general packet radio service (gprs): architecture, interfaces, and deployment

WIRELESS COMMUNICATIONS AND MOBILE COMPUTINGWirel. Commun. Mob. Comput.2001;1:77–92

REVIEW

General Packet Radio Service (GPRS): architecture,interfaces, and deployment‡

Yi-Bing Lin*,†Department of Computer Science &Information EngineeringNational Chiao Tung UniversityHsinchu, TaiwanRepublic of China

Herman C.-H. RaoAT&T Research Labs334 Szcchaun Rd, Sec. 1PanchiaTaipei Hsien, TaiwanRepublic of China

Imrich ChlamtacCenter for Advanced TelecommunicationsSystems and ServicesThe University of Texas at DallasP.O. Box 830688RichardsonTX 75083-0688, U.S.A.

Summary

This article provides an overview of General PacketRadio Service (GPRS). GPRS reuses the existingGSM infrastructure to provide end-to-endpacket-switched services. Benefits of GPRS includeefficient radio usage, fast set-up/access time andhigh bandwidth with multiple timeslots. GPRS alsoprovides a smooth path for GSM evolution to thethird generation mobile network. Specifically, athird generation network can continue to utilize theGPRS IP backbone network. We describe the GPRSnetwork nodes and the interfaces among these nodes.Deployment issues for GPRS are also elaborated.Copyright 2001 John Wiley & Sons, Ltd.

KEY WORDSpacket control unitgateway GPRS support nodegeneral packet radio serviceGPRS tunneling protocolGSMserving GPRS support node

ŁCorrespondence to: Yi-Bing Lin, Department of Computer Science & Information Engineering, National Chiao TungUniversity, Hsinchu, Taiwan, Republic of China.†E-mail: [email protected]‡This article is a concise version of Chapter 18 [Chapter title: General Packet Radio Service (GPRS)] of the bookWirelessand Mobile Network Architecturespublished by John Wiley & Sons, Inc., New York (ISBN 0-471-39492-0).Contract/grant sponsor: MOE Program of Excellence Research; contract/grant number: 89-E-FA04-4

Copyright 2001 John Wiley & Sons, Ltd.

78 Y.-B. LIN, H. C.-H. RAO AND I. CHLAMTAC

1. Introduction

In early 2000, only a small portion of GSM sub-scribers used data services because existing GSMsystems do not support easy access, high data rateand attractive prices. GSM operators must offer betterservices to stimulate the demand. The solution is Gen-eral Packet Radio Service (GPRS). GPRS reuses theexisting GSM infrastructure to provide end-to-endpacket-switched services. GPRS standardization wasinitiated by ETSI/SMG in 1994. The main set ofGPRS specifications was approved by SMG #25 in1997, and was complete in 1999. GPRS productswere developed in 1999, and service deployment hasbeen in progress [1]. GPRS core network has alsobeen developed with IS-136 TDMA systems [2], andis anticipated to evolve as the core network for thethird generation mobile systems.

To accommodate GPRS, new radio channels aredefined, and the allocation of these channels is flexi-ble: one to eight timeslots can be allocated to a useror several active users can share a single timeslot,where the uplink and the downlink are allocated sepa-rately. The radio resources can be shared dynamicallybetween speech and data services as a function oftraffic load and operator preference. Various radiochannel coding schemes are specified to allow bitrates from 9 Kb s�1 to more than 150 Kb s�1 peruser. GPRS fast reservation is designed to start packettransmission within 0.5–1 s. GPRS security function-ality is equivalent to the existing GSM security, wherea ciphering algorithm is optimized for packet datatransmission. By allowing information to be deliveredmore quickly and efficiently, GPRS is a relatively

inexpensive mobile data service compared to ShortMessage Service (SMS) and Circuit Switched Data.

An excellent survey on GPRS can be found in Ref-erence [3], which elaborated on GPRS architectureand air interface. In the remainder of this article, weprovide a GPRS overview complementary to Refer-ence [3]. We only briefly describe the air interfacethat has been covered completely in Reference [3]and discuss the air interface for enhanced GPRS. Wewill emphasize more on the individual protocols inthe signaling plane, the industrial solutions of theGPRS network components, GPRS charging, and thedevelopment efforts from GSM to GPRS. We firstdescribe the GPRS architecture, then elaborate onGPRS nodes and the interfaces among these nodes.We also describe the GPRS solutions and develop-ment of several GPRS vendors. We assume that thereader is familiar with GSM.

2. GPRS Architecture

Figure 1 shows the GPRS network nodes and thecorresponding interfaces, where SMS-related compo-nents and the Equipment Identity Register are notshown. In this architecture, MS, BSS, Mobile Switch-ing Center/Visitor Location Register (MSC/VLR) andHome Location Register (HLR) in the existing GSMnetwork are modified. For example, the HLR isenhanced with GPRS subscriber information. Two-new network nodes are introduced in GPRS. TheServing GPRS Support Node (SGSN) is the GPRSequivalent to the MSC. The Gateway GPRS SupportNode (GGSN) provides interworking with external

Fig. 1. GPRS architecture. BSS: Base Station System; GGSN: Gateway GPRS Support Node; HLR: Home LocationRegister; MS: Mobile Station; MSC: Mobile Switching Center; SGSN: Serving GPRS Support Node; PDN: Packet Data

Network; VLR: Visitor Location Register.

Copyright 2001 John Wiley & Sons, Ltd. Wirel. Commun. Mob. Comput.2001;1:77–92

GENERAL PACKET RADIO SERVICE (GPRS) 79

packet-switched networks, and is connected withSGSNs via an IP-based GPRS backbone network.

The MS and the BSS communicates through theUm interface. The BSS and the SGSN are connectedby the Gb interface with Frame Relay. Within thesame GPRS network, SGSNs/GGSNs are connectedthrough the Gn Interface. When SGSN and GGSNare in different GPRS networks, they are intercon-nected via the Gp interface. The GGSN connects tothe external networks through the Gi interface. TheMSC/VLR communicates with the BSS using theexisting GSM A interface, and with the SGSN usingthe Gs interface. The HLR connects to SGSN with theGr interface and to GGSN with Gc interfaces. Both Grand Gc follow GSM Mobile Application Part (MAP)protocol defined in GSM 09.02 [4]. The HLR andthe VLR are connected through the existing GSM Dinterface. Interfaces A, Gs, Gr, Gc, and D are usedfor signaling without involving user data transmissionin GPRS. Note that the A interface is used for bothsignaling and voice transmission in GSM. InterfacesUm, Gb, Gn, Gp and Gi are used for both signalingand transmission in GPRS.

GPRS transmission plane is shown in Figure 2,which consists of a layered protocol structure for userinformation transfer and the associated control proce-dures (e.g., flow control, error detection, error correc-tion and error recovery). Figure 3 shows the GPRSsignaling plane that consists of protocols for con-trol and support of the transmission plane functions.Among these protocols, the GPRS specific protocolsinclude SNDCP, LLC, RLC, MAC, BSSGP, BSSAP+and GTP. PLL, RFL, GMM/SM and MAP are GSMprotocols (note that GMM/SM and MAP also containGPRS-specific protocols). TCAP, SCCP and MTP are

SS7 layers. Other protocols are standard data proto-cols. Details of the layers of the transmission and thesignaling planes will be elaborated in Section 4.

The GPRS relay functions in Figure 2 merit furtherdiscussion. In the BSS, this function relays Logi-cal Link Control (LLC) Packet Data Units (PDUs)between the Um and Gb interfaces. In the SGSN, thisfunction relays (PDP) PDUs between the Gb and Gninterfaces. The Gb/Gn relay function adds sequencenumbers to PDP PDUs received from the Sub-Network Dependent Convergence Protocol (SNDCP)and from the Gi reference point, respectively. Totransparently transport PDP PDUs between externalnetworks and MSs, the PDP PDUs are encapsu-lated and decapsulated for routing as described inSection 4.3.

Before we elaborate on GPRS mechanisms, we firstintroduce three GPRS terms: Mobility Management(MM) context, Packet Data Protocol (PDP) contextand Quality of Service (QoS) profile. The MM con-text consists of the MM state and other MM-relatedinformation stored in MS and SGSN as described inSections 3.1 and 3.3.1.

The MM states specify the MM activities of anMS. The state is IDLE if the MS is not attachedto the GPRS mobility management. The state isSTANDBY if the MS is attached to GPRS mobilitymanagement but has not obtained detailed locationinformation. The state is READY if the locationinformation for the MS has been identified on celllevel. Note that a GPRS MS can be IMSI and/orGPRS attached. The IMSI attach is the same as thatfor a GSM MS. In GPRS attach procedure, both theMS and the SGSN are moved to the READY state,an MM context is created in each of MS and SGSN,

Fig. 2. GPRS transmission plane. BSSGP: BSS GPRS Protocol; FR: Frame Relay; GTP: GPRS Tunneling Protocol; LLC:Logical Link Protocol; MAC: Medium Access Control; NS: Network Service; RFL: Radio Physical Layer; PLL: Physical

Link Layer; RLC: Radio Link Control; SNDCP: SubNetwork Dependent Convergence; UDP: User Datagram Protocol;TCP: Transmission Control Protocol.



Fig. 3. GPRS signaling plane. BSSAPC: Base Station System Application PartC; GMM: GPRS Mobility Management;MAP: Mobile Application Part; MTP: Message Transfer Part; SCCP: Signaling Connection Control Part; SM: Session

Management; TCAP: Transaction Capabilities Application Part.

and authentication/ciphering may be performed. AtGPRS attach, a logical link is established betweenMS and SGSN.

The PDP contexts are stored in the MS, HLR,SGSN and GGSN as described in Sections 3.1, 3.3and 3.4, which contain mapping and routing informa-tion for packet transmission between MS and GGSN.For each GPRS communication of an MS, a PDPcontext is created to characterize the session. Afterthe PDP context activation, the MS is known to theGGSN and communication to external networks ispossible. An MS may have several activated PDPcontexts if the terminal supports several IP addresses.When the MS is detached from GPRS, all PDPcontexts are deactivated. A PDP context can be inone of the two PDP states: ACTIVE or INACTIVE.An MS in STANDBY or READY MM state mayactivate a PDP context and moves its PDP state fromINACTIVE to ACTIVE. The ACTIVE PDP contextbecomes INACTIVE when the PDP context is deacti-vated. A QoS profile is maintained in the PDP contextto indicate radio and network resources required fordata transmission. The QoS attributes include:

ž Precedence class specifies three transmission pri-ority levels. During congestion, the packets withlower priorities are discarded.

ž Delay class specifies four delay levels. In 128-octettransfer, for example, the expected transfer speedsfor delay classes 1–3 are less than 0.5 s, 5 s, and50 s, respectively. Delay class 4 supports best-effort transmission without specifying the transferspeed constraints.ž Reliability class defines residual error rates for data

loss, out-of-sequence delivery and corrupted data.There are five reliability classes. Reliability class1 supports acknowledgement for GTP mode, LLCframe mode and RLC block mode, and the LLCdata are protected. Reliability class 5 does notsupport acknowledgment and the LLC data are notprotected.ž Peak throughput class specifies the expected maxi-

mum data transmission rate. There are nine classesranging from 8 Kb s�1 to 2048 Kb s�1.ž Mean throughput class specifies the average data

transmission rate. There are 19 classes rangingfrom the best effort to 111 Kb s�1.

We will elaborate on the usage of the MM/PDPcontexts and QoS profile in subsequent sections.

3. GPRS Network Nodes

This section discusses the GPRS network nodes: MS,BSS, SGSN, GGSN, HLR, and MSC/VLR. We also



describe the solutions for these nodes provisioned byequipment suppliers.

3.1. Mobile station

A GPRS MS consists of Mobile Terminal (MT) andTerminal Equipment (TE). An MT communicates withthe BSS over the air. The MT is equipped with soft-ware for GPRS functionality, which establishes linksto SGSN. A TE can be a computer attached to the MT.Existing GSM MS does not support GPRS. For exam-ple, GPRS MS utilizes Automatic Re-transmission(ARQ) at the data link layer to re-transmit the errorframes. In GSM, no re-transmission is provided in aGSM voice channel. With multiple timeslots, GPRSmay provide high transmission rate. GSM only offerssingle timeslot for voice.

Three MS operation modes are introduced in GPRS07.60 [5]. Class A mode of operation allows simulta-neous circuit-switched and packet-switched services.Duplexer is required to support this mode. ClassB mode of operation provides automatic choice ofcircuit-switched or packet-switched service, but onlyone at a time. A Class B MS involved in packettransfer can receive a page for circuit-switched activ-ity. In this case, the MS suspends the data transferfor the duration of the circuit-switched connectionand afterwards resumes the data transfer elaboratedin Section 4.4. Class C mode of operation supportspacket-switched data only. Neither Class B nor ClassC mode requires duplexer.

The MSs access the GPRS services that are withor without GPRS-aware Subscriber Identity Modules(SIMs). An MS maintains MM and PDP contexts tosupport GPRS mobility management. Some of theMM context fields stored in GPRS-aware SIM arelisted next:

ž International Mobile Subscriber Identity (IMSI)that uniquely identifies the MS. IMSI is used asthe key to search the databases in VLR, HLR, andGSN.ž Packet Temporary Mobile Subscriber Identity (P-

TMSI) which is the GPRS equivalent of TMSIin GSM.ž Address of the routing area where the MS resides.

A routing area is a subset of a location area definedin GSM.ž Current ciphering key Kc and its Ciphering Key

Sequence Number (CKSN).

If the SIM is not GPRS-aware then the abovefields are stored in the mobile equipment. The mobile

equipment also stores several non-SIM-related fieldslisted next (a partial list):

ž MM state (either IDLE, STANDBY or READY asdescribed in Section 2).ž Identity of the cell where the MS resides.ž Ciphering algorithm that is defined in GSM

01.61 [6].ž Radio access classmark for the radio capabilities

(e.g., multiple timeslot capability and power class)and SGSN classmark for network-related capabili-ties (e.g., ciphering capability).

For data routing purpose, the MS maintains PDPcontexts including (a partial list):

ž PDP type (either X.25, PPP, or IP).ž PDP address (e.g., an X.121 address).ž PDP state (either ACTIVE or INACTIVE as des-

cribed in Section 2).ž Dynamic-Address-Allowed that specifies whether

the MS is allowed to use a dynamic address.ž Requested and negotiated QoS profiles described

in Section 2.

GSM chips for GPRS MS are available. For exam-ple, SMARTi (PMB6250) [7] is a single chip GSMmulti-band transceiver that supports multi-timeslotsdata, which can be used to support GPRS MS.Another example is Lucent’s Sceptre 3 system-on-a-chip solution that enables full GPRS to 115.2 Kb s�1.By integrating digital signal processing, micropro-cessing, read only memory, random access memory,and laser programmability on one chip, such tech-nologies provide flexible architecture for future MSdesign.

3.2. Base station system

To accommodate GPRS, the Base Transceiver Station(BTS) and the Base Station Controller (BSC) in theBSS are modified, and a new component Packet Con-trol Unit (PCU) is introduced. The BTS is modifiedto support new GPRS channel coding schemes. TheBSC forwards circuit-switched calls to the MSC, andpacket-switched data (through PCU) to the SGSN.Every BSC can only connect to one SGSN. The Gbinterface described in Section 4.2 is implemented toaccommodate functions such as paging and mobil-ity management for GPRS. The BSS should alsomanage GPRS-related radio resources such as allo-cation of packet data traffic channels in cells. As will



be described in Section 4.1, the Um radio interfaceis modified to support GPRS features. To supportGPRS traffic, the transmission capacity of the BSSis increased through standard upgrade process.

The PCU is viewed as the equivalence of aTranscoder and Rate Adaptor Unit (TRAU) for thepacket data services. The PCU is either located locallywith the BTS or remotely located in the BSC orthe SGSN. Most vendors follow the remote PCUoptions so that no hardware modifications to theBTS/BSC are required. In the remote options, exist-ing Abis interface between the BTS and the BSC isreused, and the GPRS data/signaling messages aretransferred in modified TRAU frames with a fixedlength of 320 bits (20 ms). The PCU is responsiblefor the Medium Access Control and Radio Link Con-trol layer functions such as packet segmentation andreassembly, packet data traffic channel management(e.g., access control, scheduling, and ARQ), and radiochannel management (e.g., power control, congestioncontrol, broadcast control information).

In Nortel’s solution, existing GSM BTS and sec-ond generation BSC 12 000 are reused with upgradedsoftware. The PCU and Gb functions are implementedin PCUSN based on Nortel’s GSM Passport Plat-form. The PCUSN concentration capability is up to12 BSCs per cabinet. Similarly, in Alcatel’s solution[8], both BTS and BSC are reused with upgraded soft-ware. The PCU and Gb functions are implemented inA935 Multi-Functional Server (MFS) that can con-nect up to 22 BSSs and supports 480 activated GPRSradio channels per BSC. Ericsson follows one PCUper BSC design. A PCU can cover 512 BTSs and upto 4096 GPRS radio channels (1750 channels practi-cally).

3.3. GPRS support node

Two kinds of GSNs are introduced in GPRS: Serv-ing GSN (SGSN) and Gateway GSN (GGSN). Thefunctionality of SGSN and GGSN can be combinedin a physical node (e.g., Symmetry’s UWS-GSN [9]and Ericsson’s Combined SGSN/GGSN or CGSN[10]) or distributed in separated nodes (e.g., Nor-tel, Motorola/Cisco, and Alcatel solutions). A GSNis typically implemented at multiple processor systemplatform with hardware redundancy and robust soft-ware infrastructure that support uninterrupted opera-tion [10]. A vendor may develop SGN with variouscapacities. Ericsson, for example, has developed twoGSN models. Model GSN-25 is a small-capacity GSNused to enable fast deployment of the GPRS service.

With the same capability as GSN-25, Model GSN-100 provides larger capacity in terms of throughputand number of attached users.

3.3.1. Serving GPRS support node

The role of SGSN is equivalent to that of MSC/VLRin the current GSM network. SGSN connects BSSto GSGN, which provides ciphering, mobility man-agement (e.g., inter-SGSN routing area update andinter-PLMN roaming), charging and statistics collec-tion (i.e., support of billing records). To provide ser-vices to a GPRS MS, the SGSN establishes an MMcontext containing mobility and security informationfor the MS. At PDP context activation, the SGSNestablishes a PDP context that will be used to routedata between the MS and the GGSN. SGSN main-tains MM/PDP context information when the MS isin STANDBY or READY MM states. For an MS, theSGSN MM context includes:

ž IMSI, P-TMSI and Mobile Station ISDN Number(MSISDN).ž MM state.ž Routing area identity and cell identity.ž Address of the VLR currently serving the MS.ž IP address of the new SGSN where the buffered

packets should be forwarded to.ž Authentication and ciphering parameters.ž Current ciphering key Kc and the selected cipher-

ing algorithm.ž MS radio access capabilities and GPRS network

access capabilities.ž MNRG (Mobile Station Not Reachable for GPRS

flag) indicating whether activity from the MSshould be reported to the HLR.ž NGAF (Non-GPRS Alert Flag) indicating whether

activity from the MS should be reported to theVLR.ž PPF (Paging Proceed Flag) indicating whether pag-

ing for GPRS and non-GPRS services can be ini-tiated.

Each MM context associates with zero or more ofthe following PDP contexts (a partial list):

ž PDP context identifier, PDP type, PDP address, andPDP state.ž Access point name to the external data network.ž Subscribed, requested, and negotiated QoS profiles.ž IP address of the GGSN currently used by the

activated PDP context.



ž Identifier of the charging records generated bySGSN and GGSN.

Most vendors developed SGSN based on existingmultiple processor system products where the controlprocessors are configured with hot standby redun-dancy. Lucent’s solution supports 40 000 attachedusers and 4000 simultaneous active GPRS data ses-sions. For Nortel Passport 8380G and SymmetryUWS-GMS, the number of GPRS users that can beattached to an SGSN is 50 000, the number of PDPcontext activation is 20 000, the number of SS7 linksto VLR/HLR is eight, the number of E1 links forGb interface is 10, and the throughput is 20 Mb s�1.In Alcatel’s SGSN solution, the number of attachedusers is 52 000–96 000, the number of SS7 signalinglinks is eight, the number of E1 links to Gb interfaceis 16, and the throughput is 16–48 Mb s�1. It is clearthat Alcatel’s SGSN has larger capacity than Norteland Symmetry’s SGSNs. This design is owing to thefact that in Alcatel’s solution, every GPRS network issupported by one SGSN. Thus large-capacity SGSNis required.

3.3.2. Gateway GPRS support node

GGSN is mainly provisioned by router, which sup-ports traditional gateway functionality such as publish-ing subscriber addresses, mapping addresses, routingand tunneling packets, screening messages, and count-ing packets. A GGSN may contain DNS functions tomap routing area identifiers with serving SGSNs, andDynamic Host Configuration Protocol (DHCP) func-tions to allocate dynamic IP addresses to MSs.

The GGSN maintains activated PDP context fortunneling the packets of the attached MS to thecorresponding SGSN. The information items include(a partial list):

ž IMSI.ž PDP type, PDP address.ž Dynamic address indication.ž QoS profile negotiated.ž IP address of the SGSN currently serving this MS.ž Access point name of the external data network.ž Charging id.ž MNRG flag indicating whether the MS is marked

as not reachable for GPRS at the HLR.

Note that the GGSN does not need to recordsubscribed and requested QoS profiles. Both of themare maintained in the SGSN.

Most suppliers use existing router platforms to pro-vide this function. For example, Alcatel GGSN isdeveloped based on Cisco 7200 series router, Nokia’sGGSN is developed based on its commercial IProuting platform, and Nortel GGSN is developedbased on Bay CES 4500. Existing GGSN imple-mentations typically support 5000–48 000 simultane-ous data tunnels, and 25 000–48 000 simultaneouslyattached data users. The switching capability is, forexample, 20 Mb s�1 in the Alcatel solution.

3.4. HLR and VLR

To accommodate GPRS subscription and routinginformation, new fields in the MS record areintroduced in HLR, which are accessed by SGSNand GGSN using the IMSI as the index key. Thesefields are used to map an MS to one or more GGSNs,update the SGSN of the MS at attach and detach,and store a fixed IP address and QoS profile fora transmission path. In the HLR, the GSN-relatedinformation includes:

ž IMSI and MSISDN.ž SS7 address of the SGSN that serves the MS.ž IP address of the SGSN that serves the MS.ž MS Purged for GPRS flag that indicates if the

MM and PDP contexts of the MS are deleted fromthe SGSN.ž MNRG that indicates if the MS is not reachable

for GPRS service.ž GGSN-list that provides a GGSN IP address list to

be contacted for MS activity when MNRG is set.

The PDP context-related information includes PDPcontext identifier, PDP type, PDP address, QoS pro-file subscribed, and the access point to the externalpacket data network.

In MSC/VLR, a new filed SGSN number is addedto indicate the SGSN currently serving the MS. TheMSC/VLR may contact SGSN to request locationinformation or paging for voice calls. It also performssignaling coordination for class B mobile through theGs interface and suspends/resumes GPRS activitiesthrough the A and Gb interfaces.

4. GPRS Interfaces

This section describes the GPRS interfaces shown inFigure 1: Um, Gb, Gn, Gp, Gs and Gi.



4.1. Um interface

Um is the radio interface between MS and BTS.GPRS radio technology is based on GSM radio archi-tecture, which introduces new logical channel struc-ture to control signaling and traffic flow in the Umradio interface. In this subsection, we elaborate onthe Um channel structure, Um protocol layers andthe enhanced Um for GPRS.

4.1.1. Radio channel structure

The physical channel dedicated to packet data trafficis called a Packet Data Channel (PDCH). Differentpacket data logical channels can occur on the samePDCH. The logical channels are described below.

GPRSutilizesPacketDataTrafficChannel (PDTCH)for data transfer. High spectral efficiency is achievedthrough timeslot sharing where multiple users mayshare one PDTCH. Furthermore, a user may simulta-neously occupy multiple PDTCHs.

Several Packet Common Control Channels (PCC-CHs) are introduced in GPRS. The Packet RandomAccess Channel is the only uplink PCCCH, whichis sent from the MS to the BTS to initiate uplinktransfer for data or signaling. The following downlinkPCCCHs are sent from the BTS to the MS.

ž Packet Paging Channel pages an MS for bothcircuit-switched and packet data services.ž Packet Access Grant Channel is used in the packet

transfer establishment phase for resource assign-ment.ž Packet Notification Channel is used to send a Point-

To-Multipoint Multicast (PTM-M) notification to agroup of MSs prior to a PTM-M packet transfer.ž Packet Broadcast Control Channel, GPRS

(PBCCH) broadcasts system information specificfor packet data. If PBCCH is not allocated, thepacket data specific system information is broad-cast on the existing GSM BCCH channel.

Several Packet Dedicated Control Channels aredefined in GPRS:

ž Packet Associated Control Channel (PACCH)conveys signaling information such as powercontrol, resource assignment and reassignmentinformation. The PACCH shares resources withPDTCHs. An MS currently involved in packettransfer can be paged for circuit-switched serviceson PACCH.

ž Packet Timing Advance Control Channel in theUplink direction (PTCCH/U) is used by an MS totransmit a random access burst. With this infor-mation, the BSS estimates timing advance. Inthe downlink, the BSS uses PTCCH/D to trans-mit timing advance information updates to sev-eral MSs.

The GPRS common control signaling is conveyedon PCCCH. If PCCCH is not allocated, the existingGSM Common Control Channel is used. Two conceptsare employed for GPRS channel management. In themaster–slave concept, a master PDCH accommodatesPCCCHs to carry all necessary control signaling forinitiating packet transfer. Other PDCHs serve as slavesfor user data transfer (PDTCH) and for dedicated sig-naling. In the capacity-on-demand concept, PDCHsare dynamically allocated based on the actual amountof packet transfers. Also, the number of allocatedPDCHs in a cell can be increased or decreased accord-ing to traffic change. GPRS performs fast release ofPDCH to share the pool of radio resources for bothpacket and circuit-switched services.

4.1.2. Um protocol layers

As shown in Figure 2, Um protocol layers includePhysical RF, Physical Link Layer (PLL), and RadioLink Control/Medium Access Control (RLC/MAC)layers. Physical RF layer performs modulation/demo-dulation received from/sent to PLL. PLL providesservices for information transfer over a physical chan-nel including data unit framing, data coding, and thedetection and correction of physical medium trans-mission errors.

The RLC/MAC layer provides services for infor-mation transfer over the GPRS physical layer. Thesefunctions include backward error correction proce-dures enabled by the selective retransmission of erro-neous blocks. RLC is responsible for block segmen-tation and reassembly, buffering and re-transmissionwith backward error correction. MAC is responsiblefor channel access (scheduling, queueing, contentionresolution), PDCH multiplexing, and power control.

Four GPRS coding schemes CS1, CS2, CS3 andCS4 are defined, whose characteristics are listed inTable I. Initially only CS1 and CS2 will be developed.The table indicates that the GPRS channel codingschemes increase data rate at the cost of decreas-ing protection (correction capability). These codingschemes also reduce worst link budget and cell range.For GSM, the worst link budget is 142.5 dB and the



Table I. Characteristics of the GPRS coding schemes.

Coding scheme CS1 CS2 CS3 CS4

User data rate 9.05 Kb s�1 13.4 Kb s�1 15.6 Kb s�1 21.4 Kb s�1

Correction capability Highest NoneWorst link budget 135 dB 133 dB 131 dB 128.5 dBMaximum cell range 450 m 390 m 350 m 290 m

maximum cell range is 730 m. On the other hand,the GPRS worst link budget is 135–128.5 dB andthe maximum cell range is 450–290 m.

4.1.3. Enhanced data rates for GSM evolution

With GSM radio technology, GPRS only provideslimited data capacity. To increase the GSM data rate,Enhanced Data Rates for GSM Evolution (EDGE)was introduced under the same GSM frame struc-ture. EDGE standardization has been in progress,and the EDGE Link Quality Control (LQC) schemewas agreed in the first quarter of 1999. Based onEDGE, Enhanced GPRS (EGPRS) provides user datarates two to three times higher than GPRS (upto 470 Kb s�1 for indoor and 144 Kb s�1 for out-door) and its spectrum efficiency is two-to-six timeshigher than GPRS (up to 0.7 b s�1 Hz�1 per site).These goals are achieved by utilizing 8-PSK modu-lation and adapting user rate to channel quality. Tobe compatible with GMSK used in GSM and IS-136, EDGE accommodates both GMSK and 8-PSK.EDGE follows the same TDMA format, carrier spac-ing (200 KHz), symbol rate (271 Ksymb s�1), burstformat and training sequences used in GSM. EGPRSalso utilizes same spectrum and same amount of ISIfrom modulation as GSM does. Thus EDGE can reuseGSM sites and frequency plan.

At the physical layer, blind detection of modulationis used to avoid signaling before changing modula-tion and parallel equalizations. In EDGE transmit-ter design, the Power Amplifiers (PAs) utilize lin-ear modulation that is more complex than constantenvelop modulation used in GSM. For the same PA,maximum average power for 8-PSK is lower than thatfor GMSK. The Peak to Average Ratio (PAR) for 8-PSK is 3.2dB while the PAR for GMSK is 0dB. Theequalizer in EDGE receiver design is different fromthat in GSM. For GMSK, 5 tap equalizing windowis enough. For 8-PSK, 7-8 tap equalizing window isnecessary. For the same ISI, 8-PSK is more sensitiveto interference than GMSK is.

EDGE LQC combines link adaption and incre-mental redundancy. Link adaption selects modulation

and coding schemes (MCSs) based on link qualitymeasurements. In the link adaption procedure, the MSfirst measures the downlink performance, and reportsthe results to the BSS. The BSS measures the uplinkperformance, chooses the uplink and downlink MCSsbased on the collected measurements. The BSS sendsan uplink MCS command to the MS. Based on thecommand, the MS selects appropriate MCS for uplinktransmission.

Incremental redundancy increases robustness forretransmission in EDGE. Specifically, the more oftena data block is retransmitted, the more overhead(increments to acknowledgement and signaling) isadded to the data block. Incremental redundancyrelies little on measurements, which is mandatory forEDGE MSs.

To implement EDGE, the BTS in the existing GSMstandard should be modified [11]. Specifically, thetransceiver units should be enhanced to accommo-date new power amplifiers in the transmitter andnew equalizers in the receiver. Furthermore, trans-mission capacity between BTS and BSC must beincreased. EGPRS supports maximum bit rate up to8ð 59.2 Kb s�1 at the RLC/MAC layer. This highdata rate is achieved under good propagation con-ditions. Thus EDGE and EGPRS is appropriate forindoor environments with pico- and micro-cells.

4.2. Gb interface

The Gb interface connects the BSS and the SGSN,which allows many users to be multiplexed overthe same physical resource. Unlike GSM A interfacewhere the resources of a circuit-switched connec-tion are dedicated to a user throughout the wholesession, GPRS Gb interface only allocates resourcesto a user during the periods when data are actuallydelivered. As shown in Figure 2, the Gb interface pro-tocol layers (from the highest to the lowest) includeLCC, SNDCP, Base Station System GPRS Protocol(BSSGP), Network Service (NS) Layer, Link Layer2, and Physical Layer. The Gb link layer 2 estab-lishes Frame Relay virtual circuits between SGSNand BSS. On these virtual circuits, the NS transports



BSSGP Packet Data Units (PDUs) between a BSSand an SGSN. On the BSS side, the Relay functionis required to provide buffering and parameter map-ping between the RLC/MAC (for the Um interfacebetween the BSS and the MS) and the BSSGP (forthe Gb interface between the BSS and the SGSN).LLC is a sublayer of layer 2. The purpose of LLCis to convey information between layer-3 entities inthe MS and SGSN. LLC provides one or more logi-cal link connections with sequence control (to main-tain the sequential order of frames across a logicallink connection), flow control, detection of trans-mission, format and operational errors on a logicallink connection and recovery from detected transmis-sion, format, and operational errors. LLC maintainsa ciphered data link between an MS and an SGSN,which is independent of the underlying radio inter-face protocols. This connection is maintained as theMS moves between cells served by the same SGSN.When the MS moves to a new SGSN, the existingconnection is released and a new logical connectionis established with the new SGSN. The LLC layersupports several QoS delay classes with differenttransfer delay characteristics described in Section 2.The LLC layer supports transmission with both unac-knowledged and acknowledged modes. LLC providesservice to the GPRS Mobility Management (GMM)protocol in the signaling plane (see Figure 3). GPRSMobility Management (GMM) uses the services ofthe LLC layer to transfer messages between the MSand the SGSN. GMM includes functions such asattach and authentication, and transport of sessionmanagement messages for functions such as PDPcontext activation and deactivation. In the transmis-sion plane in Figure 2, the SNDCP above the LLCperforms multiplexing of data coming from the dif-ferent sources to be sent across LLC. It also per-forms segmentation and reassembly, compression ofredundant protocol information and user data. GPRSsupports several network layer protocols providingprotocol transparency for the users of the service.Introduction of new network layer protocols to betransferred over GPRS will be possible without anychanges to GPRS. SNDCP ensures that all functionsrelated to transfer of network layer PDU are carriedout in a transparent way by the GPRS network enti-ties. Based on compression techniques, the SNDCPalso provides functions that help to improve channelefficiency. The set of protocol entities above SNDCPconsists of commonly used network protocols. Theyall use the same SNDCP entity, which then performsmultiplexing of data coming from different sources

to be sent using the service provided by the LLClayer.

4.2.1. Network service

The NS Layer delivers encapsulated packets betweenSGSN and BSS that are connected directly by a framerelay link or indirectly through cascading links in aframe relay network. Each physical frame relay linksupports one or more Network Service Virtual Links(NS-VLs). The NS-VLs are connected to constructan end-to-end virtual path between the BSS andSGSN. This path is called Network Service VirtualConnection (NS-VC). The NS manages NS-VCs withoperations such as

ž Blocking (when a NS-VC is not available)ž Unblocking (when a NS-VC becomes available

again)ž Reseting (when, e.g., a new NS-VC is set up)ž Testing (to check that end-to-end communication

exists between peer NS entities on a given NS-VC)

The NS also performs load sharing to distributethe packet traffic among the unblocked NS-VCs ofthe same BVC. The NS utilizes UNITDATA serviceprimitive to transfer packets, CONGESTION serviceprimitive to report congestion, and STATUS primitiveto inform the NS user of events such as change in theavailable transmission capabilities.

A group of NS-VCs supports a BSSGP Vir-tual Connection (BVC) used to transport packetsbetween NS users. BVCs provide communicationpaths between BSSGP entities. Each BVC is usedin the transport of BSSGP PDUs between peer point-to-point functional entities, peer point-to-multipointfunctional entities and peer signaling functional enti-ties. For every BVC, QoS profile and the MS identifi-cation are used to create queues and contexts in boththe SGSN and the BSS. The flow control mechanismis exercised based on these queues and contexts to beelaborated next.

4.2.2. BSS GPRS protocol

BSSGP provides the radio-related QoS and routinginformation required to transmit user data betweena BSS and an SGSN. It also enables the SGSN andBSS to operate node management control functions. Ifan SGSN simultaneously communicates with multipleBSSs, then there is one BSSGP protocol machine inthe SGSN corresponding to each of the BSSs. Several



service models are supported by BSSGP: BSSGP/RL,GMM, and NM.

BSSGP service model in the SGSN controls thetransfer of LLC frames across the Gb interface. Relay(RL) service model in the BSS controls the transferof LLC frames between the RLC/MAC function andBSSGP. Examples of the RL/BSSGP service primi-tives provided by the BSSGP are DL-UNITDATA andUL- UNITDATA. An UL-UNITDATA PDU is deliv-ered from the BSS to the SGSN. A DL-UNITDATAPDU is delivered from the SGSN to the BSS. Besidesthe user information (an LLC packet), the PDUcontains RLC/MAC-related information such as MSradio access capability, QoS profile, and the PDU life-time. If the PDU is queued in the BSS longer than thePDU lifetime, then it is discarded at the BSS. Basedon the QoS profile, a layer-3 signaling PDU may betransmitted over the Um interface with higher protec-tion compared with a data PDU. The PDU is eitheracknowledged using RLC/MAC ARQ functionalityor unacknowledged using RLC/MAC unitdata func-tionality.

GPRS Mobility Management (GMM) service mo-del performs mobility management functions betweenan SGSN and a BSS. Examples of the GMM ser-vice primitives provided by the BSSGP are PAG-ING, SUSPEND, and RESUME. The PAGING pro-cedure is invoked by SGSN to inform the BSS forpacket-switched (if initiated by the SGSN) or circuit-switched (if initiated by an MSC/VLR) transmissions.In this procedure, the SGSN will instruct the BSS topage one or more cells. To suspend a GPRS service,an MS initiates the SUSPEND procedure by request-ing the BSS to send a SUSPEND PDU to the SGSN.On the other hand, when an MS resumes its GPRSservice, the BSS instructs the MS to update the rout-ing area. Alternatively, the BSS may send a RESUMEPDU to the SGSN to indicate that the MS should beresumed for GPRS service.

Network Management (NM) service model handlesfunctions related to Gb interface and BSS/SGSN nodemanagement. Examples of the NM service primi-tives provided by the BSSGP are FLOW-CONTROL-BVC and FLOW-CONTROL-MS used to control thedownlink loading of the BSS per BVC and per MS,respectively. No flow control is performed in theuplink direction because the packet sending rate atthe MS is anticipated to be lower than the packetprocessing rate on the network side. There is a down-link buffer for each BVC. If a PDU in the down-link is not transferred to the MS before its lifetimeexpires, the PDU is deleted from the BVC downlink

buffer and this action is reported to the SGSN. Tocontrol downlink transmission at the SGSN, a flowcontrol message FLOW-CONTROL-BVC (FLOW-CONTROL-MS) with parameters such as the bucketsize and the bucket leak rate for a given BVC (MS)are sent from the BSS to the SGSN.

4.3. Gn and Gp interfaces

Both Gn and Gp interfaces utilize the GPRS Tun-neling Protocol (GTP). GTP tunnels user data andsignaling messages between GSNs. In the Gn inter-face, the GSNs are within the same GPRS network.On the other hand, Gp involves the GSNs in differ-ent GPRS networks. Basically Gp is the same as Gnexcept that extra security functionality is required forinter-network communications over the Gp interface.The security functionality is based on mutual agree-ments between operators. With GTP, an SGSN maycommunicate with multiple GGSNs and a GGSN mayconnect to many SGSNs. MS, BSS, MSC/VLR, andHLR are unaware of the existence of GTP.

In the transmission plane in Figure 2, GTP issupported by Transmission Control Protocol (TCP)for connection-oriented transmission and is supportedby User Datagram Protocol (UDP) for connection-less transmission. GTP transmission uses a tunnelingmechanism to carry user data packets. A tunnel isa two-way point-to-point path. Tunneling transfersencapsulated data between GSNs from the point ofencapsulation to the point of decapsulation. GTP fol-lows out-of-band signaling where the signaling pathis logically separated from the data tunnels. In thesignaling plane, GTP is supported by UDP, whichenables the SGSN to provide GPRS network accessfor an MS.

More than one path may be established betweentwo GSNs either in the same network or in differentnetworks, and a path may be used by one or moretunnels. A GTP tunnel is defined by the associatedPDP contexts in two GSN nodes and is identifiedwith a Tunnel ID. GTP performs (1) path manage-ment, (2) tunnel management, (3) location manage-ment, and (4) mobility management. In path man-agement, the GSNs exchange the EchoRequest andResponse message pair to quickly detect failuresoccurring in the path.

Location management is required if a GGSN doesnot support SS7 MAP for communications with HLR.In this case, the interaction between the GGSN andthe HLR is done indirectly through a specific GSNthat performs GTP-MAP protocol conversion.



Tunnel management and mobility management aredescribed in the following subsections.

4.3.1. GTP tunnel management

GTP tunnel management creates, updates, and deletestunnels. Some of the tunnel management messagesare described below.

To activate a PDP context for an MS, CreatePDPContextRequest and Response message pair is exch-anged between SGSN and GGSN. The SGSN selectsthe IP address of a GGSN from a list maintainedin the Domain Name Service (DNS) server, andsends the CreatePDP ContextRequest message tothat GGSN. If the GGSN does not respond, the SGSNcontinues to send the request message to the nextGGSN in the DNS list until the request message isaccepted or the list is exhausted. Upon receipt ofthis message, the GGSN creates a PDP context entryfor the MS and generates a charging identification.The new entry allows the GGSN to route and chargepackets between the SGSN and the external PDPnetwork. Based on the capabilities and current loadof the GGSN, the negotiated QoS may be morerestricted than the requested QoS specified by SGSN.The GGSN returns a CreatePDPContextResponsemessage to the SGSN. The message indicates whetherTCP or UDP will be used to transport user data. Notethat only one path is used between any given GSN-pair to tunnel end user traffic in both directions.

To update the routing area information or a PDPcontext, an SGSN sends the UpdatePDPContextRequest message to a GGSN. The message includesthe new SGSN address, tunnel identification andQoS negotiated. Upon receipt of this message, theGGSN may reject the update request if QoS nego-tiated received from the SGSN is not compati-ble (for example, the reliability class is insufficientto support the PDP type). The GGSN may alsorestrict QoS negotiated based on its capabilities andthe current load. If the GGSN returns a negativeUpdatePDP ContextResponse message, the SGSNdeactivates the PDP context. GTP may also use thismessage pair to redistribute PDP contexts for loadbalancing.

To detach an MS or to deactivate a PDP context, anSGSN and a GGSN exchange the DeletePDPCon-text Request and Response message pair. This actiondeactivates an activated PDP context. To activate aPDP context, the GGSN sends the PDUNotificationRequest message to the SGSN indicated by the HLR,i.e., the SGSN serving the MS. When receiving this

message, the SGSN is responsible for requesting theMS to activate the indicated PDP context, and repliesPDU Notification Response message to the GGSN.

4.3.2. GTP mobility management

GTP mobility management supports functions suchas GPRS attach, GPRS routing area update and acti-vation of PDP contexts. Some of them are describednext.

When an MS moves from the old SGSN to thenew SGSN, it sends P-TMSI to the new SGSN. Thenew SGSN then exchanges the IdentificationRequestand Response message pair with the old SGSN toobtain the IMSI of the MS. The IMSI is used toretrieve the MS record in the HLR. The Identifica-tion Request and Response message pair is equivalentto the MAPSEND IDENTIFICATION message pairin GSM.

SGSNContextRequest message is sent from thenew SGSN to the old SGSN to obtain the MM and allactive PDP contexts of an MS. The message includesold routing area identification, old P-TMSI, newSGSN Address and so on. Upon receipt of the mes-sage, the old SGSN sends the requested contexts (MMcontext, PDP contexts, and LLC Acknowledgement)to the new SGSN by the SGSNContextResponsemessage.

After the new SGSN receives these contexts, itacknowledges the old SGSN by sending the SGSNContextAcknowledge message. This message imp-lies that the new SGSN is ready to receive thedata packets for the corresponding MS. Then theold SGSN starts to forward user data packets to thenew SGSN.

4.4. Gs interface

The Gs interface connects the databases inthe MSC/VLR and the SGSN, which does notinvolve user data transmission. Base Station SystemApplication PartC (BSSAPC) implements thefunctionality for the Gs interface. As shown inFigure 3, BSSAPC utilizes SS7 Signaling ConnectionControl Part [12] as the lower layer protocol.

The BSSAPC procedures coordinate the locationinformation of MSs that are both IMSI and GPRSattached. It is also used to convey some GSM pro-cedures via the SGSN. The paging, suspend, resume,and location update procedures are described here.Other Gs procedures can be found in Reference [13].

The paging procedure for the MSC/VLR-based ser-vices allows the VLR to utilizes GPRS to page a



Class A or a B MS that is simultaneously IMSI andGPRS attached. By doing so, the system needs notrepeat paging of an MS for both GSM and GPRS ser-vices, and the overall paging load on the radio inter-face is expected to be reduced. The VLR initiates thisprocedure by sending the GPRSPAGING message tothe SGSN. This message has a structure similar to thePAGING message delivered on the A interface. Whenthe SGSN receives the GPRSPAGING message, itchecks if the MS is GPRS attached and is knownby the SGSN. If so, the SGSN sends the Gb PAG-ING message to the BSS as described in Section 4.2.The SGSN then forwards the paging result back tothe VLR. If the MS does not respond, the VLR orthe BSS should retransmit the paging message. TheSGSN is not responsible for retransmission of the GbPAGING message.

To perform the circuit-switched activity for aClass B MS that is simultaneously IMSI and GPRSattached, the VLR uses the Suspend procedure toinform the SGSN to suspend the GPRS activities ofthe MS. When the MS sends the circuit-switchedactivity request to the VLR, the VLR waits for aperiod T6-1, then it sends a SUSPEND messageto the corresponding SGSN. Upon receipt of theSUSPEND message, the SGSN suspends the GPRSactivity of the MS. If the MS sends GPRS signal-ing message to the SGSN within a period T6-3, theSGSN accepts the message, and may inform the VLRof the situation. On the other hand, if the MS isunknown to the SGSN, then the SGSN returns aSUSPENDFAILURE message to the VLR. If theVLR does not receive a SUSPENDFAILURE mes-sage within a period T6-2, then the suspend action isconsidered successful. Otherwise, the action fails.

Upon release of the circuit-switched activity for aClass B MS that is simultaneously IMSI and GPRSattached, the VLR sends a RESUME message tothe SGSN to resume the GPRS activities for theMS. If the VLR does not receive a response mes-sage from the SGSN within a period T7, then theVLR repeats sending the RESUME message upto N7 times. In the normal situation, the SGSNresumes the GPRS activities for the MS and returns aRESUMEACKNOWLEDGE message to the VLR.

In GPRS location update, an MS sends a LocationUpdating Request to the SGSN through the Um andGb interfaces. If this request is accepted by the SGSN,the SGSN sends a GPRSLOCATION UPDATINGRequest message to the VLR. The VLR checks if theIMSI is known. If not, the VLR retrieves the MMcontext of the MS from the HLR. If the SGSN does

not hear from the VLR within a period T8-1 or if theVLR replies a GPRSLOCATION UPDATING Re-ject message, the SGSN informs the MS that loca-tion update fails. If the update is successful, theVLR returns a GPRSLOCATION UPDATING Ac-cept message to the SGSN, and the SGSN sendsa positive Location Updating Response message tothe MS.

4.5. Gi interface

GPRS interworks with Public Switched Data Network(PSDN) and Packet Data Network (PDN) through theGi Interface. In the Gi interface, GGSN serves as theaccess point of the GPRS network to the external datanetwork. The interworking models to PSDN includeX.25 and X.75, and the interworking models to PDNinclude IP and Point-to-Point Protocol (PPP).

Both X.75 and X.25 are supported for GPRS inter-working with PSDN. In these models, an MS isassigned an X.121 address to be identified by PSDN.This X.121 address is either permanently allocated bythe PSDN operator following the PSDN numberingplan or dynamically assigned by the GPRS network atPDP context activation. In the latter case, the GPRSnetwork maintains a free pool of X.121 addresses tobe allocated to the MSs.

GPRS interworks with intranets or the Internetbased on the Internet Protocol (IP), either IPv4 orIPv6. Viewing a GGSN as a normal IP router, theexternal IP network considers the GPRS network asjust another IP network. The GPRS operator maymaintain a firewall to restrict the usage of IP appli-cations. In the firewall, GPRS may perform screen-ing specified by the operator or the subscriber. Thisfeature is important to avoid unsolicited mobile ter-minated connection (such as junk mails being sent tothe MSs).

Either the external IP network or the GPRS net-work (specifically, the GGSN) manages a DNS. TheIP address of an MS can be statically assignedat the subscription or dynamically assigned at thePDP context activation. If the IP address is dynam-ically allocated, the address assignment procedureis performed by the GGSN or an external DHCPserver.

GPRS may transparently access the Internet andnon-transparently access an intranet and/or ISP. Inthe transparent Internet access, the IP address of anMS is allocated from the GPRS operator’s address-ing space. This address is used for packet forward-ing between the Internet and the GGSN and among



the GGSNs. The MS needs not send any authen-tication request at PDP context activation and theGGSN needs not involve in user authentication andauthorization. Domain name services are provided byGPRS in this case.

In non-transparent access to an Intranet or ISP, theIP address of an MS is allocated from the Intranet/ISPaddress space where the address allocation serverbelongs to the Intranet/ISP. At PDP context activa-tion, the MS must be authenticated by the Intranet/ISPusing a security protocol agreed upon by the GPRSoperator and the Intranet/ISP. Domain name servicesare provided by the Intranet/ISP.

GPRS may provide connection to intranet/ISPthrough the transparent Internet access where a bearerservice is provided to tunnel a private Intranet usingprotocols such as IPsec. In this case, GPRS involvesin the security processes by using IPsec security orheader authentication for user authentication and forthe confidentiality of user data.

5. Evolving from GSM to GPRS

By reusing GSM infrastructure, most GPRS imple-mentation costs of the existing GSM nodes are soft-ware related. As illustrated in Table II, major hard-ware impact on the GSM network is limited to theaddition of a PCU-model to the BSC and the intro-duction of two new node types: SGSN and GGSN.GPRS software upgrade can be performed efficiently.In many vendor solutions, GPRS software can beremotely downloaded to BTSs, so that no site vis-its are needed. In the MS development, a majorchallenge is to resolve power consumption issue. Tosupport data-related features (e.g., multiple times-lots transmission), GPRS MS consumes much morepower than a standard GSM MS.

The GPRS protocols have a good characteristicin that each layer can be reused to support fea-tures in different GPRS nodes. The GPRS stack is

Table II. GSM network elements impact by GPRS.

Element Software Hardware

MS Upgraded required Upgraded requiredBTS Upgraded required No changeBSC Upgraded required PCU interfaceTRAU No change No changeMSC/VLR Upgraded required No changeHLR Upgraded required No changeSGSN New NewGGSN New New

designed so that multiple copies of every layer canbe distributed across multiple processors. Thus, it cansmoothly scale the network capacity to handle largevolumes of data. For example Reference [14], thesame SNDCP code can support both SGSN and MS.In other words, the SGSN code can be reused in theMS. GPRS protocol products can be implemented ingeneral computer languages. For example, Trilliumdelivers its GPRS protocol software in standard Cprogramming language. Lucent/Optimary GmbH pro-vides GPRS protocol stack customization with manmachine interface, which is designed to be modularand portable.

GPRS is typically deployed in two phases. Phase 1deployment implements basic GPRS featuresincluding:

ž Standard packet services delivery, i.e., point-to-point packet bearer service.ž Support for CS-1 and CS-2 channel coding

schemes.ž GPRS internal network interfaces such as Gn, Gb,

Gp, and Gs.ž Flexible radio resource allocation, i.e., multiple

users per timeslot and multiple timeslots per user.ž Support for Classes B and C MSs.ž GPRS charging, e.g., packet-based billing and

QoS-based billing.ž GSM-based services such as SMS over GPRS.ž IP and X.25 interfaces to packet data network.ž Static and dynamic IP address allocation.ž Anonymous access.ž Security, i.e., authentication and ciphering.

In Phase 1 development, most vendors cover partsof, or all above, features with some variations. Forexample, Nortel’s Phase 1 development also consid-ers advanced virtual private network features. Alca-tel’s Phase 1 deployment covers the entire networkwith a limited investment such as BSS softwareupdate, a single A935 MFS per MSC site, and oneSGSN and one GGSN for the entire network. In Erics-son’s Phase 1 development, the applications are basedon IP, X.25 and SMS.

GPRS Phase 2 development includes the followingfeatures:

ž Enhanced QoS support in GPRS.ž Unstructured octet stream GPRS PDP type.ž Access to ISPs and Intranets.ž GPRS prepaid.ž GPRS advice of charge.



ž Group call.ž Point to multi-point services.

In Nortel’s Phase 2 development, the capacitiesof SGSN and GGSN will be significantly increased.Inter-SGSN handoff is implemented and inter-GPRSnetwork roaming is supported. In Ericsson’s Phase 2development, the enhanced applications will includePTM services, multicast and group call. In Alcatel’sPhase 2 development, 935 MFS will be smoothlyupgraded, the SGSN and GGSN capacity will beincreased, and security will be enhanced.

6. Summary

This article provided an overview of GPRS. Wedescribed the GPRS architecture and the related inter-faces. Based on our discussion, benefits of GPRSinclude efficient radio usage, fast set-up/access time,and high bandwidth with multiple timeslots. GPRSreuses GSM infrastructure so that both circuit-switchedand packet-switched services co-exist under one sub-scription. GPRS also provides a smooth path to evolvefrom GSM to the third generation mobile network.Specifically, a third generation network can continueto utilize the GPRS IP backbone network. However,GPRS has its limitations. For example, based on exist-ing GSM technology, radio resources for GPRS in acell is limited and the GPRS data rate is probably toolow for many data applications. This problem can beresolved by introducing the EDGE or the third gener-ation radio technologies.

Although GPRS is an emerging technology drivenby the equipment suppliers instead of the push fromthe customers, it has generated strong interest amongservice providers. Owing to the explosive growth ofInternet applications, it is believed that data accessis an important trend for mobile services. An obvi-ous advantage of GPRS is that no dial-up modemconnection is required to access data. After PDPcontext activation, the MS becomes an ‘always-on’device that facilitates instant connections. This featureis required for mobile computing where informationshould be sent or received immediately as the needarises. Several potential GPRS applications have beenidentified [8]:

ž Vertical applications for specific data communica-tion requirements of companies: since a mobilephone can be an always-on device, a GPRS MScan always be connected to deliver information.Examples include traffic management (e.g., fleet

management, vehicle tracking, vehicle control andguidance) and monitoring automation (e.g., teleme-try and security).

ž Horizontal applications for individual users: inthis type of application, the mobile phone is amedia device so that the moving users can receiveservices such as entertainment (e.g., games andmusic), location information (e.g., restaurants, cin-ema, hotels, and parking), and so on.

GPRS also allows commerce transactions when thecustomers are in motion. Examples include on-linebanking transactions, stock transactions, gambling,ticketing (e.g., for cinema, flights, and trains), on-line shopping, and so on.

We note that the high immediacy feature of GPRSis essential for commerce transactions where it isunacceptable to keep the customers waiting.

For a moving user, GPRS is ideal for immediatehandling of functions such as quick access toPIM information, E-mail, or note taking, whicheliminates the need of accessing to the desktopor laptop’s full-featured applications and the broadrange of peripherals and services.

Many GPRS contracts are awarded in Asia, Aus-tralia, and Europe. The reader is referred to Refer-ence [15] for the details. For an in-depth reading ofGPRS documents, the reader is recommended to startwith the general descriptions [3, 16, 17] and thencontinue on specific topics such as Um [6, 18–20],Gb [21, 22], [23] Gs [13] Gn/Gp [24], Gi [25], andcharging [26]. Also, vendors’ view of GPRS can befound on several Web sites [7, 9, 14, 27]. For theGPRS suppliers’ market share, the reader is referredto Reference [15].

Acknowledgment

Lin’s work was sponsored in part by MOE Programof Excellence Research under contract 89-E-FA04-4, CCL/ITRI under contract 2-10B, FarEastone NSCThe Lee and MTI Center for Networking Research,NCTU.

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20. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Mobile Station (MS)—Serving GPRSSupport Node (SGSN) Subnetwork Dependent ConvergenceProtocol (SNDCP).Technical Report Recommendation GSM04.65, Version 6.2.1, ETSI, 1997.

21. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Base Station System (BSS)—ServingGPRS Support Node (SGSN) Interface; Gb Interface Layer 1.Technical Report Recommendation GSM 08.14 Version 6.0.0,ETSI, 1997.

22. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Base Station System (BSS)—ServingGPRS Support Node (SGSN) interfaceen–Network Service.Technical Report Recommendation GSM 08.16, Version 7.1.0,ETSI, 1998.

23. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Base Station System (BSS)—ServingGPRS Support Node (SGSN) BSS GPRS Protocol (BSSGP).Technical Report Recommendation GSM 08.18, Version 7.0.0,ETSI, 1998.

24. ETSI/TC. GPRS Tunnelling Protocol (GTP) across the Gn andGp Interface.Technical Report Recommendation, GSM 09.60,Version 7.0.0 (Phase 2C), ETSI, 1998.

25. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Interworking between the Public LandMobile Network (PLMN) Supporting GPRS and Packet DataNetworks (PDN). Technical Report Recommendation GSM09.61, Version 7.1.0, ETSI, 1998.

26. ETSI/TC. European Digital Cellular Telecommunications Sys-tem (Phase 2C); GPRS Charging.Technical Report Recom-mendation GSM 12.15, ETSI, 1998.

27. Nokia. http://www.nokia.com/networks/17/gprs/gprs1.html.


general packet radio service (gprs): architecture, interfaces, and deployment

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