 05.03 specification belongs to the physical layer on the radio path, and deals with channel coding issues.
In Release 4 of the 3GPP recommendations, the former GSM 01, 02, 03, 04, 05, and 06 series were kept for all GSM features that have not evolved with the third generation. From Release 4, these GSM series numbers were replaced by new series numbers, to be compliant with the 3GPP numbering. New series numbers can easily be deduced by adding 40 to the GSM series. Thus the 05 series describing the physical layer on the radio path become the 45 series from release 4. Also, the specification numbers in each series are deduced from the previous numbers by inserting a 0 as the first number. Thus, the R99 05.03 becomes the 45.003 from R4. Note that the different releases are continuously maintained.
The GPRS feature was introduced in Release 97 of the 3GPP recommendations. The GPRS recommendations are organized in three stages, as are all 3GPP recommendations:
Stage 1: Description of GPRS services;
Stage 2: Description of GPRS general architecture;
Stage 3: Detailed description of different equipment implemented for GPRS with their external interfaces.
The Stage 1 GPRS recommendations describe the services that will be provided by GPRS. The service description is given in the 02 series recommendations for Release 97 and Release 98 of the 3GPP recommendations and in the 22 series recommendations from Release 99. The evolution of GPRS services is discussed in Working Group 1 (WG1) of TSG SA within the 3GPP.
The stage 2 GPRS recommendations describe the general architecture of GPRS, with nodes implemented in the network and the interface mechanisms between these nodes. The description of the architecture is given in the 03 series of the recommendations for Release 97 and Release 98 of the 3GPP recommendations and in 23 series recommendations from Release 99. The evolution of GPRS architecture is discussed in WG2 of TSG SA within the 3GPP.
The stage 3 GPRS recommendations describe in a detailed manner the equipment and its external interfaces implemented in the network for GPRS (see 04, 05, 06, and 08 series recommendations up to Release 99, and then 44, 45, and 46 series recommendations from Release 4). The evolution of the behavior of this equipment is discussed in working groups of several associated TSGs.
The GPRS provides a set of GSM services for data transmission in packet mode within a PLMN. In packet-switched mode, no permanent connection is established between the mobile and the external network during data transfer. Instead, in circuit-switched mode, a connection is established during the transfer duration between the calling entity and the called entity. In packet-switched mode, data is transferred in data blocks, called packets. When the transmission of packets is needed, a channel is allocated, but it is released immediately after. This method increases the network capacity. Indeed, several users can share a given channel, since it is not allocated to a single user during an entire call period.
One of the main purposes of GPRS is to facilitate the interconnection between a mobile and the other packet-switched networks, which opens the doors to the world of the Internet. With the introduction of packet mode, mobile telephony and Internet converge to become mobile Internet technology. This technology introduced in mobile phones allows users to have access to new value-added services, including:
Client-server services, which enable access to data stored in databases. The most famous example of this is access to the World Wide Web (WWW) through a browser.
Messaging services, intended for user-to-user communication between individual users via storage servers for message handling. Multimedia Messaging Service (MMS) is an example of a well-known messaging application.
Real-time conversational services, which provide bidirectional communication in real-time. A number of Internet and multimedia applications require this scheme such as voice over IP and video conferencing.
Tele-action services, which are characterized by short transactions and are required for services such as SMS, electronic monitoring, surveillance systems, and lottery transactions.
GPRS allows for radio resource optimization by using packet switching for data applications that may present the following transmission characteristics:
Infrequent data transmission, as when the time between two transmissions exceeds the average transfer delay (e.g., messaging services);
Frequent transmission of small data blocks, in processes of several transactions of less than 500 octets per minute (e.g., downloading of several HTML pages from a browsing application);
Infrequent transmission of larger data blocks, in processes of several transactions per hour (e.g., access of information stored in database centers);
Asymmetrical throughput between uplink and downlink, such as for data retrieval in a server where the uplink is used to send signaling commands and the downlink is used to receive data as a response of the request (e.g., WEB/WAP browser).
As the GPRS operator optimizes radio resources by sharing them between several users, he is able to propose more attractive fees for data transmission in GPRS mode than in circuit-switched mode. Indeed, the invoicing in circuit-switched mode takes into account the connection time between the calling user and the called user. Studies on data transmission show that data are exchanged from end to end during 20% of a circuit-switched connection time. For example, a user browses the WWW, downloads an HTML page identified by a uniform resource locator (URL), reads the content of the HTML page, then downloads a new HTML page to read. In this example no data is exchanged from end to end between the two HTML page downloads. For this type of application, a more appropriate invoicing would take into account the volume of data exchanged instead of the circuit-switched connection time. In packet mode, the GPRS user may be invoiced according to the requested service type, the volume of data exchanged.
2.2 GPRS MS Classes
Three GPRS classes have been defined: class A, class B, and class C.
The class A mobile can support simultaneously a communication in circuit-switched mode and another one in packet-switched mode. It is also capable of detecting in idle mode an incoming call in circuit or packet-switched mode.
The class B mobile can detect an incoming call in circuit-switched mode or in packet-switched mode during the idle mode but cannot support them simultaneously. The circuit and packet calls are performed sequentially. In some configurations desired by the user, a GPRS communication may be suspended in order to perform a communication in circuit-switched mode and then may be resumed after the communication release in circuit-switched mode.
The class C mobile supports either a communication in circuit-switched mode or in packet-switched mode but is not capable of simultaneously supporting communications in both modes. It is not capable of simultaneously detecting the incoming calls in circuit-switched and packet-switched mode during idle mode. Thus a class C mobile is configured either in circuit-switched mode or in packet-switched mode. The mode configuration is selected either manually by the user or automatically by an application.
A mobile defined in class A or class B is IMSI attached for GPRS services, and non-GPRS services while a mobile defined in class C is IMSI attached if it operates in circuit-switched mode or IMSI attached for GPRS services if it operates in packet-switched mode. (Note: An MS that is IMSI attached means that it is attached to the GSM network.)
2.3 Client-Server Relation
The GPRS packet-transmission mode relies on the "client/server" principle from the computer world, rather than the "calling/called" principle in use in the telephony domain. The client sends a request to the server, which processes the request and sends the result to the client. Thus the mobile may be configured according to the application either in client mode or in server mode.
The mobile may be configured in client mode to have access to the Internet or an intranet or database by initiating a GPRS communication. Usually, the GPRS mobile is configured as a client. Figure 2.1 shows a GPRS MS configured in client mode.
Figure 2.1: GPRS mobile configured as a client.
The mobile may also be configured in server mode for vertical application to telemetry monitoring. In this type of application, the mobile may be connected to different pieces of equipment, such as a camera for monitoring or a captor for measurements. The mobile may configure a piece of equipment in order to process the request and then send back the result to the client. In order to interpret a request from a client, the mobile must be able to route information from the network toward the recipient application. In server mode, the MS must be IMSI attached for GPRS services in order to receive the requests from a client.
2.4 Quality of Service
The network associates a certain quality of service (QoS) with each data transmission in GPRS packet mode. The appropriate QoS is characterized according to a number of attributes negotiated between the MS and the network. Figure 2.2 characterizes the application in terms of error tolerance and delay requirements.
Figure 2.2: Applications in terms of QoS requirements. (From- [1].)
A first list of attributes is defined in Release 97/98 of the 3GPP recommendations. It was replaced in the release 99 by new attributes.
2.4.1 Attributes in Release 97/98
In Release 97/98 of the 3GPP recommendations, QoS is defined according to the following attributes:
Precedence class. This indicates the packet transfer priority under abnormal conditions, as for example during a network congestion load.
Reliability class. This indicates the transmission characteristics; it defines the probability of data loss, data delivered out of sequence, duplicate data delivery, and corrupted data. This parameter enables the configuration of layer 2 protocols in acknowledged or unacknowledged modes.
Peak throughput class. This indicates the expected maximum data transfer rate across the network for a specific access to an external packet switching network (from 8 to 2,048 Kbps).
Mean throughput class. This indicates the average data transfer rate across the network during the remaining lifetime of a specific access to an external packet switching network (best effort, from 0.22 bps to 111 Kbps).
Delay class. This defines the end-to-end transfer delay for the transmission of service data units (SDUs) through the GPRS network. The SDU represents the data unit accepted by the upper layer of GPRS and conveyed through the GPRS network. Table 2.1 shows the delay classes.
Table 2.1: Delay Classes Delay (Maximum Values)
SDU size: 128 octets
SDU size: 1,024 octets
Delay Class
Mean Transfer Delay (s)
95 Percentile Delay (s)
Mean Transfer Delay (s)
95 Percentile Delay (s)
1. (Predictive)
< 0.5
< 1.5
< 2
< 7
2. (Predictive)
< 5
< 25
< 15
< 75
3. (Predictive)
< 50
< 250
< 75
< 375
4. (Best Effort)
Unspecified
From: [2].
The delay class for data transfer gives some information about the number of resources that have to be allocated for a given service. Predictive value in delay class means that the network is able to ensure an end-to-end delay time for the transmission of SDUs; best effort means that the network is not able to ensure a value for an end-to-end transfer delay; in this case transmission of SDUs depends on network load.
2.4.2 Attributes in Release 99
The attributes of GPRS QoS were modified in Release 99 of the 3GPP recommendations in order to be identical to the ones defined for UMTS. The attributes described below apply to both GPRS and UMTS standards. Table 2.2 gives the characteristics of the different classes.
Table 2.2: Traffic Classes Traffic Class
Real-Time Conversational
Real-Time Streaming
Interactive Best Effort
Background Best Effort
Fundamental Characteristics
No transfer delay variation between the sender and the receiver; stringent and low delay transfer
No transfer delay variation between the sender and the receiver
Request response pattern; preserve pattern content
No time constraint; preserve pattern content
Example of Applications
Conversational voice and videophone
One-way video, audio streaming, still image, and bulk data
Web browsing, voice messaging and dictation, server access, and e-commerce
E-mail, SMS, and fax
Four classes of traffic have been defined for QoS:
Conversational class. These services are dedicated to bidirectional communication in real time (e.g., voice over IP and videoconferencing).
Streaming class. These services are dedicated to unidirectional data transfer in real time (e.g., audio streaming, one-way video).
Interactive class. These services are dedicated to the transport of human or machine interaction with remote equipment (e.g., Web browsing, access to a server, access to a database).
Background class. These services are dedicated to machine-to-machine communication that is not delay sensitive (e.g., e-mail and SMS).
Table 2.3 lists the expected performance for conversational services.
Table 2.3: End User Performance Expectations-Conversational/Real-Time Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
End-to-End One-Way Delay
Delay Variation Within a Call
Information Loss
Audio
Conversational voice
Two-way
4-25 Kbps
<150 ms preferred
<400 ms limit Note 1
< 1 ms
< 3% of frame error rate
Video
Videophone
Two-way
32-384 Kbps
< 150 ms preferred
< 400 ms limit
Lip-synch: <100 ms
< 1% of frame error rate
Data
Telemetry - two-way control
Two-way
<28.8 Kbps
< 250 ms
N/A
Zero
Data
Interactive games
Two-way
< 1 KB
< 250 ms
N/A
Zero
Data
Telnet
Two-way (asymmetric)
< 1 KB
< 250 ms
N/A
Zero
From: [1].
Table 2.4 lists the expected performance for streaming services.
Table 2.4: End User Performance Expectations-Streaming Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
One-Way Delay
Delay Variation
Information Loss
Audio
High-quality streaming audio
Primarily oneway
32-128 Kbps
< 10 s
< 1 ms
<1% FER
Video
One-way
One-way
32-384 Kbps
< 10 s
<1% FER
Data
Bulk data transfer/ retrieval
Primarily oneway
< 10 s
N/A
Zero
Data
Still image
One-way
< 10 s
N/A
Zero
Data
Telemetry-monitoring
One-way
<28.8 Kbps
< 10 s
N/A
Zero
From: [1].
Table 2.5 lists the expected performance for interactive services.
Table 2.5: End User Performance Expectations-Interactive Services Key Performance Parameters and Target Values
Medium
Application
Degree of Symmetry
Data Rate
One-Way Delay
Delay Variation
Information Loss
Audio
Voice messaging
Primarily no way
4-13 Kbps
< 1 sec for playback
< 2 sec for record
< 1 ms
< 3% FER
Data
Web browsing-HTML
Primarily oneway
< 4 sec/page
N/A
Zero
Data
Transaction services - high priority (e.g., e-commerce and ATM)
Two-way
< 4 sec
N/A
Zero
Data
E-mail (server access)
Primarily oneway
< 4 sec
N/A
Zero
From: [1].
The Release 99 of 3GPP recommendations defines attributes for QoS such as traffic class, delivery order, SDU format information, SDU error ratio, maximum SDU size, maximum bit rate for uplink, maximum bit rate for downlink, residual bit error ratio, transfer delay, traffic-handling priority, allocation/retention priority, and guaranteed bit rate for uplink and guaranteed bit rate for downlink.
Traffic class indicates the application type (conversational, streaming, interactive, background).
Delivery order indicates if there is in-sequence SDU delivery or not.
Delivery of erroneous SDUs indicates if erroneous SDUs are delivered or discarded.
SDU format information indicates the possible exact sizes of SDUs.
SDU error ratio indicates the maximum allowed fraction of SDUs lost or detected as erroneous.
Maximum SDU size indicates the maximum allowed SDU size (from 10 octets to 1,520 octets).
Maximum bit rate for uplink indicates the maximum number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
Maximum bit rate for downlink indicates the maximum number of bits delivered by the network within a period of time (from 0 to 8,640 Kbps).
Residual bit error ratio indicates the undetected bit error ratio for each subflow in the delivered SDUs.
Transfer delay indicates the maximum time of SDU transfer for 95th percentile of the distribution of delay for all delivered SDUs.
Traffic-handling priority indicates the relative importance of all SDUs belonging to a specific GPRS bearer compared with all SDUs of other GPRS bearers.
Allocation/retention priority indicates the relative importance of resource allocation and resource retention for the data flow related to a specific GPRS bearer compared with the data flows of other GPRS bearers (useful when resources are scarce).
Guaranteed bit rate for uplink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
Guaranteed bit rate for downlink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).
2.5 Third-Generation Partnership Project
The GPRS recommendations belong to the GSM recommendations. The maintenance of all GSM recommendations is now handled within the Third Generation Partnership Project (3GPP) organization, the partners of which are:
ETSI, the European standardization entity;
Association of Radio Industries and Businesses (ARIB) and Telecommunication Technology Committee (TTC), the Japanese standardization entities;
Telecommunication Technology Association (TTA), the Korean standardization entity;
T1, the American standardization entity;
China Wireless Telecommunication Standard (CWTS) group, the Chinese standardization entity.
These standardization bodies have decided to collaborate within the 3GPP organization in order to produce specifications for a third-generation mobile system. At the beginning, the 3GPP organization was in charge of all specifications related to the third-generation mobile system for radio access technologies called Universal Terrestrial Radio Access (UTRA) and for evolution of GSM core networks. Since August 2000, specifications related to GSM radio access are also the responsibility of the 3GPP.
The 3GPP takes the place of the former Special Mobile Group (SMG) GSM organization. The 3GPP is organized around technical specification groups (TSGs) that deal with the following subjects:
TSG SA (Service Architecture), dealing with service, architecture, security, and speech coding aspects;
TSG RAN (Radio Access Network), focusing on UTRA radio access technologies;
TSG CN (Core Network), dealing with core network specifications;
TSG T (Terminal), covering applications, tests for 3G mobiles, and the USIM card;
TSG GERAN (GSM EDGE Radio Access Network), focusing on GSM radio interface, A and Gb interfaces.
Thus GSM evolutions as GPRS are treated in all TSGs except TSG RAN, which deals exclusively with UTRAN access technologies such as frequency division duplex (FDD), time-division duplex (TDD), and CDMA 2000. The TSG GERAN deals exclusively with the GSM radio interface evolutions and with A and Gb interfaces.
The 3GPP recommendations are ranked according to a version reference. Each new version of 3GPP recommendations contains a list of new features or a list of improvements on existing features. Initially, the GSM recommendations versions were referenced in the following order: Phase 1; Phase 2; Release 96; Release 97; Release 98; Release 99. As the reference year for the new version of phase 2++ recommendations no longer matched the release year of these specifications, it was decided that the versions following Release 99 will be referenced according to a version number, Release 4 being the first new version reference.
The GSM recommendations are organized up to Release 99 in the following series
01 series: General;
02 series: Service Aspects;
03 series: Network Aspects;
04 series: MS-BS Interface and Protocols;
05 series: Physical Layer on the Radio Path;
06 series: Speech Coding Specification;
07 series: Terminal Adaptors for MSs;
08 series: BS-MSC Interface;
09 series: Network Interworking;
11 series: Equipment and Type Approval Specification;
12 series: Operation and Maintenance.
Each of the series contains a list of specifications identified by numbers. A given specification is therefore defined by its series number, followed by a recommendation number. For example, the 05.03 specification belongs to the physical layer on the radio path, and deals with channel coding issues.
In Release 4 of the 3GPP recommendations, the former GSM 01, 02, 03, 04, 05, and 06 series were kept for all GSM features that have not evolved with the third generation. From Release 4, these GSM series numbers were replaced by new series numbers, to be compliant with the 3GPP numbering. New series numbers can easily be deduced by adding 40 to the GSM series. Thus the 05 series describing the physical layer on the radio path become the 45 series from release 4. Also, the specification numbers in each series are deduced from the previous numbers by inserting a 0 as the first number. Thus, the R99 05.03 becomes the 45.003 from R4. Note that the different releases are continuously maintained.
The GPRS feature was introduced in Release 97 of the 3GPP recommendations. The GPRS recommendations are organized in three stages, as are all 3GPP recommendations:
Stage 1: Description of GPRS services;
Stage 2: Description of GPRS general architecture;
Stage 3: Detailed description of different equipment implemented for GPRS with their external interfaces.
The Stage 1 GPRS recommendations describe the services that will be provided by GPRS. The service description is given in the 02 series recommendations for Release 97 and Release 98 of the 3GPP recommendations and in the 22 series recommendations from Release 99. The evolution of GPRS services is discussed in Working Group 1 (WG1) of TSG SA within the 3GPP.
The stage 2 GPRS recommendations describe the general architecture of GPRS, with nodes implemented in the network and the interface mechanisms between these nodes. The description of the architecture is given in the 03 series of the recommendations for Release 97 and Release 98 of the 3GPP recommendations and in 23 series recommendations from Release 99. The evolution of GPRS architecture is discussed in WG2 of TSG SA within the 3GPP.
The stage 3 GPRS recommendations describe in a detailed manner the equipment and its external interfaces implemented in the network for GPRS (see 04, 05, 06, and 08 series recommendations up to Release 99, and then 44, 45, and 46 series recommendations from Release 4). The evolution of the behavior of this equipment is discussed in working groups of several associated TSGs.
Introduction to the GSM System
Chapter 1: Introduction to the GSM System
This chapter provides an overview of the GSM cellular system, with a focus on the radio interface. The purpose is not to give a detailed description of the many features supported by the system, but to summarize the elementary concepts of GSM, as an aid to reader comprehension of the subsequent chapters. In-depth presentation of the GSM system can be found in [1, 2].
1.1 Introduction
1.1.1 Birth of the GSM System
The first step in the history of GSM development was achieved back in 1979, at the World Administrative Radio Conference, with the reservation of the 900-MHz band. In 1982 at the Conference of European Posts and Telegraphs (CEPT) in Stockholm, the Groupe Spécial Mobile was created, to implement a common mobile phone service in Europe on this 900-MHz frequency band. Currently the acronym GSM stands for Global System for Mobile Communication; the term "global" was preferred due to the intended adoption of this standard in every continent of the world.
The proposed system had to meet certain criteria, such as:
Good subjective speech quality (similar to the fixed network);
Affordability of handheld terminals and service;
Adaptability of handsets from country to country;
Support for wide range of new services;
Spectral efficiency improved with respect to the existing first-generation analog systems;
Compatibility with the fixed voice network and the data networks such as ISDN;
Security of transmissions.
Digital technology was chosen to ensure call quality.
The basic design of the system was set by 1987, after numerous discussions led to the choice of key elements such as the narrowband time-division multiple access (TDMA) scheme, or the modulation technique. In 1989 responsibility for the GSM was transferred to the European Telecommunication Standards Institute (ETSI). ETSI was asked by the EEC to unify European regulations in the telecommunications sector and in 1990 published phase I of the GSM system specifications (the phase 2 recommendations were published in 1995).
The first GSM handset prototypes were presented in Geneva for Telecom '91, where a GSM network was also set up. Commercial service had started by the end of 1991, and by 1993 there were 36 GSM networks in 22 countries. The system was standardized in Europe, but is now operational in more than 160 countries all over the world, and was adopted by 436 operators.
The growth of subscribers has been tremendous, reaching 500 million by May 2001.
1.1.2 The Standard Approach
The thousands of pages of GSM recommendations, designed by operators and infrastructure and mobile vendors, provide enough standardization to guarantee proper interworking between the components of the system. This is achieved by means of the functional and interface descriptions for each of the different entities.
The GSM today is still under improvement, with the definition of new features and evolution of existing features. This permanent evolution is reflected in the organization of the recommendations, first published as phase I, then phase II and phase II+, and now published with one release each year (releases 96, 97, 98, 99, and releases 4 and 5 in 2000 and 2001).
As stated, responsibility for the GSM specifications was carried by ETSI up to the end of 1999. During 2000, the responsibility of the GSM recommendations was transferred to the Third Generation Partnership Project (3GPP). This world organization was created to produce the third-generation mobile system specifications and technical reports. The partners have agreed to cooperate in the maintenance and development of GSM technical specifications and technical reports, including evolved radio access technologies [e.g., General Packet Radio Service (GPRS) and Enhanced Data rates for Global Evolution (EDGE)]. The structure and organization of the 3GPP is further described in Section 2.5.
1.2 General Concepts
1.2.1 Analog Versus Digital Telephony Systems
First-generation systems were analog. During the early 1980s these systems underwent rapid development in Europe. Although the NMT system was used by all the Nordic countries, and the TACS system in the United Kingdom and Italy, there was a variety of systems and no compatibility among them. Compared with these systems, the main advantages offered by GSM, which is the most important of the second-generation digital systems, are:
Standardization;
Capacity;
Quality;
Security.
Standardization guarantees compatibility among systems of different countries, allowing subscribers to use their own terminals in those countries that have adopted the digital standard. The lack of standardization in the first-generation system limited service to within the borders of a country. Mobility is improved, since roaming is no longer limited to areas covered by a certain system (see Section 1.2.6). Calls can be charged and handled using the same personal number even when the subscriber moves from one country to another.
Standardization also allows the operator to buy entities of the network from different vendors, since the functional elements of the network and the interfaces between these elements are standardized. This means that a mobile phone from any manufacturer is able to communicate with any network, even if this network is built with entities from different vendors. This leads to a large economy of scale and results in cost reduction for both the operator and the subscriber. Furthermore, the phone cost is also reduced, because as GSM is an international standard, produced quantities are greater and the level of competition is high.
With respect to capacity, the use of the radio resource is much more efficient in a digital system such as GSM than in an analog system. This means that more users can be allocated in the same frequency bandwidth. This is possible with the use of advanced digital techniques, such as voice compression algorithms, channel coding, and multiple access techniques. Note that capacity gains are also achieved with radio frequency reuse, which had also been used in analog systems. Frequency reuse means that a given carrier can be employed in different areas, as explained in Section 1.2.2.
The quality in digital transmission systems is better, thanks to the channel coding schemes that increase the robustness in the face of noise and disturbances such as interference caused by other users or other systems. The quality improvement is also due to the improved control of the radio link, and adaptations to propagation conditions, with advanced techniques such as power control or frequency hopping. This will be explained in greater detail in Section 1.5.6.3.
In terms of security, powerful authentication and encryption techniques for voice and data communications are enabled with GSM, which guarantees protected access to the network, and confidentiality.
1.2.2 Cellular Telephony
In mobile radio systems, one of the most important factors is the frequency spectrum. In order to make the best use of the bandwidth, the system is designed by means of the division of the service area into neighboring zones, or cells, which in theory have a hexagonal shape. Each cell has a base transceiver station (BTS), which to avoid interference operates on a set of radio channels different from those of the adjacent cells. This division allows for the use of the same frequencies in nonadjacent cells. A group of cells that as a whole use the entire radio spectrum available to the operator is referred to as a cluster. The shape of a cell is irregular, depending on the availability of a spot for the BTS, the geography of the terrain, the propagation of the radio signal in the presence of obstacles, and so on.
In dense urban areas, for instance, where the mobile telephony traffic is important, the diameter of the cells is often reduced in order to increase capacity. This is allowed since the same frequency channels are used in a smaller area. On the other hand, reducing the cell diameter leads to a decrease in the distance necessary to reuse the frequencies (that is, the distance between two cochannel cells), increasing cochannel interference. In order to minimize the level of interference, several techniques are used on the radio interface.
A basic example of cluster organization is shown in Figure 1.1. In this example, we see a reuse pattern for seven different frequencies, f1 to f7. These frequencies correspond to the beacon carrier of each cell, on which signaling information about the cell is broadcast (see Section 1.2.7). It can be seen from this figure that a given carrier can be reused in two separate geographical areas, as long as these areas are far enough from each other to reduce the effect of interference. With this technique of dividing the area in cells and clusters, the operator can increase the area it is able to cover with a limited frequency bandwidth.
Figure 1.1: Example of a cell planning.
1.2.3 Public Land Mobile Network
A public land mobile network (PLMN) is a network established for the purpose of providing land mobile telecommunications services to the public. It may be considered as an extension of a fixed network, such as the Public Switched Telephone Network (PSTN), or as an integral part of the PSTN.
1.2.4 Multiband Mobile Phones
Because of the increasing demand on the mobile networks, today the mobile stations (MSs) tend to be multiband. Indeed, to avoid network saturation in densely populated regions, mobile phones capable of supporting different frequency bands have been implemented, to allow for the user making communications in any area, at any time.
A dual-band phone can operate in two different frequency bands of the same technology, for instance in the 900-MHz and 1800-MHz frequency bands of the GSM system. Triple-band mobile phones have also come on the market, with the support of GSM-900 (900-MHz GSM band), DCS-1800 (1800-MHz GSM band), and PCS-1900 (1900-MHz GSM band), for example. Note that DCS-1800 and PCS-1900 are never deployed in the same country, and therefore this kind of phone can be used by travelers who want to have service coverage in a large number of countries.
1.2.5 SIM Card
One of the most interesting innovations of GSM is that the subscriber's data is not maintained in the mobile phone. Rather a "smart card," called a subscriber identity module (SIM) card, is used.
The SIM is inserted in the phone to allow the communications. A user may thus make telephone calls with a mobile phone that is not his own, or have several phones but only one contract. It is for example possible to use a SIM card in a different mobile when traveling to a country that has adopted the GSM on a different frequency band. A European can therefore rent a PCS1900 phone when traveling to the United States, while still using his own SIM card, and thus may receive or send calls. The SIM is used to keep names and phone numbers, in addition to those that are already kept in the phone's memory.
The card is also used for the protection of the subscriber, by means of a ciphering and authentication code.
1.2.6 Mobility
GSM is a cellular telephony system that supports mobility over a large area. Unlike cordless telephony systems, it provides location, roaming, and handover.
1.2.6.1 Location Area
The ability to locate a user is not supported in first-generation cellular systems. This means that when a mobile is called, the network has to broadcast the notification of this call in all the radio coverage. In GSM, however, location areas (LAs), which are groups of cells, are defined by the operator. The system is able to identify the LA in which the subscriber is located. This way, when a user receives a call, the notification (or paging) is only transmitted in this area. This is far more efficient, since the physical resource use is limited.
1.2.6.2 Roaming
In particular, the GSM system has the capability of international roaming, or the ability to make and receive phone calls to and from other nations as if one had never left home. This is possible because bilateral agreements have been signed between the different operators, to allow GSM mobile clients to take advantage of GSM services with the same subscription when traveling to different countries, as if they had a subscription to the local network. To allow this, the SIM card contains a list of the networks with which a roaming agreement exists.
When a user is "roaming" to a foreign country, the mobile phone automatically starts a search for a network stipulated on the SIM card list. The choice of a network is performed automatically, and if more than one network is given in the list, the choice is based on the order in which the operators appear. This order can be changed by the user. The home PLMN is the network in which the user has subscribed, while the visited PLMN often refers to the PLMN in which the user is roaming. When a user receives a call on a visited PLMN, the transfer of the call from the home PLMN to the visited PLMN is charged to the called user by his operator.
1.2.6.3 Handover
When the user is moving from one cell to the other during a call, the radio link between BTS 1 and the MS can be replaced by another link, between BTS 2 and the MS. The continuity of the call can be performed in a seamless way for the user. This is called handover. With respect to dual-band telephones, one interesting feature is called the dual-band handover. It allows the user in an area covered both by the GSM-900 and by the DCS-1800 frequency bands, for instance, to be able to transfer automatically from one system to the other in the middle of a call.
1.2.7 Beacon Channel
For each BTS of a GSM network, one frequency channel is used to broadcast general signaling information about this cell. This particular carrier frequency is called a beacon channel, and it is transmitted by the BTS with the maximum power used in the cell, so that every MS in the cell is able to receive it.
1.2.8 MS Idle Mode
When it is not in communication, but still powered on, the MS is said to be in idle mode. This means that it is in a low consumption mode, but synchronized to the network and able to receive or initiate calls.
1.3 GSM Services
In the specification of a telecommunication standard such as GSM, the first step is of course the definition of the services offered by the system. GSM is a digital cellular system designed to support a wide variety of services, depending on the user contract and the network and mobile equipment capabilities.
In GSM terminology, telecommunication services are divided into two broad categories:
Bearer services are telecommunication services providing the capability of transmission of signals between access points [the user-network interfaces (UNIs) in ISDN]. For instance, synchronous dedicated packet data access is a bearer service.
Teleservices are telecommunication services providing the complete capability, including terminal equipment functions, for communication between users according to protocols established by agreement between network operators.
In addition to these services, supplementary services are defined that modify or supplement a basic telecommunication service.
1.3.1 Bearer Services
There exist several categories of bearer services:
Unrestricted digital information (UDI) is designed to offer a peer-to-peer digital link.
The 3.1 kHz is external to the PLMN and provides a UDI service on the GSM network, interconnected with the ISDN or the PSTN by means of a modem.
PAD allows an asynchronous connection to a packet assembler/disassembler (PAD). This enables the PLMN subscribers to access a packet-switched public data network (PSPDN).
Packet enables a synchronous connection to access a PSPDN network and alternate speech and data, providing the capability to switch between voice and data during a call.
Speech followed by data first provides a speech connection, and then allows to switch during the call for a data connection. The user cannot switch back to speech after the data portion.
1.3.2 Teleservices
In terms of application, teleservices correspond to the association of a particular terminal to one or several bearer services. They provide access to two kinds of applications:
Between two compatible terminals;
From an access point of the PLMN to a system including high-level functions, for example, a server.
Of course, the most basic teleservice supported by GSM is digital voice telephony, based on transmission of the digitally encoded voice over the radio. The voice service also includes emergency calls, for which the nearest emergency-service provider is notified by dialing three digits.
The other teleservices that are defined for a PLMN are:
Data services, with data rates ranging from 2.4 Kbps to 14.4 Kbps. These services are based on circuit-switched technology. Circuit switched means that during the communication, a circuit is established between two entities for the transfer of data. The physical resource is used during the whole duration of the call.
Short message service (SMS), which is a bidirectional service for short alphanumeric (up to 160 bytes) messages.
Access to a voice message service.
Fax transmission.
1.3.3 Supplementary Services
Supplementary services include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), caller identification, call waiting, multiparty conversations, charging information, and call barring of outgoing or incoming calls. These call-barring features can be used for example when roaming in another country, if the user wants to limit the communication fees.
1.4 Network Architecture
The structure of a GSM network relies on several functional entities, which have been specified in terms of functions and interfaces. It involves three main subsystems, each containing functional units and interconnected with the others through a series of standard interfaces.
The main parts of a GSM network, as shown in Figure 1.2, are listed below. (In the figure, the lines between the entities represent the interfaces.)
Figure 1.2: General architecture of a GSM network.
The MS, the handheld mobile terminal;
The base station subsystem (BSS), which controls the radio link with the MS;
The network and switching subsystem (NSS), which manages the function of connection switching to other fixed public network or mobile network subscribers, and handles the databases required for mobility management and for the subscriber data.
As can be seen from the figure, in a PLMN, the radio access network part (BSS) is logically separated from the core network (NSS), in order to ease the standardization of the different functions.
1.4.1 MS
The MS is made up of the mobile equipment (ME), and a SIM. It performs the following functions:
Radio transmission and reception;
Source and channel coding and decoding, modulation and demodulation;
Audio functions (amplifiers, microphone, earphone);
Protocols to handle radio functions: power control, frequency hopping, rules for access to the radio medium;
Protocol to handle call control and mobility;
Security algorithms (encryption techniques).
As mentioned, the SIM enables the user to have access to subscribed services irrespective of a specific terminal. The insertion of the SIM card into any GSM terminal allows the user to receive calls on that terminal, to make calls from that terminal, and to use the other subscribed services. The ME is identified with an international mobile equipment identity (IMEI).
The SIM card contains, among other information, the international mobile subscriber identity (IMSI) used to identify the subscriber to the system, and a secret key for authentication. The IMEI and the IMSI are independent, thereby allowing personal mobility.
1.4.2 BSS
The BSS is composed of several base station controllers (BSCs) and BTS. These two elements communicate across the Abis interface.
The BTS contains the radio transceivers, responsible for the radio transmissions with the MS. This includes the following functions:
Modulation and demodulation;
Channel coding and decoding;
Encryption process;
RF transmit and receive circuits (power control, frequency hopping, management of antenna diversity, discontinuous transmission).
Several types of BTS exist: the normal BTS, the micro BTS, and the pico BTS. The micro BTS is different from a normal BTS in two ways. First, the range requirements are reduced, and the close proximity requirements are more stringent. Second, the micro BTS is required to be small and affordable in order to allow external street deployment in large numbers. The pico BTS is an extension of the micro BTS concept to the indoor environments. The RF performances of these different BTSs are slightly different.
The BSC manages the radio resources for one or more BTSs. It handles the management of the radio resource, and as such it controls the following functions: allocation and release of radio channels, frequency hopping, power control algorithms, handover management, choice of the encryption algorithm, and monitoring of the radio link.
1.4.3 Network Subsystem
The mobile services switching center (MSC) is the central part of the network subsystem (NSS). It is responsible for the switching of calls between the mobile users (between different BSCs or toward another MSC) and between mobile and fixed network users. It manages outgoing and incoming calls from various types of networks, such as PSTN, ISDN, and PDN. It also handles the functionality required for the registration and authentication of a user, and the mobility operations. This includes location updating, inter-MSC handovers, and call routing.
The BSS communicates with the MSC across the A interface.
Associated with the MSC, two databases, the home location register (HLR) and the visitor location register (VLR), provide the call-routing and roaming capabilities. The HLR contains all the administrative information related to the registered subscribers within the GSM network. This includes the IMSI, which unequivocally identifies the subscriber within any GSM network, the MS ISDN number (MSISDN), and the list of services subscribed by the user (such as voice, data service). The HLR also stores the current location of the MS, by means of the address of the VLR in which it is registered.
The VLR temporarily keeps the administrative data of the subscribers that are currently located in a given geographical area under its control.
Each functional entity may be implemented as an independent unit, but most of the time, the VLR is colocated with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR. The MSC contains no information about particular MSs, but rather, the information is stored in the location registers.
Two other registers are used for authentication and security purposes:
The equipment identity register (EIR) is a database that contains a list of all valid ME on the network, where each MS is identified by its IMEI. An IMEI is marked as invalid if it has been reported stolen.
The authentication center (AuC) is a protected database that contains a copy of the secret key stored in each subscriber's SIM card, for authentication and encryption over the radio channel. The AuC verifies if a legitimate subscriber has requested a service. It provides the codes for both authentication and encryption to avoid undesired violations of the system by third parties.
Two other important entities of the NSS are the operations and maintenance center (OMC) and the network management center (NMC). These entities perform the functions relative to the network management (NM), such as the configuration of the system (locally or remotely), maintenance and tests of the pieces of equipment, billing, statistics on the performance, and gathering of all information related to subscriber traffic necessary for invoicing and administration of subscribers.
1.5 Radio Interface
1.5.1 General Characteristics
Currently, there are several types of networks in the world using the GSM standard, but at different frequencies.
The GSM-900 is the most common in Europe and the rest of the world. Its extension is E-GSM.
The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover urban areas. It was also introduced to avoid saturation problems with the GSM-900.
The PCS-1900 is used primarily in North America.
The GSM-850 is under development in America.
The GSM-400 is intended for deployment in Scandinavian countries in the band previously used for the analog Nordic Mobile Telephony (NMT) system.
The system is based on frequency-division duplex (FDD), which means that the uplink (radio link from the mobile to the network-that is, mobile transmit, base receive), and downlink (from the network to the mobile-that is, base transmit, mobile receive) are transmitted on different frequency bands. For instance, in the 900-MHz E-GSM band, the block 880-915 MHz is used for transmission from mobiles to network, and the block 925-960 MHz is used for the transmission from network to mobiles. Table 1.1 gives a summary of uplink and downlink frequency bands for the different GSM systems.
Table 1.1: GSM System Frequency Bands Uplink Band
Downlink Band
GSM-900
890-915 MHz
935-960 MHz
E-GSM-900
880-915 MHz
925-960 MHz
DCS-1800
1,710-1,785 MHz
1,805-1,880 MHz
PCS-1900
1,850-1,910 MHz
1,930-1,990 MHz
GSM-400
GSM-450
450,4-457,6 MHz
460.4-467.6 MHz
GSM-480
478.8-486 MHz
488.8-496 MHz
GSM-850
824-849 MHz
869-894 MHz
Operators may implement networks that operate on a combination of the frequency bands listed above to support multiband mobile terminals.
There are different ways of sharing the physical resource among all the users in a radio system, and this is called the multiple-access method. The multiple-access scheme defines how simultaneous communications share the GSM radio spectrum. The various multiple-access techniques in use in radio systems are frequency-division multiple access (FDMA), TDMA, and code-division multiple access (CDMA). GSM is based on both FDMA and TDMA techniques (see Figure 1.3).
Figure 1.3: TDMA and FDMA.
FDMA consists in dividing the frequency band of the system into several channels. In GSM, each RF channel has a bandwidth of 200 kHz, which is used to convey radio modulated signals, or carriers. Each pair of uplink/ downlink channels is called an absolute radio frequency channel (ARFC) and is assigned an ARFC number (ARFCN). The mapping of each ARFCN on the corresponding carrier frequency is given in [3].
TDMA is the division of the time into intervals: within a frequency channel, the time is divided into time slots. This division allows several users (eight) to be multiplexed on the same carrier frequency, each user being assigned a single time slot. A packet of data information, called a burst, is transmitted during a time slot. The succession of eight time slots is called a TDMA frame, and each time slot belonging to a TDMA frame is identified by a time slot number (TN), from 0 to 7.
1.5.2 Logical Channels
The association of a radio frequency channel and a time slot-the pair ARFCN and TN-uniquely defines a physical channel on both the uplink and the downlink.
On top of the physical channels, logical channels ar mapped to convey the information of voice, data, and signaling. This signaling information is used for setting up a call, or to adapt the link to rapidly changing radio conditions, or to manage handovers, to give a few examples. Logical channels can be seen as pipes, each one used for a different purpose by the higher layers of the system.
Two types of logical channels exist, traffic channels and control channels. Among the control channels, according to their functions, four classes are defined: broadcast, dedicated, common, and associated. A broadcast channel is used by the network (in downlink only) to send general information to the MSs. A channel is said to be dedicated if only one MS can transmit or receive in the ARFCN-TN defining this channel, and common if it carries information for several mobiles. An associated control channel is allocated to one mobile, in addition to a dedicated channel, and carries signaling for the operation of this channel.
The broadcast channels are transmitted on the beacon carrier frequency presented in Section 1.2.7. The purposes of the beacon are:
To allow a synchronization in time and frequency of the MSs to the BTS. This synchronization is needed by the MS to access the services of a cell. The frequency and time synchronization procedures that are performed by the mobile are explained in Section 1.5.7.
To help the mobile in estimating the quality of the link during a communication, by measurements on the received signal from the BTS it is transmitting to, and from the other BTSs of the geographical area. These measurements are used by the network to determine when a handover is necessary, and to which BTS this handover should apply.
To help the mobile in the selection of a cell when it is in idle mode (that is, not in communication, but still synchronized to the system and able to receive an incoming call or to initiate a call). This selection is performed on the basis of the received power measurements made on the adjacent cells' beacon channels.
To access the general parameters of the cell needed for the procedures applied by the MS, or general information concerning the cell, such as its identification, the beacon frequencies of the surrounding cells, or the option supported by the cell (services).
To allow these various operations, the logical channels transmitted on the beacon are:
The broadcast control channel (BCCH), which continually broadcasts, on the downlink, general information on the cell, including base station identity, frequency allocations, and frequency-hopping sequences. The information is transmitted within system information (SI) blocks, which can be of different types according to the information that is carried out. The frequency with which an SI is retransmitted on the BCCH varies with the type of information.
The frequency control channel (FCCH), used by the MS to adjust its local oscillator (LO) to the BTS oscillator, in order to have a frequency synchronization between the MS and the BTS.
The synchronization channel (SCH), used by the MS to synchronize in time with the BTS, and to identify the cell.
As listed below, four channels comprise the common control channels (CCCH). Among these, the first three are used for the MS-initiated call or for call paging (notification of an incoming call toward the MS):
The random access channel (RACH) is used for the MS access requests to the network, for the establishment of a call, based on a slotted aloha method.
The paging channel (PCH) is defined to inform the MS of an incoming call.
The access grant channel (AGCH) is used to allocate some physical resource to a mobile for signaling, following a request on the RACH.
The cell broadcast channel (CBCH) may be used to broadcast specific news to the mobiles of a cell.
The dedicated control channels are:
The stand-alone dedicated control channel (SDCCH), utilized for registration, authentication, call setup, and location updating.
The slow associated control channel (SACCH), which carries signaling for the TCH or SDCCH with which it corresponds. The information that is transmitted on this channel concerns the radio link control (RLC), such as the power control on the corresponding TCH or SDCCH, or the time synchronization between the MS and the BTS.
The fast associated control channel (FACCH), carries the signaling that must be sent by the network to the MS to notify that a handover is occurring.
The TCHs can be of several types, according to the service that is accessed by the subscribers: voice or data, with various possible data rates.
Table 1.2 summarizes the purpose of the different logical channels. In this table, UL stands for uplink, and DL for downlink.
Table 1.2: The Logical Channels and Their Purpose Logical Channel
Abbreviation
Uplink/ Downlink
Task
Broadcast channel (BCH)
Broadcast control channel
BCCH
DL
System Information broadcast
Frequency correction channel
FCCH
DL
Cell frequency synchronization
Synchronization channel
SCH
DL
Cell time synchronization and identification
Common control channel (CCCH)
Paging channel
PCH
DL
MS paging
Random access channel
RACH
UL
MS random access
Access grant channel
AGCH
DL
Resource allocation
Cell broadcast channel
CBCH
DL
Short messages broad cast
Dedicated control channel
Standalone dedicated control channel
SDCCH
UL/DL
General signaling
Slow associated control channel
SACCH
UL/DL
Signaling associated with the TCH
Fast associated control channel
FACCH
UL/DL
Handover signaling
Traffic channel (TCH)
Full speech
TCH/FS
UL/DL
Full-rate voice channel
Half rate
TCH/HS
UL/DL
Half-rate voice channel
2.4 Kbps, 4.8 Kbps, 9.6 Kbps, and 14.4 Kbps full-rate data channels
TCH/F2.4 TCH/F4.8 TCH/F9.6 TCH/F14.4
UL/DL
Full-rate data channels
2.4-Kbps- and 4.8-Kbps-rate data channels
TCH/H2.4 TCH/H4.8
UL/DL
Half-rate data channels
1.5.3 Mapping of Logical Channels onto Physical Channels
1.5.3.1 TDMA Time Structure
The basic time unit is the time slot. Its duration is 576.9 μs = 15/26 ms, or 156.25 symbol periods (a symbol period is 48/13 μs). The piece of information transmitted during a time slot is called a burst. As we saw in Section 1.5.1, the GSM multiple access scheme is TDMA, with eight time slots per carrier. A sequence of eight time slots is called a TDMA frame, and has a duration of 4.615 ms. The time slots of a TDMA frame are numbered from 0 to 7, as shown in Figure 1.4. Note that the beginning and end of TDMA frames in uplink and downlink are shifted in time: Time slot number 0 on the uplink corresponds to time slot 3 in the downlink. This allows some time for the mobile to switch from one frequency to the other.
Figure 1.4: Slot numbering within the TDMA frame.
As seen earlier, a physical channel is defined as a sequence of TDMA frames, a time slot number (from 0 to 7) and a frequency. It is bidirectional, with the same TN in uplink and in downlink. In order to support cryptographic mechanisms, a long time-structure has been defined. It is called a hyperframe and has a duration of 3 hours, 28 minutes, 53 seconds, and 760 ms (or 12,533.76 seconds). The TDMA frames are numbered within the hyperframe. The numbering is done with the TDMA frame number (FN) from 0 to 2,715,647.
One hyperframe is subdivided into 2,048 superframes, which have a duration of 6.12 seconds. The superframe is itself subdivided into multi-frames. In GSM, there are two types of multiframes defined, containing 26 or 51 TDMA frames.
The 26 multiframe has a duration of 120 ms, and comprises 26 TDMA frames This multiframe is used to carry TCH, SACCH, and FACCH. The 51 multiframe is made up of 51 TDMA frames. Its duration is 235.4 ms (3,060/13 ms). This multiframe is used to carry BCH, CCCH, and SDCCH (with its associated SACCH). Note that a superframe is composed of twenty-six 51-multiframes, or of fifty-one 26-multiframes. This hierarchical time structure is summarized in Figure 1.5.
Figure 1.5: Hierarchical structure of a hyperframe.
1.5.3.2 Mapping of the TCH and SACCH on the 26-Multiframe
The TCHs are bidirectional channels mapped onto the 26-multiframe. Two types of channels must be distinguished: full-rate and half-rate channels, and therefore two different mappings of the TCH on the multiframe are possible:
A full-rate traffic channel (TCH/FS, for full speech) uses one time slot per TDMA frame, for each frame of the multiframe, except the frames 12 and 25 (see Figure 1.6). The TDMA frame 12 is used to carry the SACCH/FS, and the TDMA frame 25 is an idle frame, which means that no channel is transmitted during this entire TDMA frame.
Figure 1.6: Mapping of a TCH/FS and SACCH/FS on the 26-multiframe.
A half-rate traffic channel (TCH/HS) uses one time slot every two TDMA frames, due to the fact that it carries data from a half-rate voice coder. As shown in Figure 1.7, two half-rate channels can be mapped on the same time slot, one using TDMA frames 0, 2, 4, 6, 8, 10, 13, 15, 17, 19, 21, and 23 and the other one using frames 1, 3, 5, 7, 9, 11, 14, 16, 18, 20, 22, and 24. The SACCH/HS channel associated with the first TCH subchannel is transported on TDMA frame 12, and the SACCH/HS associated with the second subchannel is on time slot 25.
Figure 1.7: Mapping of a TCH/HS and SACCH/HS on the 26-multiframe.
1.5.3.3 Mapping of the FACCH on the 26-Multiframe
The FACCH is associated with a TCH, and is required to support the highspeed signaling needed during call establishment, subscriber authentication, and handover management. The occurrence of the FACCH is not fixed in the multiframe, as it is for the SACCH. Rather, the FACCH occurs on a TDMA frame that is reserved for a TCH. The multiplexing of TCH and FACCH is possible by means of the frame stealing. This means that a speech frame carried over a TCH can be replaced by a FACCH frame. This is signaled to the receiver by means of stealing flags, as described in Section 1.5.4. This principle allows for a fast signaling channel without significant loss on the quality of speech, if it is not performed too often.
1.5.3.4 Mapping of the SDCCH on the 51-Multiframe
The SDCCH is a signaling channel that carries the higher layers of control information. A SACCH is associated with a SDCCH.
Two different multiplexings on the 51 multiframe are possible:
SDCCH with SACCH alone. An SDCCH channel is mapped on four TDMA frames of a 51 multiframe. As a result, eight SDCCH channels, dedicated to eight different MSs are mapped onto a 51 multiframe. The TDMA frames not occupied by an SDCCH are used by the eight associated SACCH channels. The mapping of these associated channels is performed on two consecutive 51 multi-frames, as shown in Figure 1.8(b).
SDCCH with SACCH multiplexed together with CCCH, BCCH, SCH. and FCCH. This case is described in the next subsection and in Figure 1.8(c).
Figure 1.8: Channel associations on the 51-multiframe. (a) BCCH + CCCH, (b) 8 SDCCH/8, and (c) BCCH + CCCH 4 SDCCH/4. (From- [4].)
Note that the frame-stealing concept, used on the TCH to allocate FACCH frames, is not in use in the case of an SDCCH. This is due to the fact that sufficiently fast signaling can be transmitted over an SDCCH to carry on a handover procedure.
1.5.3.5 Mapping of the Broadcast Channels and CCCH on the 51 Multiframe
The broadcast channels and the CCCH (i.e., PCH, RACH, AGCH) are all multiplexed on the 51 multiframe, on the beacon carrier frequency. The FCCH is sent on downlink time slot 0, on the TDMA frames 0, 10, 20, 30, and 40 of the 51 multiframe. The SCH is also mapped on slot 0, in the TDMA frames immediately following an FCCH frame (i.e., frames 1, 11, 21, 31, 41).
The BCCH is associated with time slot 0 as well, in the TDMA frames that are not occupied by the FCCH and the SCH, and can also be mapped on time slots 2, 4, and 6 in some configurations.
The AGCH and PCH are dynamically multiplexed on the multiframe according to the network load. They are mapped on time slot 0, together with the FCCH, SCH, and BCCH, and optionally on time slots 2, 4, and 6 (also possibly used for the BCCH). Note that the BCCH and CCCH are multiplexed dynamically. The exact mapping is conveyed to the MS by means of SI blocks, sent over the BCCH.
One of these broadcast parameters also indicates whether or not the CCCH are combined with SDCCH and SACCH onto the same basic physical channel (the second type of mapping of SDCCH/SACCH presented in Section 1.5.3.4).
The mapping of the RACH channel is simple: every uplink slot corresponding to a downlink FCCH, SCH, BCCH, PCH, and AGCH can be used for a RACH. If the SDCCH and SACCH are multiplexed with the CCCH, the number of available random-access channel blocks are reduced. Figure 1.8(a, c) shows possible configurations with the multiplexing BCCH/ CCCH and BCCH/CCCH/SDCCH/SACCH.
1.5.3.6 Summary of Logical Channel Combinations
The different allowed combinations of logical channels on a physical channel are as follows:
TCH/F + FACCH/F + SACCH/TF;
TCH/H+ FACCH/H + SACCH/TH;
FCCH + SCH + BCCH + CCCH;
FCCH + SCH + BCCH + CCCH + SDCCH + SACCH;
BCCH + CCCH;
SDCCH+ SACCH.
Other logical channels exist for packet-switched services (GPRS, EDGE, or DTM), and will be described later.
1.5.4 Voice Digital Communication Chain
The functions performed by the physical layer during transmission are presented in this section, and the TCH/FS voice channel will be used for the sake of example. The steps of the communication chain, as outlined below, are presented in Figure 1.9.
Figure 1.9: The voice transmission chain.
1.5.4.1 Source Coding
First of all, speech must be digitized. On the basis of subjective speech quality and complexity issues, a regular pulse excited-linear predictive coder (RPE-LPC) with a long-term predictor loop was chosen. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual (the difference between the predicted and actual sample), represent the signal. Speech is divided into 20-ms samples, each of which is encoded as 260 bits, giving a total bit rate of 13 Kbps. This is the so-called full-rate speech coding. Several other voice codecs are defined (half rate, enhanced full rate, adaptive multi-rate codecs), but these elements are beyond the scope of this book.
1.5.4.2 Channel Coding
In order to protect the voice against noise, interference, and multipath radio propagation conditions, the encoded speech transmitted over the radio interface is protected from errors. Convolutional encoding and block interleaving are used to achieve this protection. As discussed above, the speech codec produces a 260-bit block for every 20-ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are therefore divided into three classes:
Class Ia: 50 bits, most sensitive to bit errors;
Class Ib: 132 bits, moderately sensitive to bit errors;
Class II: 78 bits, least sensitive to bit errors.
Class Ia bits have a 3-bit cyclic redundancy code added for error detection. If an error is detected, the frame is determined to be incomprehensible, and it is discarded. In such a case it is replaced by a slightly attenuated version of the previous correctly received frame.
These 53 bits, together with the 132 class Ib bits and a 4-bit tail sequence (a total of 189 bits), are input into a 1/2-rate convolutional encoder of constraint length 4. Each input bit is encoded as 2 output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits. The 78 remaining class II bits, which are unprotected, are added to these 378 bits. A total of 456 encoded bits are therefore produced for every 20ms speech sample. This represents a bit rate of 22.8 Kbps. In order to protect against burst errors common to the radio interface, each block of 456 bits is interleaved.
1.5.4.3 Interleaving
The 456 coded bits are permutated, and divided into eight blocks of 57 bits, and these blocks are transmitted in eight consecutive bursts, as described in Figure 1.10. Since each burst can carry two 57-bit blocks, each burst carries traffic from two different speech frames. The benefit of doing this is that it provides time diversity. If a sequence of several consecutive bits is corrupted by the degraded propagation conditions (fading, for example) during a given period of time, interleaving ensures that the errors will be randomly distributed over the block of 456 bits. This property is required for a better performance of the decoding algorithm. The decoding of convolutional codes is indeed often performed by the Viterbi algorithm (refer to the case study on this subject in Chapter 4), for which the errors occur in packets. Thus, a random distribution of the corrupted bits over a block at the input of the decoder leads to an increased decoding efficiency, and so to better performance in terms of bit error rate.
Figure 1.10: Interleaving scheme on the TCH.
1.5.4.4 Burst Formatting
We have seen that the coded bits are interleaved and transmitted over bursts. The format of the burst-carrying TCH traffic, or normal burst (NB), is shown in Figure 1.11. NBs are used on most of the logical channels, except the RACH, SCH, and FCCH. The NB is constituted of two data blocks of 57 bits, carrying the coded voice samples, as described above. The middle of the burst contains the training sequence of 26 bits, known by the receiver, used to estimate the distortion introduced by the radio channel and for time synchronization. At the beginning and at the end of the burst, two sequences of 3 bits, known as the tail bits, all equal to 0, are used. Two bits, one before and one after the training sequence, form the stealing flags (SF). In Section 1.5.3.3 we saw that those bits allow the FACCH and TCH to be multiplexed, with the frame-stealing concept. When a speech frame is stolen for the transmission of a FACCH block, this is signaled by the SF bits. When one stealing bit is equal to one, this means that half of the burst is used for FACCH; otherwise it is used for TCH. An example of the multiplexing of FACCH and TCH is shown in Figure 1.12. Note that a FACCH is transported over eight half bursts. The burst duration is 148 bits, or 546.46 μs, and it is followed by a guard period of 8.25 bits, to allow for the burst ramping up and down between the time slots, as represented in Figure 1.11.
Figure 1.11: Structure of an NB.
Figure 1.12: Example of frame stealing- multiplexing of a TCH and an FACCH.
1.5.4.5 Ciphering
For security reasons, ciphering is used to modify the data parts of the burst, with a binary addition between a pseudorandom bit sequence and the 114 data bits. The same operation is performed by the receiver for deciphering. The pseudorandom bit sequence is different in the uplink and in the downlink.
1.5.4.6 Differential Encoding
All the bits di of the burst are differentially encoded. The output of the differential encoder is
(1.1)
where ⊕ denotes modulo 2 addition.
The result is mapped onto +1 and -1 values, to form the modulating data value αi input to the modulator, as follows:
(1.2)
1.5.4.7 GMSK Modulation
The digital signal is modulated onto the analog carrier frequency using Gaussian-filtered minimum shift keying (GMSK), with a symbol period of 48/13 μs (i.e., 270.8333 kHz). This modulation was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, power consumption for the MS, and limited out of channels emissions. These radio emissions, outside of the allocated channel, must be strictly controlled so as to limit adjacent channel interference and allow for the coexistence of GSM and the other systems. The spectrum due to the modulation mask requirement is presented in Figure 1.13, along with an ideal GMSK spectrum.
Figure 1.13: Ideal GMSK spectrum and required spectrum mask.
On the receive section, the opposite operations are performed, namely, demodulation, burst deformatting, de-interleaving, channel decoding, and source decoding.
1.5.4.8 Demodulation
Because of the various obstacles in the environment, many reflected signals, each with a different time delay and phase, arrives at the receiver. This is called multipath, and it is time variant, since the terminal is mobile, by definition. To cope with these time-varying propagation conditions, the receiver uses an equalizer. An equalizer is an algorithm that uses the receive sampled symbols to estimate the sequence of bits that was transmitted by the peer entity, by suppressing the intersymbol interference (ISI). Equalization is therefore used to extract the desired signal from the unwanted reflections. To do this, the receiver uses a known sequence of transmitted bits, the training sequence, to estimate the channel impulse response (CIR). This known signal is the 26-bit training sequence transmitted in the middle of every NB. The CIR is then used by the equalizer to retrieve the transmitted symbols. The estimation of the CIR also allows the finest time synchronization of the mobile to the BTS (the mobile can detect and correct a delay of several symbol periods). Note that the actual implementation of the equalizer is not specified in the GSM system, and it is up to the mobile phone or BTS vendor to implement a solution that will achieve the specified receiver performance (see Section 1.5.6.2).
1.5.5 Bursts Format
There are four burst formats the purposes of which are defined as follows:
The NB is used to carry information on traffic and control channels, except for RACH, SCH, and FCCH. It contains 114 encrypted symbols and includes a guard time of 8.25 symbol duration (=30,46 μs). A training sequence of 26 symbols is present in the middle of the burst (see Figure 1.11, Section 1.5.4).
The frequency correction burst (FB), as shown in Figure 1.14(a), contains a sequence of 142 fixed bits. This sequence is made of alternating ones and zeros (1, 0, 1, 0, ... 1, 0), so that after the differential encoding and GMSK modulation, the RF signal is equivalent to an unmodulated carrier, shifted by 67.7 kHz above the carrier frequency. This characteristic is used to help the mobile synchronize in frequency with the BTS, as explained in Section 1.5.7. The FB is transmitted over the FCCH.
The synchronization burst (SB) is needed for time synchronization of the mobile on the SCH. It contains a long training sequence and carries the information of the TDMA FN and base station identity code (BSIC), as can be seen in Figure 1.14(b).
The access burst (AB), presented in Figure 1.14(c), is used for random access (or the RACH) and is characterized by a longer guard period (68.25 bit duration or 252 μs), allowing the estimation of the timing advance (TA) by the BTS (see Section 1.5.6.3).
Figure 1.14: The (a) FB, (b) SB, and (c) AB burst structures.
In Figure 1.14, we see a guard period at the end of a burst. During this period, the transmission is attenuated in several steps, as specified by the power-versus-time mask specification (see the example of NB, Figure 1.11).
1.5.6 RF Characteristics
1.5.6.1 Transmission Characteristics
Several classes of mobiles are defined, according to their maximum output power capability, as shown in Table 1.3. In GSM-900, most of the mobiles available on the market are class 4 handheld terminals, while class 2 terminals are used as vehicle-mounted equipment. The class 4 and 5 MSs are denoted as "small MS." In DCS-1800, the typical class is class 1.
Table 1.3: MS Power Classes Power Class
GSM-400, GSM-900, GSM-850
Nominal Maximum Output Power
DCS-1800
Nominal Maximum Output Power
PCS-1900
Nominal Maximum Output Power
1
-
1W (30 dBm)
1W(30dBm)
2
8W (39 dBm)
0.25W (24 dBm)
0.25W (24 dBm)
3
5W (37 dBm)
4W (36 dBm)
2W (33 dBm)
4
2W (33 dBm)
5
0.8W (29 dBm)
These output power levels are maximum values, and can be reduced according to the commands that are sent by the network to the MSs. With these network commands, the MS operates at the lowest power level that maintains an acceptable signal quality.
These commands are based on the measurements that are performed by the MS and by the BTS. For instance, with a class 4 MS, the range of transmission can be several kilometers, but if the MS is getting closer to the BTS, it may receive a request from the network to decrease its output power level. This procedure, called power control, improves the performance of the system by reducing the interference caused to the other users. Moreover, it is a means of prolonging the battery life of the mobile. The power level can be stepped up or down in steps of 2 dB from the maximum power (depending on the MS class) down to a minimum of 5 dBm in GSM-400/900/850, and 0 dBm in DCS-1800/PCS-1900. The transmission of power control commands by the BTS is explained in Section 1.5.6.3.
For the BTS transceiver (TRX), the power classes are given in Table 1.4.
Table 1.4: TRX Power Classes TRX Power Class
GSM-400, GSM-900, GSM- 850
Maximum Output Power
DCS-1800 and PCS-1900
Maximum Output Power
1
320 (< 640)W
20(<40)W
2
160(<320)W
10(<20)W
3
80(<160)W
5(<10)W
4
40(<80)W
2.5(<5)W
5
20(<40)W
6
10(<20)W
7
5(<10)W
8
2.5(<5)W
As an option, the BSS can utilize downlink RF power control, with up to 15 steps of power control levels with a step size of 2 dB. Note that this power control on the downlink is not used on the beacon frequency, which is always transmitted with constant output power.
Many other requirements on the transmit section are defined in the GSM specifications, such as the spectrum due to modulation constraint (see Figure 1.13), the modulation accuracy, the transmitter frequency error, and the spurious emissions requirements.
1.5.6.2 Reception Characteristics
Several types of propagation models have been defined, in order to measure the mobile and BTS performances. These models represent several environments:
Typical urban (TUx);
Rural area (RAx);
Hilly terrain (HTx).
In the above definitions, the x stands for the velocity of the mobile, in km/h. The various propagation models are represented by a number of taps, each determined by their time delay and average power. The Rayleigh distributed amplitude of each tap varies according to a Doppler spectrum.
In addition to these multipath fading channels, the static channel was defined. This is a simple single-path constant channel. With this channel, the only perturbation comes from the receiver noise of the measured equipment.
One of the most important receiver performances that is specified is the sensitivity level, which determines the minimum level for which the receiver can demodulate a signal correctly. The sensitivity requirement, in GSM, is specified as an input level, in dBm, for which the measured equipment should reach a certain performance, in terms of bit error rate. For instance, for GSM400/900/850 power classes 4 or 5 mobiles and DCS-1800/PCS-1900 classes 1 or 2 mobiles, the sensitivity level is -102 dBm. For a normal BTS (that is not a micro- or a pico-BTS) the sensitivity level is -104 dBm, for all the frequency bands.
At these levels, different performances, according to both the logical channel and the propagation channel used for the measurement, must be met. Table 1.5 shows an example of performances that are reached at the sensitivity level, for GSM-900 and GSM-850. In this table, BER stands for bit error rate, FER for frame erasure ratio (i.e., incorrect-speech-frames ratio), and RBER for residual BER (defined as the ratio of the number of errors detected over the frames defined as "good" to the number of transmitted bits in the good frames). This table is an example; similar tables exist for the other logical channels and for the different frequency bands. Note that frequency hopping may be used for the sensitivity performance measurements.
Table 1.5: Sensitivity-Level Performance Requirements Propagation Conditions
Logical Channel
Static
TU
(no FH)
TU50
(ideal FH)
RA250
(no FH)
HT100
(no FH)
FACCH/H
(FER)
0.1%
6.9%
6.9%
5.7%
10.0%
FACCH/F
(FER)
0.1%
8.0%
3.8%
3.4%
6.3%
SDCCH
(FER)
0.1%
13%
8%
8%
12%
RACH
(FER)
0.5%
13%
13%
12%
13%
SCH
(FER)
1%
16%
16%
15%
16%
TCH/F14.4
(BER)
10.0-5
2.5%
2%
2%
5%
TCH/F9.6
(BER)
10.0-5
0.5%
0.4%
0.1%
0.7%
TCH/FS
(FER)
0.1α%
6α%
3α%
2α%
7α%
Class Ib
(RBER)
0.4/α%
0.4/α%
0.3/α%
0.2/α%
0.5/α%
Class II
(RBER)
2%
8%
8%
7%
9%
Note that in this example, the parameter a is defined as 1 ≤ α ≤ 1.6 and allows a tradeoff between the number of erased speech frames (i.e., decoded as wrong, and therefore not transmitted to the voice decoder) and the quality of the nonerased frames.
Another important characteristic of the receiver concerns its performance in the presence of an interferer. This is specified either for a cochannel interference (i.e., an interference situated at the same frequency as the signal of interest) or an adjacent channel interference (situated at 200 or 400 kHz from the carrier of interest). The level of the useful signal is set 20 dB higher than for the sensitivity evaluation, and a GMSK interfering signal is added, either at the same frequency or with an offset of 200 or 400 kHz from the carrier. For the cochannel test, the carrier to interference ratio C/Ic is set to 9 dB. Under these conditions, the performance of Table 1.6 must be met. Again, this table does not contain all the logical channels, and concerns the GSM-900 and GSM-850 only. Similar performance requirements are defined for the other cases.
Table 1.6: Interference Performance Requirements Propagation Conditions
Type of Channel
TU3
(NoFH)
TU3
(Ideal FH)
TU50
(No FH)
TU50
(Ideal FH)
RA250
(No FH)
FACCH/H
(FER)
22%
6.7%
6.7%
6.7%
5.7%
FACCH/F
(FER)
22%
3.4%
9.5%
3.4%
3.5%
SDCCH
(FER)
22%
9%
13%
9%
8%
RACH
(FER)
15%
15%
16%
16%
13%
SCH
(FER)
17%
17%
17%
17%
18%
TCH/F 14.4
BER)
10%
3%
4.5%
3%
3%
TCH/F 9.6
(BER)
8%
0.3%
0.8%
0.3%
0.2%
TCH/FS
(FER)
21α%
3α%
6α%
3α%
3α%
Class Ib
(RBER)
2/α%
0.2/α%
0.4/α%
0.2/α%
0.2/α%
Class II
(RBER)
4%
8%
8%
8%
8%
This table is also applied in the case of an adjacent channel interference. In this case, the C/I is set to -9 dB if the interferer is 200 kHz from the carrier, and -41 dB if it is 400 kHz from the carrier.
1.5.6.3 Control of the Radio Link
This section describes some of the procedures that are in use to improve the efficiency of the system, by adapting the transmission between the mobile and the BTS to the continuously varying radio environment.
Compensation for the Propagation Delay
Due to the distance between the MS and the BTS, there is a propagation delay that is equal to d/ c seconds, where d is the MS to BTS distance in meters, and c is the speed of light (c = 3 · 108 m.s-1). Without any compensation of this delay, the bursts transmitted by two different MSs, in the same TDMA frame on two consecutive slots, could interfere with one another.
Let us take the example of one MS situated 25 km away from the BTS, transmitting on time slot 0 of a given channel frequency. Another MS is located, say, 1 km away from the BTS, and transmitting on time slot 1 of that same frequency. The second MS transmission will experience a very short delay (around 3.33 μs), but the burst on time slot 0, from MS 1, will be received by the BTS 83.33 μs after it has been transmitted. This means that at the BTS receiver, the burst on time slot 0 will interfere with the beginning of the burst of time slot 1, for a period of about 80 μs. This example is represented in Figure 1.15.
Figure 1.15: Propagation delay difference between two MSs transmitting to the same BTS.
In order to cope with this problem, the network manages a parameter for each mobile called the TA. This parameter represents the transmission delay between the BTS and the MS, added to the delay for the return link.
The estimation of the delay is performed by the BTS upon reception of an AB on the RACH. As described in Section 1.5.5, this burst is characterized by a longer guard period (68.25-bit duration or 252 μs) to allow burst transmission from a mobile that does not know the TA at the first access. The received AB allows the BTS to estimate the delay by means of a correlation with the training sequence.
The TA value, between 0 and 63 symbol periods (i.e., between 0 and 232.615 μs by steps of 48/13 μs), is transmitted on the AGCH. It allows the MS to advance its time base, so that the burst received at the BTS arrives exactly three time slots after the BTS transmit burst, as shown in Figure 1.16. A distance of 35 km between the MS and the BTS is therefore possible. (The 232.675 μs allows to compensatefor a distance of around 70 km, including the forward and return links.)
Figure 1.16: Correction of MS transmission timing to compensate for propagation delay.
After this first propagation delay estimation, the BTS continuously monitors the delay of the NBs sent by from the MS on the other logical channels. If the delay changes by more than one symbol period, a new value of the TA is signaled to the MS on the SACCH.
MS Power Control
As explained in Section 1.5.6.1, the MS can vary its transmit output power from a maximum defined by its power class, by steps of 2 dB. During a communication, the MS and BTS measure the received signal strength and quality (based on the bit error ratio) and pass the information to the BSC, which ultimately decides if and when the power level should be changed. A command is then sent to the MS on the SACCH.
Power control is a difficult mechanism to implement, since there is a possibility of instability. This arises from having MS in cochannel cells, alternatively increasing their power in response to increased cochannel interference. Suppose that mobile A increases its power because the corresponding BTS receives a cochannel interference caused by mobile B, in another cell. Then the BTS receiving the signal from mobile B might request mobile B to increase its power, and so forth. This is the reason why some coordination is required at the BSC level.
Note that for an access request on the RACH, the MS uses the maximum power level defined by the parameter MS_TXPWR_MAX_CCH broadcast by the network.
Frequency Hopping
The radio environment depends on the radio frequency. In order to avoid important differences in the quality of the channels, a feature called slow frequency hopping (FH) was introduced. The slow FH changes the frequency with every TDMA frame, which also has the effect of reducing the cochannel interference. This capability is optionally used by the operator, and is not necessarily implemented in all the cells of the network, but it must be supported by all the MSs. The main advantage of FH is to provide diversity on one transmission link (especially to increase the efficiency of coding and interleaving for slowly moving MSs) and also to average the quality on all the communications through interference diversity.
The principle of slow FH is that every mobile transmits its time slots according to a sequence of frequencies that it derives from an algorithm. The FH sequences are orthogonal inside one cell (i.e., no collisions occur between communications of the same cell) and independent from one cell to a cochannel cell (i.e., a cell using the same set of RF channels or cell allocation). The hopping sequence is derived by the mobile from parameters broadcast at the channel assignment, namely, the mobile allocation (set of N frequencies on which to hop), the hopping sequence number (HSN) of the cell (which allows different sequences on cochannel cells), and the index offset (to distinguish the different mobiles of the cell using the same mobile allocation) or mobile allocation index offset (MAIO). Based on these parameters and on the FN, the MS knows which frequency to hop in each TDMA frame.
It must be noted that the basic physical channel supporting the BCCH does not hop.
1.5.7 MS Cell Synchronization Procedure
In synchronizing to a cell, the MS first searches for the FB on the FACCH. This allows a first timing synchronization, but most of all, it allows the mobile to adjust its oscillator to be synchronized in the frequency domain with the BTS. This is possible because, as described in Section 1.5.6, the fixed sequence of the FB has been chosen so that the modulating bit sequence at the GMSK modulator input is constant. This results in a continuous π/2 phase rotation, which in the frequency domain is equivalent to an unmodulated carrier with a +1 625/24 kHz frequency offset, above the nominal carrier frequency. Once the MS has identified the FB, it uses this property to estimate its frequency drift with regard to the BTS.
In the TDMA frame immediately following the occurrence of an FB, on the same carrier frequency, the SB is transmitted over the air interface, on the SCH. This burst is identified by the MS with its extended 64-bit training sequence code. The MS received samples of the SB, correlated with the known training sequence, allow for the timing of the mobile to be adjusted to the base station with good precision. At this point, the MS and the BTS are synchronized in the time domain, except that the propagation delay between them is not compensated. This is performed with the TA scheme, as discussed in Section 1.5.6.3, when the MS sends an AB on the RACH.
The decoding of the SB enables a logical synchronization of the mobile to the cell, since it gives the elements to estimate the TDMA FN (see Section 1.5.3.1), which allows the MS to determine the position of the SCH in the hyperframe. The SCH also contains the BTS identity code (BSIC): these 6 bits (before channel coding) consist of the PLMN color code with range 0 to 7 and of the BS color code with range 0 to 7 (3 bits each).
1.5.8 Summary of MS Operations in Idle Mode
In idle mode, as opposed to dedicated mode, the mobile has no channel of its own. It is required to
Synchronize in time and frequency to a given cell, selected as the best suitable cell with regard to a set of criteria (based on the beacon received power at the MS). This is termed "camping onto" a cell. This process of evaluating different cells and choosing the best suitable one is called cell selection, or reselection if it is performed again, due to the degradation of the link quality with the previously selected cell. The MS during idle mode continuously measures the radio link quality of the serving cell and the surrounding cells, so that cell reselection criteria are evaluated periodically.
Listen to possible incoming calls from the network. The notification of an incoming call is usually referred to as paging.
1.5.8.1 Selection of the PLMN
When the mobile is switched on, the first operation that it performs is identification or selection of a PLMN. Most of the time, the PLMN will be the home PLMN (i.e., the network to which the user has subscribed). In such a case, no selection is needed, as information about the network is stored in the SIM card. If it is not the case, because the user is traveling in a different area, the MS will scan all the frequencies in order to detect the surrounding beacon channels (detection of FB and SB, as described in Section 1.5.7). The MS is then able to decode the PLMN identifiers, and either choose the first PLMN in the priority ordered list of the SIM card, or ask the user which PLMN is preferred among all the detected PLMNs. The selection is then stored, in order to be used at the next terminal switch on. In any case, the user can explicitly ask for a given PLMN selection.
1.5.8.2 Principles of Cell Selection and Reselection
Once the PLMN is selected, the MS must select a cell. Two scenarios are possible:
The beacon channel frequencies are stored in the MS, because it has already performed a selection in the previous terminal activity. In this case, the MS will perform measurement on these frequencies, to determine which cell is the most suitable with regard to certain criteria. Once the best cell is selected, the MS performs registration and "camps on" this cell. Note that if the stored frequency list of beacon carriers is not detected by the MS, it will perform a PLMN selection as described above.
It is the first time the PLMN is accessed. The carriers of the system are all scanned, in order to detect the beacon channels, and the received signal strength of these channels is added in an ordered list. Once this is achieved, the cell selection can be performed, as in the previous case. In order to speed up the process, a list of the RF channels containing BCCH carriers of the same PLMN is broadcast in the system information messages.
When an MS is camping on a cell, it can receive paging blocks on the PCH, or initiate call setup for outgoing calls by sending an AB on the RACH. It still regularly monitors the signal level on the surrounding beacon carriers, and evaluates the reselection criteria. The reselection is triggered if one of the following events occurs:
The path loss criterion parameter C1 indicates that the path loss to the cell has become too high.
There is a downlink signaling failure (i.e., the success rate of the MS in decoding signaling blocks drops too low).
The cell camped on has become barred (this means that the operator decides not to allow MSs to camp on this cell).
There is a better cell, in terms of the path loss criterion C2 in the same registration area.
A random access attempt is still unsuccessful after a given number of repetitions, specified by a broadcast parameter.
The criteria for cell selection and reselection (path loss criterion C1 and reselection criterion C2) are based on the measurements performed by the MS on the BCCH frequency. (As stated earlier, the beacon frequency is transmitted with its maximum output power by the BTS.) Details on these criteria for cell selection are given in Chapter 5.
1.5.8.3 Monitoring of Paging Blocks
We discussed the mapping of the CCCH on the 51 multiframe in Section 1.5.3.5, and in particular the case of the PCH. This logical channel is used to convey paging blocks on the downlink. These blocks are used to notify the MS of an incoming call. In order to conserve the MS's power, a PCH is divided into subchannels, each corresponding to a group of MSs. Each MS will then only "listen" to its subchannel and will stay in the sleep mode during the other subchannels of the PCH. This is called the discontinuous reception (DRX) mode. The mobile knows in which group it belongs by determining the parameter CCCH_GROUP. It is estimated with an algorithm, which inputs are the mobile IMSI and the parameter BS_CC_CHANS, broadcast on the BCCH. This parameter defines the number of basic physical channels supporting CCCH.
Mobiles in a specific CCCH_GROUP will listen for paging messages and make random accesses only on the specific CCCH to which the CCCH_GROUP belongs. This algorithm is detailed in Chapter 4. Note that the MS is not authorized to use the DRX mode of operation while performing the cell-selection algorithm.
1.5.9 Measurements Performed by MS During Communication
When assigned a TCH or SDCCH, during the time slots that are not used for these channels and for the associated SACCH, the MS performs measurements on all the adjacent BCCH frequencies. These measurements are then sent to the network by means of the SACCH, and are interpreted by the NSS for the power control and handover procedures. Measurements are performed in each TDMA frame, (see Figure 1.17) and are referred to as monitoring, which consists of estimating the receive signal strength on a given frequency. The list of frequencies to be monitored is broadcast on the BCCH, by means of the BCCH allocation (BA) list, which contains up to 32 frequencies. The frequencies are monitored one after the other, and the measured samples are averaged prior to the reporting to the network, on an uplink SACCH block, under form of a value called RXLEV. The MS therefore measures the received signal level from surrounding cells by tuning and listening to their BCCH carriers. This can be achieved without interbase station synchronization. The measurements are reported at every reporting period.
Figure 1.17: Monitoring during a TDMA frame.
For a TCH/FS, the reporting period duration is 104 TDMA frames (480 ms).
It is essential that the MS identify which surrounding BSS is being measured in order to ensure reliable handover. Because of frequency reuse with small cluster sizes, the BCCH carrier frequency may not be sufficient to uniquely identify a surrounding cell. The cell in which the MS is situated may have more than one surrounding cell using the same BCCH frequency. It is therefore necessary for the MS to synchronize to (using the method explained in Section 1.5.7) and demodulate surrounding BCCH carriers to identify the BSIC in the SB. In order to do so, the MS uses the idle frames. These frames are termed "search" frames. Note that a window of nine consecutive slots is needed to find time slot 0 on the BCCH frequency (remember that time slot 0 carries the SCH and FCCH), since the beacon channels are not necessarily synchronized with one another. One important characteristic to notice is that the SCH and FCCH are mapped onto the 51 multi-frame, and that the idle frame of the mobile during communication is occurs in on the 26 multiframe. Since 26 and 51 are mutually prime numbers, this means a search frame will be available every 26 modulo 51 frame on the beacon channel.
For instance, let us imagine that an idle frame occurs in the frame 0 of the 51 multiframe. The next idle frames will be programmed on frames 26, 1, 27, 2, and so on. Therefore, after a certain number of search frames, the MS will necessarily decode an FB and an SB.
Another measured parameter during a TCH or SDCCH is the RXQUAL, which represents an indication of the quality of the received link, in terms of BER. For each channel, the measured received signal quality is averaged on that channel over the reporting period of length one SACCH multiframe defined above.
This chapter provides an overview of the GSM cellular system, with a focus on the radio interface. The purpose is not to give a detailed description of the many features supported by the system, but to summarize the elementary concepts of GSM, as an aid to reader comprehension of the subsequent chapters. In-depth presentation of the GSM system can be found in [1, 2].
1.1 Introduction
1.1.1 Birth of the GSM System
The first step in the history of GSM development was achieved back in 1979, at the World Administrative Radio Conference, with the reservation of the 900-MHz band. In 1982 at the Conference of European Posts and Telegraphs (CEPT) in Stockholm, the Groupe Spécial Mobile was created, to implement a common mobile phone service in Europe on this 900-MHz frequency band. Currently the acronym GSM stands for Global System for Mobile Communication; the term "global" was preferred due to the intended adoption of this standard in every continent of the world.
The proposed system had to meet certain criteria, such as:
Good subjective speech quality (similar to the fixed network);
Affordability of handheld terminals and service;
Adaptability of handsets from country to country;
Support for wide range of new services;
Spectral efficiency improved with respect to the existing first-generation analog systems;
Compatibility with the fixed voice network and the data networks such as ISDN;
Security of transmissions.
Digital technology was chosen to ensure call quality.
The basic design of the system was set by 1987, after numerous discussions led to the choice of key elements such as the narrowband time-division multiple access (TDMA) scheme, or the modulation technique. In 1989 responsibility for the GSM was transferred to the European Telecommunication Standards Institute (ETSI). ETSI was asked by the EEC to unify European regulations in the telecommunications sector and in 1990 published phase I of the GSM system specifications (the phase 2 recommendations were published in 1995).
The first GSM handset prototypes were presented in Geneva for Telecom '91, where a GSM network was also set up. Commercial service had started by the end of 1991, and by 1993 there were 36 GSM networks in 22 countries. The system was standardized in Europe, but is now operational in more than 160 countries all over the world, and was adopted by 436 operators.
The growth of subscribers has been tremendous, reaching 500 million by May 2001.
1.1.2 The Standard Approach
The thousands of pages of GSM recommendations, designed by operators and infrastructure and mobile vendors, provide enough standardization to guarantee proper interworking between the components of the system. This is achieved by means of the functional and interface descriptions for each of the different entities.
The GSM today is still under improvement, with the definition of new features and evolution of existing features. This permanent evolution is reflected in the organization of the recommendations, first published as phase I, then phase II and phase II+, and now published with one release each year (releases 96, 97, 98, 99, and releases 4 and 5 in 2000 and 2001).
As stated, responsibility for the GSM specifications was carried by ETSI up to the end of 1999. During 2000, the responsibility of the GSM recommendations was transferred to the Third Generation Partnership Project (3GPP). This world organization was created to produce the third-generation mobile system specifications and technical reports. The partners have agreed to cooperate in the maintenance and development of GSM technical specifications and technical reports, including evolved radio access technologies [e.g., General Packet Radio Service (GPRS) and Enhanced Data rates for Global Evolution (EDGE)]. The structure and organization of the 3GPP is further described in Section 2.5.
1.2 General Concepts
1.2.1 Analog Versus Digital Telephony Systems
First-generation systems were analog. During the early 1980s these systems underwent rapid development in Europe. Although the NMT system was used by all the Nordic countries, and the TACS system in the United Kingdom and Italy, there was a variety of systems and no compatibility among them. Compared with these systems, the main advantages offered by GSM, which is the most important of the second-generation digital systems, are:
Standardization;
Capacity;
Quality;
Security.
Standardization guarantees compatibility among systems of different countries, allowing subscribers to use their own terminals in those countries that have adopted the digital standard. The lack of standardization in the first-generation system limited service to within the borders of a country. Mobility is improved, since roaming is no longer limited to areas covered by a certain system (see Section 1.2.6). Calls can be charged and handled using the same personal number even when the subscriber moves from one country to another.
Standardization also allows the operator to buy entities of the network from different vendors, since the functional elements of the network and the interfaces between these elements are standardized. This means that a mobile phone from any manufacturer is able to communicate with any network, even if this network is built with entities from different vendors. This leads to a large economy of scale and results in cost reduction for both the operator and the subscriber. Furthermore, the phone cost is also reduced, because as GSM is an international standard, produced quantities are greater and the level of competition is high.
With respect to capacity, the use of the radio resource is much more efficient in a digital system such as GSM than in an analog system. This means that more users can be allocated in the same frequency bandwidth. This is possible with the use of advanced digital techniques, such as voice compression algorithms, channel coding, and multiple access techniques. Note that capacity gains are also achieved with radio frequency reuse, which had also been used in analog systems. Frequency reuse means that a given carrier can be employed in different areas, as explained in Section 1.2.2.
The quality in digital transmission systems is better, thanks to the channel coding schemes that increase the robustness in the face of noise and disturbances such as interference caused by other users or other systems. The quality improvement is also due to the improved control of the radio link, and adaptations to propagation conditions, with advanced techniques such as power control or frequency hopping. This will be explained in greater detail in Section 1.5.6.3.
In terms of security, powerful authentication and encryption techniques for voice and data communications are enabled with GSM, which guarantees protected access to the network, and confidentiality.
1.2.2 Cellular Telephony
In mobile radio systems, one of the most important factors is the frequency spectrum. In order to make the best use of the bandwidth, the system is designed by means of the division of the service area into neighboring zones, or cells, which in theory have a hexagonal shape. Each cell has a base transceiver station (BTS), which to avoid interference operates on a set of radio channels different from those of the adjacent cells. This division allows for the use of the same frequencies in nonadjacent cells. A group of cells that as a whole use the entire radio spectrum available to the operator is referred to as a cluster. The shape of a cell is irregular, depending on the availability of a spot for the BTS, the geography of the terrain, the propagation of the radio signal in the presence of obstacles, and so on.
In dense urban areas, for instance, where the mobile telephony traffic is important, the diameter of the cells is often reduced in order to increase capacity. This is allowed since the same frequency channels are used in a smaller area. On the other hand, reducing the cell diameter leads to a decrease in the distance necessary to reuse the frequencies (that is, the distance between two cochannel cells), increasing cochannel interference. In order to minimize the level of interference, several techniques are used on the radio interface.
A basic example of cluster organization is shown in Figure 1.1. In this example, we see a reuse pattern for seven different frequencies, f1 to f7. These frequencies correspond to the beacon carrier of each cell, on which signaling information about the cell is broadcast (see Section 1.2.7). It can be seen from this figure that a given carrier can be reused in two separate geographical areas, as long as these areas are far enough from each other to reduce the effect of interference. With this technique of dividing the area in cells and clusters, the operator can increase the area it is able to cover with a limited frequency bandwidth.
Figure 1.1: Example of a cell planning.
1.2.3 Public Land Mobile Network
A public land mobile network (PLMN) is a network established for the purpose of providing land mobile telecommunications services to the public. It may be considered as an extension of a fixed network, such as the Public Switched Telephone Network (PSTN), or as an integral part of the PSTN.
1.2.4 Multiband Mobile Phones
Because of the increasing demand on the mobile networks, today the mobile stations (MSs) tend to be multiband. Indeed, to avoid network saturation in densely populated regions, mobile phones capable of supporting different frequency bands have been implemented, to allow for the user making communications in any area, at any time.
A dual-band phone can operate in two different frequency bands of the same technology, for instance in the 900-MHz and 1800-MHz frequency bands of the GSM system. Triple-band mobile phones have also come on the market, with the support of GSM-900 (900-MHz GSM band), DCS-1800 (1800-MHz GSM band), and PCS-1900 (1900-MHz GSM band), for example. Note that DCS-1800 and PCS-1900 are never deployed in the same country, and therefore this kind of phone can be used by travelers who want to have service coverage in a large number of countries.
1.2.5 SIM Card
One of the most interesting innovations of GSM is that the subscriber's data is not maintained in the mobile phone. Rather a "smart card," called a subscriber identity module (SIM) card, is used.
The SIM is inserted in the phone to allow the communications. A user may thus make telephone calls with a mobile phone that is not his own, or have several phones but only one contract. It is for example possible to use a SIM card in a different mobile when traveling to a country that has adopted the GSM on a different frequency band. A European can therefore rent a PCS1900 phone when traveling to the United States, while still using his own SIM card, and thus may receive or send calls. The SIM is used to keep names and phone numbers, in addition to those that are already kept in the phone's memory.
The card is also used for the protection of the subscriber, by means of a ciphering and authentication code.
1.2.6 Mobility
GSM is a cellular telephony system that supports mobility over a large area. Unlike cordless telephony systems, it provides location, roaming, and handover.
1.2.6.1 Location Area
The ability to locate a user is not supported in first-generation cellular systems. This means that when a mobile is called, the network has to broadcast the notification of this call in all the radio coverage. In GSM, however, location areas (LAs), which are groups of cells, are defined by the operator. The system is able to identify the LA in which the subscriber is located. This way, when a user receives a call, the notification (or paging) is only transmitted in this area. This is far more efficient, since the physical resource use is limited.
1.2.6.2 Roaming
In particular, the GSM system has the capability of international roaming, or the ability to make and receive phone calls to and from other nations as if one had never left home. This is possible because bilateral agreements have been signed between the different operators, to allow GSM mobile clients to take advantage of GSM services with the same subscription when traveling to different countries, as if they had a subscription to the local network. To allow this, the SIM card contains a list of the networks with which a roaming agreement exists.
When a user is "roaming" to a foreign country, the mobile phone automatically starts a search for a network stipulated on the SIM card list. The choice of a network is performed automatically, and if more than one network is given in the list, the choice is based on the order in which the operators appear. This order can be changed by the user. The home PLMN is the network in which the user has subscribed, while the visited PLMN often refers to the PLMN in which the user is roaming. When a user receives a call on a visited PLMN, the transfer of the call from the home PLMN to the visited PLMN is charged to the called user by his operator.
1.2.6.3 Handover
When the user is moving from one cell to the other during a call, the radio link between BTS 1 and the MS can be replaced by another link, between BTS 2 and the MS. The continuity of the call can be performed in a seamless way for the user. This is called handover. With respect to dual-band telephones, one interesting feature is called the dual-band handover. It allows the user in an area covered both by the GSM-900 and by the DCS-1800 frequency bands, for instance, to be able to transfer automatically from one system to the other in the middle of a call.
1.2.7 Beacon Channel
For each BTS of a GSM network, one frequency channel is used to broadcast general signaling information about this cell. This particular carrier frequency is called a beacon channel, and it is transmitted by the BTS with the maximum power used in the cell, so that every MS in the cell is able to receive it.
1.2.8 MS Idle Mode
When it is not in communication, but still powered on, the MS is said to be in idle mode. This means that it is in a low consumption mode, but synchronized to the network and able to receive or initiate calls.
1.3 GSM Services
In the specification of a telecommunication standard such as GSM, the first step is of course the definition of the services offered by the system. GSM is a digital cellular system designed to support a wide variety of services, depending on the user contract and the network and mobile equipment capabilities.
In GSM terminology, telecommunication services are divided into two broad categories:
Bearer services are telecommunication services providing the capability of transmission of signals between access points [the user-network interfaces (UNIs) in ISDN]. For instance, synchronous dedicated packet data access is a bearer service.
Teleservices are telecommunication services providing the complete capability, including terminal equipment functions, for communication between users according to protocols established by agreement between network operators.
In addition to these services, supplementary services are defined that modify or supplement a basic telecommunication service.
1.3.1 Bearer Services
There exist several categories of bearer services:
Unrestricted digital information (UDI) is designed to offer a peer-to-peer digital link.
The 3.1 kHz is external to the PLMN and provides a UDI service on the GSM network, interconnected with the ISDN or the PSTN by means of a modem.
PAD allows an asynchronous connection to a packet assembler/disassembler (PAD). This enables the PLMN subscribers to access a packet-switched public data network (PSPDN).
Packet enables a synchronous connection to access a PSPDN network and alternate speech and data, providing the capability to switch between voice and data during a call.
Speech followed by data first provides a speech connection, and then allows to switch during the call for a data connection. The user cannot switch back to speech after the data portion.
1.3.2 Teleservices
In terms of application, teleservices correspond to the association of a particular terminal to one or several bearer services. They provide access to two kinds of applications:
Between two compatible terminals;
From an access point of the PLMN to a system including high-level functions, for example, a server.
Of course, the most basic teleservice supported by GSM is digital voice telephony, based on transmission of the digitally encoded voice over the radio. The voice service also includes emergency calls, for which the nearest emergency-service provider is notified by dialing three digits.
The other teleservices that are defined for a PLMN are:
Data services, with data rates ranging from 2.4 Kbps to 14.4 Kbps. These services are based on circuit-switched technology. Circuit switched means that during the communication, a circuit is established between two entities for the transfer of data. The physical resource is used during the whole duration of the call.
Short message service (SMS), which is a bidirectional service for short alphanumeric (up to 160 bytes) messages.
Access to a voice message service.
Fax transmission.
1.3.3 Supplementary Services
Supplementary services include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), caller identification, call waiting, multiparty conversations, charging information, and call barring of outgoing or incoming calls. These call-barring features can be used for example when roaming in another country, if the user wants to limit the communication fees.
1.4 Network Architecture
The structure of a GSM network relies on several functional entities, which have been specified in terms of functions and interfaces. It involves three main subsystems, each containing functional units and interconnected with the others through a series of standard interfaces.
The main parts of a GSM network, as shown in Figure 1.2, are listed below. (In the figure, the lines between the entities represent the interfaces.)
Figure 1.2: General architecture of a GSM network.
The MS, the handheld mobile terminal;
The base station subsystem (BSS), which controls the radio link with the MS;
The network and switching subsystem (NSS), which manages the function of connection switching to other fixed public network or mobile network subscribers, and handles the databases required for mobility management and for the subscriber data.
As can be seen from the figure, in a PLMN, the radio access network part (BSS) is logically separated from the core network (NSS), in order to ease the standardization of the different functions.
1.4.1 MS
The MS is made up of the mobile equipment (ME), and a SIM. It performs the following functions:
Radio transmission and reception;
Source and channel coding and decoding, modulation and demodulation;
Audio functions (amplifiers, microphone, earphone);
Protocols to handle radio functions: power control, frequency hopping, rules for access to the radio medium;
Protocol to handle call control and mobility;
Security algorithms (encryption techniques).
As mentioned, the SIM enables the user to have access to subscribed services irrespective of a specific terminal. The insertion of the SIM card into any GSM terminal allows the user to receive calls on that terminal, to make calls from that terminal, and to use the other subscribed services. The ME is identified with an international mobile equipment identity (IMEI).
The SIM card contains, among other information, the international mobile subscriber identity (IMSI) used to identify the subscriber to the system, and a secret key for authentication. The IMEI and the IMSI are independent, thereby allowing personal mobility.
1.4.2 BSS
The BSS is composed of several base station controllers (BSCs) and BTS. These two elements communicate across the Abis interface.
The BTS contains the radio transceivers, responsible for the radio transmissions with the MS. This includes the following functions:
Modulation and demodulation;
Channel coding and decoding;
Encryption process;
RF transmit and receive circuits (power control, frequency hopping, management of antenna diversity, discontinuous transmission).
Several types of BTS exist: the normal BTS, the micro BTS, and the pico BTS. The micro BTS is different from a normal BTS in two ways. First, the range requirements are reduced, and the close proximity requirements are more stringent. Second, the micro BTS is required to be small and affordable in order to allow external street deployment in large numbers. The pico BTS is an extension of the micro BTS concept to the indoor environments. The RF performances of these different BTSs are slightly different.
The BSC manages the radio resources for one or more BTSs. It handles the management of the radio resource, and as such it controls the following functions: allocation and release of radio channels, frequency hopping, power control algorithms, handover management, choice of the encryption algorithm, and monitoring of the radio link.
1.4.3 Network Subsystem
The mobile services switching center (MSC) is the central part of the network subsystem (NSS). It is responsible for the switching of calls between the mobile users (between different BSCs or toward another MSC) and between mobile and fixed network users. It manages outgoing and incoming calls from various types of networks, such as PSTN, ISDN, and PDN. It also handles the functionality required for the registration and authentication of a user, and the mobility operations. This includes location updating, inter-MSC handovers, and call routing.
The BSS communicates with the MSC across the A interface.
Associated with the MSC, two databases, the home location register (HLR) and the visitor location register (VLR), provide the call-routing and roaming capabilities. The HLR contains all the administrative information related to the registered subscribers within the GSM network. This includes the IMSI, which unequivocally identifies the subscriber within any GSM network, the MS ISDN number (MSISDN), and the list of services subscribed by the user (such as voice, data service). The HLR also stores the current location of the MS, by means of the address of the VLR in which it is registered.
The VLR temporarily keeps the administrative data of the subscribers that are currently located in a given geographical area under its control.
Each functional entity may be implemented as an independent unit, but most of the time, the VLR is colocated with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR. The MSC contains no information about particular MSs, but rather, the information is stored in the location registers.
Two other registers are used for authentication and security purposes:
The equipment identity register (EIR) is a database that contains a list of all valid ME on the network, where each MS is identified by its IMEI. An IMEI is marked as invalid if it has been reported stolen.
The authentication center (AuC) is a protected database that contains a copy of the secret key stored in each subscriber's SIM card, for authentication and encryption over the radio channel. The AuC verifies if a legitimate subscriber has requested a service. It provides the codes for both authentication and encryption to avoid undesired violations of the system by third parties.
Two other important entities of the NSS are the operations and maintenance center (OMC) and the network management center (NMC). These entities perform the functions relative to the network management (NM), such as the configuration of the system (locally or remotely), maintenance and tests of the pieces of equipment, billing, statistics on the performance, and gathering of all information related to subscriber traffic necessary for invoicing and administration of subscribers.
1.5 Radio Interface
1.5.1 General Characteristics
Currently, there are several types of networks in the world using the GSM standard, but at different frequencies.
The GSM-900 is the most common in Europe and the rest of the world. Its extension is E-GSM.
The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover urban areas. It was also introduced to avoid saturation problems with the GSM-900.
The PCS-1900 is used primarily in North America.
The GSM-850 is under development in America.
The GSM-400 is intended for deployment in Scandinavian countries in the band previously used for the analog Nordic Mobile Telephony (NMT) system.
The system is based on frequency-division duplex (FDD), which means that the uplink (radio link from the mobile to the network-that is, mobile transmit, base receive), and downlink (from the network to the mobile-that is, base transmit, mobile receive) are transmitted on different frequency bands. For instance, in the 900-MHz E-GSM band, the block 880-915 MHz is used for transmission from mobiles to network, and the block 925-960 MHz is used for the transmission from network to mobiles. Table 1.1 gives a summary of uplink and downlink frequency bands for the different GSM systems.
Table 1.1: GSM System Frequency Bands Uplink Band
Downlink Band
GSM-900
890-915 MHz
935-960 MHz
E-GSM-900
880-915 MHz
925-960 MHz
DCS-1800
1,710-1,785 MHz
1,805-1,880 MHz
PCS-1900
1,850-1,910 MHz
1,930-1,990 MHz
GSM-400
GSM-450
450,4-457,6 MHz
460.4-467.6 MHz
GSM-480
478.8-486 MHz
488.8-496 MHz
GSM-850
824-849 MHz
869-894 MHz
Operators may implement networks that operate on a combination of the frequency bands listed above to support multiband mobile terminals.
There are different ways of sharing the physical resource among all the users in a radio system, and this is called the multiple-access method. The multiple-access scheme defines how simultaneous communications share the GSM radio spectrum. The various multiple-access techniques in use in radio systems are frequency-division multiple access (FDMA), TDMA, and code-division multiple access (CDMA). GSM is based on both FDMA and TDMA techniques (see Figure 1.3).
Figure 1.3: TDMA and FDMA.
FDMA consists in dividing the frequency band of the system into several channels. In GSM, each RF channel has a bandwidth of 200 kHz, which is used to convey radio modulated signals, or carriers. Each pair of uplink/ downlink channels is called an absolute radio frequency channel (ARFC) and is assigned an ARFC number (ARFCN). The mapping of each ARFCN on the corresponding carrier frequency is given in [3].
TDMA is the division of the time into intervals: within a frequency channel, the time is divided into time slots. This division allows several users (eight) to be multiplexed on the same carrier frequency, each user being assigned a single time slot. A packet of data information, called a burst, is transmitted during a time slot. The succession of eight time slots is called a TDMA frame, and each time slot belonging to a TDMA frame is identified by a time slot number (TN), from 0 to 7.
1.5.2 Logical Channels
The association of a radio frequency channel and a time slot-the pair ARFCN and TN-uniquely defines a physical channel on both the uplink and the downlink.
On top of the physical channels, logical channels ar mapped to convey the information of voice, data, and signaling. This signaling information is used for setting up a call, or to adapt the link to rapidly changing radio conditions, or to manage handovers, to give a few examples. Logical channels can be seen as pipes, each one used for a different purpose by the higher layers of the system.
Two types of logical channels exist, traffic channels and control channels. Among the control channels, according to their functions, four classes are defined: broadcast, dedicated, common, and associated. A broadcast channel is used by the network (in downlink only) to send general information to the MSs. A channel is said to be dedicated if only one MS can transmit or receive in the ARFCN-TN defining this channel, and common if it carries information for several mobiles. An associated control channel is allocated to one mobile, in addition to a dedicated channel, and carries signaling for the operation of this channel.
The broadcast channels are transmitted on the beacon carrier frequency presented in Section 1.2.7. The purposes of the beacon are:
To allow a synchronization in time and frequency of the MSs to the BTS. This synchronization is needed by the MS to access the services of a cell. The frequency and time synchronization procedures that are performed by the mobile are explained in Section 1.5.7.
To help the mobile in estimating the quality of the link during a communication, by measurements on the received signal from the BTS it is transmitting to, and from the other BTSs of the geographical area. These measurements are used by the network to determine when a handover is necessary, and to which BTS this handover should apply.
To help the mobile in the selection of a cell when it is in idle mode (that is, not in communication, but still synchronized to the system and able to receive an incoming call or to initiate a call). This selection is performed on the basis of the received power measurements made on the adjacent cells' beacon channels.
To access the general parameters of the cell needed for the procedures applied by the MS, or general information concerning the cell, such as its identification, the beacon frequencies of the surrounding cells, or the option supported by the cell (services).
To allow these various operations, the logical channels transmitted on the beacon are:
The broadcast control channel (BCCH), which continually broadcasts, on the downlink, general information on the cell, including base station identity, frequency allocations, and frequency-hopping sequences. The information is transmitted within system information (SI) blocks, which can be of different types according to the information that is carried out. The frequency with which an SI is retransmitted on the BCCH varies with the type of information.
The frequency control channel (FCCH), used by the MS to adjust its local oscillator (LO) to the BTS oscillator, in order to have a frequency synchronization between the MS and the BTS.
The synchronization channel (SCH), used by the MS to synchronize in time with the BTS, and to identify the cell.
As listed below, four channels comprise the common control channels (CCCH). Among these, the first three are used for the MS-initiated call or for call paging (notification of an incoming call toward the MS):
The random access channel (RACH) is used for the MS access requests to the network, for the establishment of a call, based on a slotted aloha method.
The paging channel (PCH) is defined to inform the MS of an incoming call.
The access grant channel (AGCH) is used to allocate some physical resource to a mobile for signaling, following a request on the RACH.
The cell broadcast channel (CBCH) may be used to broadcast specific news to the mobiles of a cell.
The dedicated control channels are:
The stand-alone dedicated control channel (SDCCH), utilized for registration, authentication, call setup, and location updating.
The slow associated control channel (SACCH), which carries signaling for the TCH or SDCCH with which it corresponds. The information that is transmitted on this channel concerns the radio link control (RLC), such as the power control on the corresponding TCH or SDCCH, or the time synchronization between the MS and the BTS.
The fast associated control channel (FACCH), carries the signaling that must be sent by the network to the MS to notify that a handover is occurring.
The TCHs can be of several types, according to the service that is accessed by the subscribers: voice or data, with various possible data rates.
Table 1.2 summarizes the purpose of the different logical channels. In this table, UL stands for uplink, and DL for downlink.
Table 1.2: The Logical Channels and Their Purpose Logical Channel
Abbreviation
Uplink/ Downlink
Task
Broadcast channel (BCH)
Broadcast control channel
BCCH
DL
System Information broadcast
Frequency correction channel
FCCH
DL
Cell frequency synchronization
Synchronization channel
SCH
DL
Cell time synchronization and identification
Common control channel (CCCH)
Paging channel
PCH
DL
MS paging
Random access channel
RACH
UL
MS random access
Access grant channel
AGCH
DL
Resource allocation
Cell broadcast channel
CBCH
DL
Short messages broad cast
Dedicated control channel
Standalone dedicated control channel
SDCCH
UL/DL
General signaling
Slow associated control channel
SACCH
UL/DL
Signaling associated with the TCH
Fast associated control channel
FACCH
UL/DL
Handover signaling
Traffic channel (TCH)
Full speech
TCH/FS
UL/DL
Full-rate voice channel
Half rate
TCH/HS
UL/DL
Half-rate voice channel
2.4 Kbps, 4.8 Kbps, 9.6 Kbps, and 14.4 Kbps full-rate data channels
TCH/F2.4 TCH/F4.8 TCH/F9.6 TCH/F14.4
UL/DL
Full-rate data channels
2.4-Kbps- and 4.8-Kbps-rate data channels
TCH/H2.4 TCH/H4.8
UL/DL
Half-rate data channels
1.5.3 Mapping of Logical Channels onto Physical Channels
1.5.3.1 TDMA Time Structure
The basic time unit is the time slot. Its duration is 576.9 μs = 15/26 ms, or 156.25 symbol periods (a symbol period is 48/13 μs). The piece of information transmitted during a time slot is called a burst. As we saw in Section 1.5.1, the GSM multiple access scheme is TDMA, with eight time slots per carrier. A sequence of eight time slots is called a TDMA frame, and has a duration of 4.615 ms. The time slots of a TDMA frame are numbered from 0 to 7, as shown in Figure 1.4. Note that the beginning and end of TDMA frames in uplink and downlink are shifted in time: Time slot number 0 on the uplink corresponds to time slot 3 in the downlink. This allows some time for the mobile to switch from one frequency to the other.
Figure 1.4: Slot numbering within the TDMA frame.
As seen earlier, a physical channel is defined as a sequence of TDMA frames, a time slot number (from 0 to 7) and a frequency. It is bidirectional, with the same TN in uplink and in downlink. In order to support cryptographic mechanisms, a long time-structure has been defined. It is called a hyperframe and has a duration of 3 hours, 28 minutes, 53 seconds, and 760 ms (or 12,533.76 seconds). The TDMA frames are numbered within the hyperframe. The numbering is done with the TDMA frame number (FN) from 0 to 2,715,647.
One hyperframe is subdivided into 2,048 superframes, which have a duration of 6.12 seconds. The superframe is itself subdivided into multi-frames. In GSM, there are two types of multiframes defined, containing 26 or 51 TDMA frames.
The 26 multiframe has a duration of 120 ms, and comprises 26 TDMA frames This multiframe is used to carry TCH, SACCH, and FACCH. The 51 multiframe is made up of 51 TDMA frames. Its duration is 235.4 ms (3,060/13 ms). This multiframe is used to carry BCH, CCCH, and SDCCH (with its associated SACCH). Note that a superframe is composed of twenty-six 51-multiframes, or of fifty-one 26-multiframes. This hierarchical time structure is summarized in Figure 1.5.
Figure 1.5: Hierarchical structure of a hyperframe.
1.5.3.2 Mapping of the TCH and SACCH on the 26-Multiframe
The TCHs are bidirectional channels mapped onto the 26-multiframe. Two types of channels must be distinguished: full-rate and half-rate channels, and therefore two different mappings of the TCH on the multiframe are possible:
A full-rate traffic channel (TCH/FS, for full speech) uses one time slot per TDMA frame, for each frame of the multiframe, except the frames 12 and 25 (see Figure 1.6). The TDMA frame 12 is used to carry the SACCH/FS, and the TDMA frame 25 is an idle frame, which means that no channel is transmitted during this entire TDMA frame.
Figure 1.6: Mapping of a TCH/FS and SACCH/FS on the 26-multiframe.
A half-rate traffic channel (TCH/HS) uses one time slot every two TDMA frames, due to the fact that it carries data from a half-rate voice coder. As shown in Figure 1.7, two half-rate channels can be mapped on the same time slot, one using TDMA frames 0, 2, 4, 6, 8, 10, 13, 15, 17, 19, 21, and 23 and the other one using frames 1, 3, 5, 7, 9, 11, 14, 16, 18, 20, 22, and 24. The SACCH/HS channel associated with the first TCH subchannel is transported on TDMA frame 12, and the SACCH/HS associated with the second subchannel is on time slot 25.
Figure 1.7: Mapping of a TCH/HS and SACCH/HS on the 26-multiframe.
1.5.3.3 Mapping of the FACCH on the 26-Multiframe
The FACCH is associated with a TCH, and is required to support the highspeed signaling needed during call establishment, subscriber authentication, and handover management. The occurrence of the FACCH is not fixed in the multiframe, as it is for the SACCH. Rather, the FACCH occurs on a TDMA frame that is reserved for a TCH. The multiplexing of TCH and FACCH is possible by means of the frame stealing. This means that a speech frame carried over a TCH can be replaced by a FACCH frame. This is signaled to the receiver by means of stealing flags, as described in Section 1.5.4. This principle allows for a fast signaling channel without significant loss on the quality of speech, if it is not performed too often.
1.5.3.4 Mapping of the SDCCH on the 51-Multiframe
The SDCCH is a signaling channel that carries the higher layers of control information. A SACCH is associated with a SDCCH.
Two different multiplexings on the 51 multiframe are possible:
SDCCH with SACCH alone. An SDCCH channel is mapped on four TDMA frames of a 51 multiframe. As a result, eight SDCCH channels, dedicated to eight different MSs are mapped onto a 51 multiframe. The TDMA frames not occupied by an SDCCH are used by the eight associated SACCH channels. The mapping of these associated channels is performed on two consecutive 51 multi-frames, as shown in Figure 1.8(b).
SDCCH with SACCH multiplexed together with CCCH, BCCH, SCH. and FCCH. This case is described in the next subsection and in Figure 1.8(c).
Figure 1.8: Channel associations on the 51-multiframe. (a) BCCH + CCCH, (b) 8 SDCCH/8, and (c) BCCH + CCCH 4 SDCCH/4. (From- [4].)
Note that the frame-stealing concept, used on the TCH to allocate FACCH frames, is not in use in the case of an SDCCH. This is due to the fact that sufficiently fast signaling can be transmitted over an SDCCH to carry on a handover procedure.
1.5.3.5 Mapping of the Broadcast Channels and CCCH on the 51 Multiframe
The broadcast channels and the CCCH (i.e., PCH, RACH, AGCH) are all multiplexed on the 51 multiframe, on the beacon carrier frequency. The FCCH is sent on downlink time slot 0, on the TDMA frames 0, 10, 20, 30, and 40 of the 51 multiframe. The SCH is also mapped on slot 0, in the TDMA frames immediately following an FCCH frame (i.e., frames 1, 11, 21, 31, 41).
The BCCH is associated with time slot 0 as well, in the TDMA frames that are not occupied by the FCCH and the SCH, and can also be mapped on time slots 2, 4, and 6 in some configurations.
The AGCH and PCH are dynamically multiplexed on the multiframe according to the network load. They are mapped on time slot 0, together with the FCCH, SCH, and BCCH, and optionally on time slots 2, 4, and 6 (also possibly used for the BCCH). Note that the BCCH and CCCH are multiplexed dynamically. The exact mapping is conveyed to the MS by means of SI blocks, sent over the BCCH.
One of these broadcast parameters also indicates whether or not the CCCH are combined with SDCCH and SACCH onto the same basic physical channel (the second type of mapping of SDCCH/SACCH presented in Section 1.5.3.4).
The mapping of the RACH channel is simple: every uplink slot corresponding to a downlink FCCH, SCH, BCCH, PCH, and AGCH can be used for a RACH. If the SDCCH and SACCH are multiplexed with the CCCH, the number of available random-access channel blocks are reduced. Figure 1.8(a, c) shows possible configurations with the multiplexing BCCH/ CCCH and BCCH/CCCH/SDCCH/SACCH.
1.5.3.6 Summary of Logical Channel Combinations
The different allowed combinations of logical channels on a physical channel are as follows:
TCH/F + FACCH/F + SACCH/TF;
TCH/H+ FACCH/H + SACCH/TH;
FCCH + SCH + BCCH + CCCH;
FCCH + SCH + BCCH + CCCH + SDCCH + SACCH;
BCCH + CCCH;
SDCCH+ SACCH.
Other logical channels exist for packet-switched services (GPRS, EDGE, or DTM), and will be described later.
1.5.4 Voice Digital Communication Chain
The functions performed by the physical layer during transmission are presented in this section, and the TCH/FS voice channel will be used for the sake of example. The steps of the communication chain, as outlined below, are presented in Figure 1.9.
Figure 1.9: The voice transmission chain.
1.5.4.1 Source Coding
First of all, speech must be digitized. On the basis of subjective speech quality and complexity issues, a regular pulse excited-linear predictive coder (RPE-LPC) with a long-term predictor loop was chosen. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual (the difference between the predicted and actual sample), represent the signal. Speech is divided into 20-ms samples, each of which is encoded as 260 bits, giving a total bit rate of 13 Kbps. This is the so-called full-rate speech coding. Several other voice codecs are defined (half rate, enhanced full rate, adaptive multi-rate codecs), but these elements are beyond the scope of this book.
1.5.4.2 Channel Coding
In order to protect the voice against noise, interference, and multipath radio propagation conditions, the encoded speech transmitted over the radio interface is protected from errors. Convolutional encoding and block interleaving are used to achieve this protection. As discussed above, the speech codec produces a 260-bit block for every 20-ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are therefore divided into three classes:
Class Ia: 50 bits, most sensitive to bit errors;
Class Ib: 132 bits, moderately sensitive to bit errors;
Class II: 78 bits, least sensitive to bit errors.
Class Ia bits have a 3-bit cyclic redundancy code added for error detection. If an error is detected, the frame is determined to be incomprehensible, and it is discarded. In such a case it is replaced by a slightly attenuated version of the previous correctly received frame.
These 53 bits, together with the 132 class Ib bits and a 4-bit tail sequence (a total of 189 bits), are input into a 1/2-rate convolutional encoder of constraint length 4. Each input bit is encoded as 2 output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits. The 78 remaining class II bits, which are unprotected, are added to these 378 bits. A total of 456 encoded bits are therefore produced for every 20ms speech sample. This represents a bit rate of 22.8 Kbps. In order to protect against burst errors common to the radio interface, each block of 456 bits is interleaved.
1.5.4.3 Interleaving
The 456 coded bits are permutated, and divided into eight blocks of 57 bits, and these blocks are transmitted in eight consecutive bursts, as described in Figure 1.10. Since each burst can carry two 57-bit blocks, each burst carries traffic from two different speech frames. The benefit of doing this is that it provides time diversity. If a sequence of several consecutive bits is corrupted by the degraded propagation conditions (fading, for example) during a given period of time, interleaving ensures that the errors will be randomly distributed over the block of 456 bits. This property is required for a better performance of the decoding algorithm. The decoding of convolutional codes is indeed often performed by the Viterbi algorithm (refer to the case study on this subject in Chapter 4), for which the errors occur in packets. Thus, a random distribution of the corrupted bits over a block at the input of the decoder leads to an increased decoding efficiency, and so to better performance in terms of bit error rate.
Figure 1.10: Interleaving scheme on the TCH.
1.5.4.4 Burst Formatting
We have seen that the coded bits are interleaved and transmitted over bursts. The format of the burst-carrying TCH traffic, or normal burst (NB), is shown in Figure 1.11. NBs are used on most of the logical channels, except the RACH, SCH, and FCCH. The NB is constituted of two data blocks of 57 bits, carrying the coded voice samples, as described above. The middle of the burst contains the training sequence of 26 bits, known by the receiver, used to estimate the distortion introduced by the radio channel and for time synchronization. At the beginning and at the end of the burst, two sequences of 3 bits, known as the tail bits, all equal to 0, are used. Two bits, one before and one after the training sequence, form the stealing flags (SF). In Section 1.5.3.3 we saw that those bits allow the FACCH and TCH to be multiplexed, with the frame-stealing concept. When a speech frame is stolen for the transmission of a FACCH block, this is signaled by the SF bits. When one stealing bit is equal to one, this means that half of the burst is used for FACCH; otherwise it is used for TCH. An example of the multiplexing of FACCH and TCH is shown in Figure 1.12. Note that a FACCH is transported over eight half bursts. The burst duration is 148 bits, or 546.46 μs, and it is followed by a guard period of 8.25 bits, to allow for the burst ramping up and down between the time slots, as represented in Figure 1.11.
Figure 1.11: Structure of an NB.
Figure 1.12: Example of frame stealing- multiplexing of a TCH and an FACCH.
1.5.4.5 Ciphering
For security reasons, ciphering is used to modify the data parts of the burst, with a binary addition between a pseudorandom bit sequence and the 114 data bits. The same operation is performed by the receiver for deciphering. The pseudorandom bit sequence is different in the uplink and in the downlink.
1.5.4.6 Differential Encoding
All the bits di of the burst are differentially encoded. The output of the differential encoder is
(1.1)
where ⊕ denotes modulo 2 addition.
The result is mapped onto +1 and -1 values, to form the modulating data value αi input to the modulator, as follows:
(1.2)
1.5.4.7 GMSK Modulation
The digital signal is modulated onto the analog carrier frequency using Gaussian-filtered minimum shift keying (GMSK), with a symbol period of 48/13 μs (i.e., 270.8333 kHz). This modulation was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, power consumption for the MS, and limited out of channels emissions. These radio emissions, outside of the allocated channel, must be strictly controlled so as to limit adjacent channel interference and allow for the coexistence of GSM and the other systems. The spectrum due to the modulation mask requirement is presented in Figure 1.13, along with an ideal GMSK spectrum.
Figure 1.13: Ideal GMSK spectrum and required spectrum mask.
On the receive section, the opposite operations are performed, namely, demodulation, burst deformatting, de-interleaving, channel decoding, and source decoding.
1.5.4.8 Demodulation
Because of the various obstacles in the environment, many reflected signals, each with a different time delay and phase, arrives at the receiver. This is called multipath, and it is time variant, since the terminal is mobile, by definition. To cope with these time-varying propagation conditions, the receiver uses an equalizer. An equalizer is an algorithm that uses the receive sampled symbols to estimate the sequence of bits that was transmitted by the peer entity, by suppressing the intersymbol interference (ISI). Equalization is therefore used to extract the desired signal from the unwanted reflections. To do this, the receiver uses a known sequence of transmitted bits, the training sequence, to estimate the channel impulse response (CIR). This known signal is the 26-bit training sequence transmitted in the middle of every NB. The CIR is then used by the equalizer to retrieve the transmitted symbols. The estimation of the CIR also allows the finest time synchronization of the mobile to the BTS (the mobile can detect and correct a delay of several symbol periods). Note that the actual implementation of the equalizer is not specified in the GSM system, and it is up to the mobile phone or BTS vendor to implement a solution that will achieve the specified receiver performance (see Section 1.5.6.2).
1.5.5 Bursts Format
There are four burst formats the purposes of which are defined as follows:
The NB is used to carry information on traffic and control channels, except for RACH, SCH, and FCCH. It contains 114 encrypted symbols and includes a guard time of 8.25 symbol duration (=30,46 μs). A training sequence of 26 symbols is present in the middle of the burst (see Figure 1.11, Section 1.5.4).
The frequency correction burst (FB), as shown in Figure 1.14(a), contains a sequence of 142 fixed bits. This sequence is made of alternating ones and zeros (1, 0, 1, 0, ... 1, 0), so that after the differential encoding and GMSK modulation, the RF signal is equivalent to an unmodulated carrier, shifted by 67.7 kHz above the carrier frequency. This characteristic is used to help the mobile synchronize in frequency with the BTS, as explained in Section 1.5.7. The FB is transmitted over the FCCH.
The synchronization burst (SB) is needed for time synchronization of the mobile on the SCH. It contains a long training sequence and carries the information of the TDMA FN and base station identity code (BSIC), as can be seen in Figure 1.14(b).
The access burst (AB), presented in Figure 1.14(c), is used for random access (or the RACH) and is characterized by a longer guard period (68.25 bit duration or 252 μs), allowing the estimation of the timing advance (TA) by the BTS (see Section 1.5.6.3).
Figure 1.14: The (a) FB, (b) SB, and (c) AB burst structures.
In Figure 1.14, we see a guard period at the end of a burst. During this period, the transmission is attenuated in several steps, as specified by the power-versus-time mask specification (see the example of NB, Figure 1.11).
1.5.6 RF Characteristics
1.5.6.1 Transmission Characteristics
Several classes of mobiles are defined, according to their maximum output power capability, as shown in Table 1.3. In GSM-900, most of the mobiles available on the market are class 4 handheld terminals, while class 2 terminals are used as vehicle-mounted equipment. The class 4 and 5 MSs are denoted as "small MS." In DCS-1800, the typical class is class 1.
Table 1.3: MS Power Classes Power Class
GSM-400, GSM-900, GSM-850
Nominal Maximum Output Power
DCS-1800
Nominal Maximum Output Power
PCS-1900
Nominal Maximum Output Power
1
-
1W (30 dBm)
1W(30dBm)
2
8W (39 dBm)
0.25W (24 dBm)
0.25W (24 dBm)
3
5W (37 dBm)
4W (36 dBm)
2W (33 dBm)
4
2W (33 dBm)
5
0.8W (29 dBm)
These output power levels are maximum values, and can be reduced according to the commands that are sent by the network to the MSs. With these network commands, the MS operates at the lowest power level that maintains an acceptable signal quality.
These commands are based on the measurements that are performed by the MS and by the BTS. For instance, with a class 4 MS, the range of transmission can be several kilometers, but if the MS is getting closer to the BTS, it may receive a request from the network to decrease its output power level. This procedure, called power control, improves the performance of the system by reducing the interference caused to the other users. Moreover, it is a means of prolonging the battery life of the mobile. The power level can be stepped up or down in steps of 2 dB from the maximum power (depending on the MS class) down to a minimum of 5 dBm in GSM-400/900/850, and 0 dBm in DCS-1800/PCS-1900. The transmission of power control commands by the BTS is explained in Section 1.5.6.3.
For the BTS transceiver (TRX), the power classes are given in Table 1.4.
Table 1.4: TRX Power Classes TRX Power Class
GSM-400, GSM-900, GSM- 850
Maximum Output Power
DCS-1800 and PCS-1900
Maximum Output Power
1
320 (< 640)W
20(<40)W
2
160(<320)W
10(<20)W
3
80(<160)W
5(<10)W
4
40(<80)W
2.5(<5)W
5
20(<40)W
6
10(<20)W
7
5(<10)W
8
2.5(<5)W
As an option, the BSS can utilize downlink RF power control, with up to 15 steps of power control levels with a step size of 2 dB. Note that this power control on the downlink is not used on the beacon frequency, which is always transmitted with constant output power.
Many other requirements on the transmit section are defined in the GSM specifications, such as the spectrum due to modulation constraint (see Figure 1.13), the modulation accuracy, the transmitter frequency error, and the spurious emissions requirements.
1.5.6.2 Reception Characteristics
Several types of propagation models have been defined, in order to measure the mobile and BTS performances. These models represent several environments:
Typical urban (TUx);
Rural area (RAx);
Hilly terrain (HTx).
In the above definitions, the x stands for the velocity of the mobile, in km/h. The various propagation models are represented by a number of taps, each determined by their time delay and average power. The Rayleigh distributed amplitude of each tap varies according to a Doppler spectrum.
In addition to these multipath fading channels, the static channel was defined. This is a simple single-path constant channel. With this channel, the only perturbation comes from the receiver noise of the measured equipment.
One of the most important receiver performances that is specified is the sensitivity level, which determines the minimum level for which the receiver can demodulate a signal correctly. The sensitivity requirement, in GSM, is specified as an input level, in dBm, for which the measured equipment should reach a certain performance, in terms of bit error rate. For instance, for GSM400/900/850 power classes 4 or 5 mobiles and DCS-1800/PCS-1900 classes 1 or 2 mobiles, the sensitivity level is -102 dBm. For a normal BTS (that is not a micro- or a pico-BTS) the sensitivity level is -104 dBm, for all the frequency bands.
At these levels, different performances, according to both the logical channel and the propagation channel used for the measurement, must be met. Table 1.5 shows an example of performances that are reached at the sensitivity level, for GSM-900 and GSM-850. In this table, BER stands for bit error rate, FER for frame erasure ratio (i.e., incorrect-speech-frames ratio), and RBER for residual BER (defined as the ratio of the number of errors detected over the frames defined as "good" to the number of transmitted bits in the good frames). This table is an example; similar tables exist for the other logical channels and for the different frequency bands. Note that frequency hopping may be used for the sensitivity performance measurements.
Table 1.5: Sensitivity-Level Performance Requirements Propagation Conditions
Logical Channel
Static
TU
(no FH)
TU50
(ideal FH)
RA250
(no FH)
HT100
(no FH)
FACCH/H
(FER)
0.1%
6.9%
6.9%
5.7%
10.0%
FACCH/F
(FER)
0.1%
8.0%
3.8%
3.4%
6.3%
SDCCH
(FER)
0.1%
13%
8%
8%
12%
RACH
(FER)
0.5%
13%
13%
12%
13%
SCH
(FER)
1%
16%
16%
15%
16%
TCH/F14.4
(BER)
10.0-5
2.5%
2%
2%
5%
TCH/F9.6
(BER)
10.0-5
0.5%
0.4%
0.1%
0.7%
TCH/FS
(FER)
0.1α%
6α%
3α%
2α%
7α%
Class Ib
(RBER)
0.4/α%
0.4/α%
0.3/α%
0.2/α%
0.5/α%
Class II
(RBER)
2%
8%
8%
7%
9%
Note that in this example, the parameter a is defined as 1 ≤ α ≤ 1.6 and allows a tradeoff between the number of erased speech frames (i.e., decoded as wrong, and therefore not transmitted to the voice decoder) and the quality of the nonerased frames.
Another important characteristic of the receiver concerns its performance in the presence of an interferer. This is specified either for a cochannel interference (i.e., an interference situated at the same frequency as the signal of interest) or an adjacent channel interference (situated at 200 or 400 kHz from the carrier of interest). The level of the useful signal is set 20 dB higher than for the sensitivity evaluation, and a GMSK interfering signal is added, either at the same frequency or with an offset of 200 or 400 kHz from the carrier. For the cochannel test, the carrier to interference ratio C/Ic is set to 9 dB. Under these conditions, the performance of Table 1.6 must be met. Again, this table does not contain all the logical channels, and concerns the GSM-900 and GSM-850 only. Similar performance requirements are defined for the other cases.
Table 1.6: Interference Performance Requirements Propagation Conditions
Type of Channel
TU3
(NoFH)
TU3
(Ideal FH)
TU50
(No FH)
TU50
(Ideal FH)
RA250
(No FH)
FACCH/H
(FER)
22%
6.7%
6.7%
6.7%
5.7%
FACCH/F
(FER)
22%
3.4%
9.5%
3.4%
3.5%
SDCCH
(FER)
22%
9%
13%
9%
8%
RACH
(FER)
15%
15%
16%
16%
13%
SCH
(FER)
17%
17%
17%
17%
18%
TCH/F 14.4
BER)
10%
3%
4.5%
3%
3%
TCH/F 9.6
(BER)
8%
0.3%
0.8%
0.3%
0.2%
TCH/FS
(FER)
21α%
3α%
6α%
3α%
3α%
Class Ib
(RBER)
2/α%
0.2/α%
0.4/α%
0.2/α%
0.2/α%
Class II
(RBER)
4%
8%
8%
8%
8%
This table is also applied in the case of an adjacent channel interference. In this case, the C/I is set to -9 dB if the interferer is 200 kHz from the carrier, and -41 dB if it is 400 kHz from the carrier.
1.5.6.3 Control of the Radio Link
This section describes some of the procedures that are in use to improve the efficiency of the system, by adapting the transmission between the mobile and the BTS to the continuously varying radio environment.
Compensation for the Propagation Delay
Due to the distance between the MS and the BTS, there is a propagation delay that is equal to d/ c seconds, where d is the MS to BTS distance in meters, and c is the speed of light (c = 3 · 108 m.s-1). Without any compensation of this delay, the bursts transmitted by two different MSs, in the same TDMA frame on two consecutive slots, could interfere with one another.
Let us take the example of one MS situated 25 km away from the BTS, transmitting on time slot 0 of a given channel frequency. Another MS is located, say, 1 km away from the BTS, and transmitting on time slot 1 of that same frequency. The second MS transmission will experience a very short delay (around 3.33 μs), but the burst on time slot 0, from MS 1, will be received by the BTS 83.33 μs after it has been transmitted. This means that at the BTS receiver, the burst on time slot 0 will interfere with the beginning of the burst of time slot 1, for a period of about 80 μs. This example is represented in Figure 1.15.
Figure 1.15: Propagation delay difference between two MSs transmitting to the same BTS.
In order to cope with this problem, the network manages a parameter for each mobile called the TA. This parameter represents the transmission delay between the BTS and the MS, added to the delay for the return link.
The estimation of the delay is performed by the BTS upon reception of an AB on the RACH. As described in Section 1.5.5, this burst is characterized by a longer guard period (68.25-bit duration or 252 μs) to allow burst transmission from a mobile that does not know the TA at the first access. The received AB allows the BTS to estimate the delay by means of a correlation with the training sequence.
The TA value, between 0 and 63 symbol periods (i.e., between 0 and 232.615 μs by steps of 48/13 μs), is transmitted on the AGCH. It allows the MS to advance its time base, so that the burst received at the BTS arrives exactly three time slots after the BTS transmit burst, as shown in Figure 1.16. A distance of 35 km between the MS and the BTS is therefore possible. (The 232.675 μs allows to compensatefor a distance of around 70 km, including the forward and return links.)
Figure 1.16: Correction of MS transmission timing to compensate for propagation delay.
After this first propagation delay estimation, the BTS continuously monitors the delay of the NBs sent by from the MS on the other logical channels. If the delay changes by more than one symbol period, a new value of the TA is signaled to the MS on the SACCH.
MS Power Control
As explained in Section 1.5.6.1, the MS can vary its transmit output power from a maximum defined by its power class, by steps of 2 dB. During a communication, the MS and BTS measure the received signal strength and quality (based on the bit error ratio) and pass the information to the BSC, which ultimately decides if and when the power level should be changed. A command is then sent to the MS on the SACCH.
Power control is a difficult mechanism to implement, since there is a possibility of instability. This arises from having MS in cochannel cells, alternatively increasing their power in response to increased cochannel interference. Suppose that mobile A increases its power because the corresponding BTS receives a cochannel interference caused by mobile B, in another cell. Then the BTS receiving the signal from mobile B might request mobile B to increase its power, and so forth. This is the reason why some coordination is required at the BSC level.
Note that for an access request on the RACH, the MS uses the maximum power level defined by the parameter MS_TXPWR_MAX_CCH broadcast by the network.
Frequency Hopping
The radio environment depends on the radio frequency. In order to avoid important differences in the quality of the channels, a feature called slow frequency hopping (FH) was introduced. The slow FH changes the frequency with every TDMA frame, which also has the effect of reducing the cochannel interference. This capability is optionally used by the operator, and is not necessarily implemented in all the cells of the network, but it must be supported by all the MSs. The main advantage of FH is to provide diversity on one transmission link (especially to increase the efficiency of coding and interleaving for slowly moving MSs) and also to average the quality on all the communications through interference diversity.
The principle of slow FH is that every mobile transmits its time slots according to a sequence of frequencies that it derives from an algorithm. The FH sequences are orthogonal inside one cell (i.e., no collisions occur between communications of the same cell) and independent from one cell to a cochannel cell (i.e., a cell using the same set of RF channels or cell allocation). The hopping sequence is derived by the mobile from parameters broadcast at the channel assignment, namely, the mobile allocation (set of N frequencies on which to hop), the hopping sequence number (HSN) of the cell (which allows different sequences on cochannel cells), and the index offset (to distinguish the different mobiles of the cell using the same mobile allocation) or mobile allocation index offset (MAIO). Based on these parameters and on the FN, the MS knows which frequency to hop in each TDMA frame.
It must be noted that the basic physical channel supporting the BCCH does not hop.
1.5.7 MS Cell Synchronization Procedure
In synchronizing to a cell, the MS first searches for the FB on the FACCH. This allows a first timing synchronization, but most of all, it allows the mobile to adjust its oscillator to be synchronized in the frequency domain with the BTS. This is possible because, as described in Section 1.5.6, the fixed sequence of the FB has been chosen so that the modulating bit sequence at the GMSK modulator input is constant. This results in a continuous π/2 phase rotation, which in the frequency domain is equivalent to an unmodulated carrier with a +1 625/24 kHz frequency offset, above the nominal carrier frequency. Once the MS has identified the FB, it uses this property to estimate its frequency drift with regard to the BTS.
In the TDMA frame immediately following the occurrence of an FB, on the same carrier frequency, the SB is transmitted over the air interface, on the SCH. This burst is identified by the MS with its extended 64-bit training sequence code. The MS received samples of the SB, correlated with the known training sequence, allow for the timing of the mobile to be adjusted to the base station with good precision. At this point, the MS and the BTS are synchronized in the time domain, except that the propagation delay between them is not compensated. This is performed with the TA scheme, as discussed in Section 1.5.6.3, when the MS sends an AB on the RACH.
The decoding of the SB enables a logical synchronization of the mobile to the cell, since it gives the elements to estimate the TDMA FN (see Section 1.5.3.1), which allows the MS to determine the position of the SCH in the hyperframe. The SCH also contains the BTS identity code (BSIC): these 6 bits (before channel coding) consist of the PLMN color code with range 0 to 7 and of the BS color code with range 0 to 7 (3 bits each).
1.5.8 Summary of MS Operations in Idle Mode
In idle mode, as opposed to dedicated mode, the mobile has no channel of its own. It is required to
Synchronize in time and frequency to a given cell, selected as the best suitable cell with regard to a set of criteria (based on the beacon received power at the MS). This is termed "camping onto" a cell. This process of evaluating different cells and choosing the best suitable one is called cell selection, or reselection if it is performed again, due to the degradation of the link quality with the previously selected cell. The MS during idle mode continuously measures the radio link quality of the serving cell and the surrounding cells, so that cell reselection criteria are evaluated periodically.
Listen to possible incoming calls from the network. The notification of an incoming call is usually referred to as paging.
1.5.8.1 Selection of the PLMN
When the mobile is switched on, the first operation that it performs is identification or selection of a PLMN. Most of the time, the PLMN will be the home PLMN (i.e., the network to which the user has subscribed). In such a case, no selection is needed, as information about the network is stored in the SIM card. If it is not the case, because the user is traveling in a different area, the MS will scan all the frequencies in order to detect the surrounding beacon channels (detection of FB and SB, as described in Section 1.5.7). The MS is then able to decode the PLMN identifiers, and either choose the first PLMN in the priority ordered list of the SIM card, or ask the user which PLMN is preferred among all the detected PLMNs. The selection is then stored, in order to be used at the next terminal switch on. In any case, the user can explicitly ask for a given PLMN selection.
1.5.8.2 Principles of Cell Selection and Reselection
Once the PLMN is selected, the MS must select a cell. Two scenarios are possible:
The beacon channel frequencies are stored in the MS, because it has already performed a selection in the previous terminal activity. In this case, the MS will perform measurement on these frequencies, to determine which cell is the most suitable with regard to certain criteria. Once the best cell is selected, the MS performs registration and "camps on" this cell. Note that if the stored frequency list of beacon carriers is not detected by the MS, it will perform a PLMN selection as described above.
It is the first time the PLMN is accessed. The carriers of the system are all scanned, in order to detect the beacon channels, and the received signal strength of these channels is added in an ordered list. Once this is achieved, the cell selection can be performed, as in the previous case. In order to speed up the process, a list of the RF channels containing BCCH carriers of the same PLMN is broadcast in the system information messages.
When an MS is camping on a cell, it can receive paging blocks on the PCH, or initiate call setup for outgoing calls by sending an AB on the RACH. It still regularly monitors the signal level on the surrounding beacon carriers, and evaluates the reselection criteria. The reselection is triggered if one of the following events occurs:
The path loss criterion parameter C1 indicates that the path loss to the cell has become too high.
There is a downlink signaling failure (i.e., the success rate of the MS in decoding signaling blocks drops too low).
The cell camped on has become barred (this means that the operator decides not to allow MSs to camp on this cell).
There is a better cell, in terms of the path loss criterion C2 in the same registration area.
A random access attempt is still unsuccessful after a given number of repetitions, specified by a broadcast parameter.
The criteria for cell selection and reselection (path loss criterion C1 and reselection criterion C2) are based on the measurements performed by the MS on the BCCH frequency. (As stated earlier, the beacon frequency is transmitted with its maximum output power by the BTS.) Details on these criteria for cell selection are given in Chapter 5.
1.5.8.3 Monitoring of Paging Blocks
We discussed the mapping of the CCCH on the 51 multiframe in Section 1.5.3.5, and in particular the case of the PCH. This logical channel is used to convey paging blocks on the downlink. These blocks are used to notify the MS of an incoming call. In order to conserve the MS's power, a PCH is divided into subchannels, each corresponding to a group of MSs. Each MS will then only "listen" to its subchannel and will stay in the sleep mode during the other subchannels of the PCH. This is called the discontinuous reception (DRX) mode. The mobile knows in which group it belongs by determining the parameter CCCH_GROUP. It is estimated with an algorithm, which inputs are the mobile IMSI and the parameter BS_CC_CHANS, broadcast on the BCCH. This parameter defines the number of basic physical channels supporting CCCH.
Mobiles in a specific CCCH_GROUP will listen for paging messages and make random accesses only on the specific CCCH to which the CCCH_GROUP belongs. This algorithm is detailed in Chapter 4. Note that the MS is not authorized to use the DRX mode of operation while performing the cell-selection algorithm.
1.5.9 Measurements Performed by MS During Communication
When assigned a TCH or SDCCH, during the time slots that are not used for these channels and for the associated SACCH, the MS performs measurements on all the adjacent BCCH frequencies. These measurements are then sent to the network by means of the SACCH, and are interpreted by the NSS for the power control and handover procedures. Measurements are performed in each TDMA frame, (see Figure 1.17) and are referred to as monitoring, which consists of estimating the receive signal strength on a given frequency. The list of frequencies to be monitored is broadcast on the BCCH, by means of the BCCH allocation (BA) list, which contains up to 32 frequencies. The frequencies are monitored one after the other, and the measured samples are averaged prior to the reporting to the network, on an uplink SACCH block, under form of a value called RXLEV. The MS therefore measures the received signal level from surrounding cells by tuning and listening to their BCCH carriers. This can be achieved without interbase station synchronization. The measurements are reported at every reporting period.
Figure 1.17: Monitoring during a TDMA frame.
For a TCH/FS, the reporting period duration is 104 TDMA frames (480 ms).
It is essential that the MS identify which surrounding BSS is being measured in order to ensure reliable handover. Because of frequency reuse with small cluster sizes, the BCCH carrier frequency may not be sufficient to uniquely identify a surrounding cell. The cell in which the MS is situated may have more than one surrounding cell using the same BCCH frequency. It is therefore necessary for the MS to synchronize to (using the method explained in Section 1.5.7) and demodulate surrounding BCCH carriers to identify the BSIC in the SB. In order to do so, the MS uses the idle frames. These frames are termed "search" frames. Note that a window of nine consecutive slots is needed to find time slot 0 on the BCCH frequency (remember that time slot 0 carries the SCH and FCCH), since the beacon channels are not necessarily synchronized with one another. One important characteristic to notice is that the SCH and FCCH are mapped onto the 51 multi-frame, and that the idle frame of the mobile during communication is occurs in on the 26 multiframe. Since 26 and 51 are mutually prime numbers, this means a search frame will be available every 26 modulo 51 frame on the beacon channel.
For instance, let us imagine that an idle frame occurs in the frame 0 of the 51 multiframe. The next idle frames will be programmed on frames 26, 1, 27, 2, and so on. Therefore, after a certain number of search frames, the MS will necessarily decode an FB and an SB.
Another measured parameter during a TCH or SDCCH is the RXQUAL, which represents an indication of the quality of the received link, in terms of BER. For each channel, the measured received signal quality is averaged on that channel over the reporting period of length one SACCH multiframe defined above.
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