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.