ECPE 6504: Wireless Networks and MobileComputing

Individual Project Report

Overview and Comparisonof the Architecture and Protocols of the

Global System for MobileCommunications

and theGeneral Packet Radio Service

George C. Hadjichristofi04/25/00

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Table of Contents

1 Abstract……………………………………………………………………………….. ……. 3

2 Introduction…………………………………………………………………………………. 3

3 Overview of Wireless Wide Area Network………………………………………………. 43.1 Global System for Mobile Communications (GSM) ……………………….. 43.2 General Packet Radio Service (GPRS)……………………………………….. 5

4 Architecture Comparison …………………………………………………………………… 64.1 Global System for Mobile Communications (GSM) ……………………………64.2 General Packet Radio Service (GPRS)………………………………………….9

5 Protocol Comparison ………………………………………………………………………..125.1 Physical Layer……………………………………………………………………..135.2 Data Link Layer………………………….……………………………………… ..135.3 Network Layer..………………………….……………………………………….. 145.2 Signaling………………………………….………………………………………. 14

6 Conclusion……………….…………………………………………………………………...16

7 Works cited…………….………………………………………………………………….….17

8 Appendix ……………….…………………………………………………………………...19

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1 Abstract

The goal of this paper is to be an in-depth tutorial which will offer a brief overview and acritical comparison of the architecture and protocol stack of the Global System for MobileCommunications(GSM) and the General packet radio service(GPRS).

The paper starts by describing the need for the creation of both Systems. It then gives abrief overview of each system. The architecture comparison is done by first describingone system and then analyzing the second system while stating any differences and/orsimilarities. This way redundancy is avoided, as there is no need to restate the particularcharacteristics of each system. The protocol stack comparison is carried out by showingthe protocol stack for each system, and then stating the major differences between eachone.

The results of this research showed that GPRS is an extension of GSM. Additionalnodes and interfaces were needed to implement the extended services of packetswitching required by GPRS. Since additional nodes were used, existing protocol wereenhanced to cover for higher data rates, while at the same time preventing higher errorrates.

2 Introduction

Analog cellular telephone systems were experiencing rapid growth in Europe during theearly 1980s. Each country developed its own system, but it was incompatible witheveryone else's in equipment and operation. The mobile equipment were limited tooperation within national boundaries, and there was also a very limited market for eachtype of equipment. The Europeans realized this early on, and in 1982 the Conference ofEuropean Posts and Telegraphs (CEPT) formed a study group to study and develop apan-European public land mobile system[12].The proposed system called the Global System for Mobile communications (GSM) hadto meet certain criteria:• Good subjective speech quality• Low terminal and service cost• Support for international roaming• Support for range of new services and facilities• Spectral efficiency• Ability to support handheld terminals• ISDN compatibility

In 1989, GSM responsibility was transferred to the European TelecommunicationStandards Institute (ETSI).

Currently GSM is one of the world’s most widely deployed and fastest growing digitalcellular standard. It is one of the most successful digital mobile telecommunicationsystems. There are over 250 million GSM subscribers world-wide -two thirds of theworld’s digital mobile population - and this figure is increasing by nearly four new usersper second. GSM covers every continent, being the technology of choice for 357operators in 142 countries. The industry predicts that there will be nearly 600 millionGSM customers by 2003.

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After GSM’s successful entrance in the mobile communication mass-market, the ETSIhas been working to improve its performance and to offer new services. GSMdevelopments via GPRS provide a better use of the radio resources with regards toincrease capacity in number of subscribers, and consequently to reduce tariffs.

GPRS is a standard for wireless communications which runs at speeds up to 150 kilobitsper second, compared with current GSM’s systems' 9.6 kbs. GPRS is an efficient use oflimited bandwidth and is particularly suited for sending and receiving small bursts ofdata, such as e-mail and Web browsing, as well as large volumes of data. The GPRSservices reflect the GSM services with an exception that the GPRS has a highertransmission rate which makes a good impact in the most of the existing services andcreates the opportunity for the introduction of new services as operators.

This paper first introduces the two systems and then states and compares thearchitecture of the two systems. It then goes over the protocols of both systems anddescribes any differences and/or similarities.

3 Overview

3.1 GSM

GSM is a wireless platform that uses radio frequencies, and this way users can be fullymobile, and do wireless data computing anywhere, without worrying about adapters,telephone jacks, cables, etc. The unique roaming features of GSM allow cellularsubscribers to use their services in any GSM service area in the world in which theirprovider has a roaming agreement. GSM-enabled phones have a "smart card" insidecalled the Subscriber Identity Module (SIM). The SIM card is personalized to the user. Itidentifies the user’s account to the network and provides authentication, which allowsappropriate billing.

GSM has been designed for speech services. It uses circuit switched transmission,reserving one radio channel for the user’s traffic. It also uses cells which enables it toreuse different frequencies.

GSM, provides almost complete coverage in western Europe, and growing coverage inthe Americas, Asia and elsewhere. GSM networks presently operate in three differentfrequency ranges. These are:

GSM 900 (also called GSM) - operates in the 900 MHz frequency range and is the mostcommon in Europe and the world.GSM 1800 (also called PCN (Personal Communication Network), and DCS 1800) -operates in the 1800 MHz frequency range and is found in a rapidly-increasing numberof countries including France, Germany, UK, and Russia.GSM 1900 (also called PCS (Personal Communication Services), PCS 1900,and DCS1900) - the only frequency used in the United States and Canada for GSM.

GSM standard circuit is a digital data bearer service offering 9.6kb/s. This datatransmission in these networks is regarded as too slow and often too expensive formany applications. The cost is the total time that the user occupied that channel

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eventhough he was using the channel all the time. The performance of services such asInternet Applications in a cellular environment is typically characterized by the lowavailable bandwidth, and an inefficient use of the rare air link capacity. Furthermore, longconnection setup delay is a problem for bursty services requiring occasional datatransfers.

3.2 GPRS

GSM’s use of circuit switched systems meant that in the case of bursty traffic, the trafficchannel will be idle for some time. As the demand for data services increased, GPRSwas developed to support packet switching . The work on the GPRS specification beganin 1994 as a part of GSM phase 2+ specification. GPRS is a separate packet datanetwork within GSM which provides a packet base platform both for the data transferand signaling. GPRS is compatible with the GSM architecture. Voice and GPRS servicescoexist in the same environment with the minimum changes in the system[8].

GPRS focused strongly on the development of a service, which overcomes thesedrawbacks of a mobile Internet Access. It allows allow reduced connection setup-times,supports existing packet oriented protocols like X.25 and IP, and provides an optimizedusage of radio resources.

The main idea is to allocate resources depending on the GPRS demand. This featureoperates in a capacity-on-demand mode. The capacity-on-demand concept has beenintroduced in order to keep compatibility with the existing GSM circuit-switchedresources. Resources for GPRS may be dynamically allocated depending on how manyusers require them with a given quality of service and depending also on how manyresources are available at the moment. The operator can decide whether to permanentlydedicate some physical resources for GPRS. Load supervision is carried out in the MAClayer to monitor the load of the GPRS physical resources, and it's the function that willallow increasing or decreasing the number of allocated resources according to theexisting demand. The operator has also the choice to dedicate temporarily physicalresources for GPRS as long as no other higher-priority GSM services require them[8].

Since GPRS is packet oriented it enables volume based charging in contrast to GSMlike charging of online time. It therefore allows users to stay constantly online while onlypaying for the occasional data transfer. Another important factor is the Quality of Service(QoS) offered by these services. The QoS can be negotiated when starting the sessionand can be renegotiated if it is required. The QoS agreed between the user and thenetwork can be used to charge the service.In addition, GPRS increases the capacity of the system and reduces the idle periods ofthe radio channels. This is done by allowing for multiple users per physical channel andusing a channel only when it is needed, and releasing it immediately after thetransmission is complete.

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4. Architecture ComparisonThis section analyzes the GSM architecture first, as it was the base upon which GPRSwas built. GPRS architecture is then described while at the same time anydifferences/similarities are stated.

4.1 GSMA GSM network is composed of several subsystems whose functions and interfaces arespecified. Figure 1 shows the layout of a generic GSM network. These are the:1) base station subsystem(BSS)2) mobile station(MS)3) network and switching subsystem(NSS)4) operations subsystem(OSS)5) operations and maintenance Center(OMC)

Base Station SubsystemThe Base Station Subsystem controls the radio link with the Mobile Station. It is mainlycomposed of the Base Transceiver Station (BTS) and the Base Station Controller (BSC).The BSC-to-BTS link is called the Abis interface which is cable or an optical fiberinterface, and allows operation between components made by different suppliers.The BTS is made up of the antenna and the radio transceivers. A BTS houses the radiotranceivers that define a cell, and handles the radio-link protocols with the MobileStation. BSC manages the radio resources and handles radio-channel setup, frequencyhopping, and handovers among a number of different cells.

BASESTATIONSUBSYSTEM

BTS

BSC

BTS

HLR

BSC

EIR

VLR

AUC

MOBILESTATION

UM

ABIS

A

M E

MSC

SIM

PLMN

OMC

TRAU

TRAU

VLR

oss

PSPDN

GIWU

PSTN

GMSC

Figure 1. GSM Arcitecture [1][3][12]

NETWORKAND SWITCHINGSUBSYSTEM

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The BSC connection between the MS and the Mobile service Switching Center (MSC) isdone through the Translation and Adaptation Unit(TRAU). Usually, 20 to 30 BTS will becontrolled by one BSC.

Mobile Station

The MS, both hand-held (or portables) and traditional mobiles, is carried by thesubscriber. The MS is made up of the mobile equipment(ME), also known as theterminal, and a smart card known as the Subscriber Identity Module (SIM).The mobile equipment is uniquely identified by the International Mobile EquipmentIdentity (IMEI).

The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identifythe subscriber to the system, a secret key for authentication, and other information. GSMsubscriber information are not programmed on the mobile equipment but rather stored ina computer chip on the SIM card. The SIM card can be inserted into another GSMterminal, enabling the user to receive calls at that terminal, make calls from that terminal,and receive other subscribed services. This way personal mobility is provided as theuser can have access to subscribed services irrespective of a specific terminals.The SIM card provides subscriber account protection against unauthorized use by apassword or personal identity number. The SIM provides assistance with voice and dataencryption by deriving the variables for the encryption process.

Network Subsystem

The network subsystem includes the:1) the Mobile Switching Center(MSC)2) the Home Location Register(HLR)3) the Visitor Location Register(VLR)4) the Equipment Identity Register(EIR)5) the Authentication Register(AUC)

The central component of the Network Subsystem is the MSC. It is an advancedelectronic switch that provides all the functionality needed to handle a mobile subscriber,such as registration, authentication, location updating, handovers(mobility), and callrouting to a roaming subscriber. The MSC also has the interface to other networks suchas private land mobile networks, public switched telephone networks and integratedservices digital networks (ISDN). Signaling between functional entities in the NetworkSubsystem uses Signaling System Number 7 (SS7).The MSC is connected to the HLR. Logically there is only one HLR per GSM network,although it may be implemented as a distributed database. The HLR contains all theadministrative information of each subscriber registered in the corresponding GSMnetwork, along with the current location of the mobile. The location of the mobile istypically in the form of the signaling address of the VLR associated with the mobilestation. Each MSC will also have a VLR that contains selected administrativeinformation from the HLR, necessary for call control and provision of the subscribedservices, for each mobile currently located in the geographical area controlled by theVLR. Usually the VLR is implemented together with the MSC, so that the geographicalarea controlled by the MSC corresponds to that controlled by the VLR. This way thesignaling required is simplified.

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The MSC is also connected to the EIR and the AUC. The EIR is a database thatcontains a list of all valid mobile equipment on the network. The Authentication Center isa protected database that stores a copy of the secret key stored in each subscriber'sSIM card, which is used for authentication and encryption over the radio channel.

Operations and Maintenance Center

The OMC is the command center for monitoring every part of the network. The systemis equipped with alarms for all kinds of failures such as when a tower is being hit.

Operation Subsystem

The OSS contains contains al the parts of the network that are needed to run day to dayoperations. That includes the inventory systems, customer billing, and gateways totransport information.A higher lever overview of the GSM network in a public local mobile network is shown inFigure 2.The diagram demonstrates how the different subsystems come together.

UM or Air Interface

The air interface is the central interface of every mobile system and typically the onlyone to which a customer is exposed. GSM utilizes a combination of frequency divisionmultiple access(FDMA) and time division multiple access(TDMA).

HLR

MSC AREABSS

BSCTRAU

MSC

MSC AREAMSC AREA

MSC AREA

MSC AREA

MSC AREA

BTSBTS

BTSBTS BSS

BSSBSS

BSS

HLR

HLR

HLR

HLR

HLR

PLMN

Figure 2. GSM view of a Public Local Mobile Network[1]

MSC AREA

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Abis-Interface

The Abis-interface in the interface between the BTS and the BSC. It is a pulse codemodulation(PCM) 30 interface. The transmission rate is 2.048 Mbps which is partitionedinto 32 channels of 64 Kbps each. The compression techniques that GSM utilized packsup 8 GSM traffic channels into a single 64 Kbps channel.

A –Interface

On the physical level the A-interface consists of one or more pulse code modulation(PCM) links between the MSC and the BSC. Each one has a transmission capacity of 2Mbps.

4.2 GPRS

GPRS is an addition to the existing GSM infrastructure. As a result the GPRSarchitecture is very similar to the GSM’s. The existing GSM nodes are upgraded withGPRS functionality. The sametransmission links can be reused for both GSM andGPRS. eg the link between BSCs and BTSs.

The GSM network provided only circuit- switched services and thus two new networknodes were defined to give support for packet switching. This way packet data trafficseparated from traditional GSM speech and data traffic. The two nodes are the ServingGPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN)(see figure3). SGSN and GGSN are mobile aware routers and are interconnected via an IPbackbone network.

The SGSN is responsible for the communication between the mobile station (MS) andthe GPRS network. It carries out the basic functions of GSM’S BSC of providingauthentication, ciphering and IMEI check, mobility management, logical linkmanagement towards the MS, and charging data. It also connects to the HLR, MSC,and BSC and handles packet data traffic of GPRS users in a geographical area. Thetraffic is routed from the SGSN to the BSC via the BTS to the MS. The SGSN like theGSM’s MSC provides packet routing to and from the SGSN service area.

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The GGSN connects to outside data networks and to other GPRS networks.The GGSN provides the interface to external packet data networks like X.25 andexternal IP networks which are not supported by GSM. It routes incoming packets to theappropriate SGSN for a particular mobile station.

It also provides mobility management, access server functionality, and routing to theright SGSN and protocol conversion. The GPRS protocols are limited to just setting upan IP bearer, a logical link, between the MS and the Access Server. It translates dataformats, signaling protocols and address information permitting communication betweenthe different networks and enabling compatibility with the GSM network. GGSN is a hostowning all IP addresses of all subscribers served by the GPRS network thus replacingthe functionality of GSM’S VLR.

GPRS uses the GSM’S BSS but with enhanced functionality to support GPRS(seefigure 3). The GSM’s BSS is used as a shared resource of both circuit switched andpacket switched network elements to ensure backward compatibility and keep therequirements for the introduction of GPRS at a reasonable level.

The main change that GPRS brought compared to GSM is the addition of the packetcontrol unit (PCU) into the BSC which controls the packet channels, separating the dataflows of circuit and packet switched data. Circuit switched data are send through the A-interface on the MSC whereas packet data are send to the SGSN into the GPRSbackbone. The BSC of GSM is given new functionality for mobility management, forhandling GPRS paging. The new traffic and signaling interface from the SGSN isterminated in the BSC.

GPRS uses the MSC/VLR interface provided by GSM, between the MSC and SGSN co-ordinated signaling for mobile stations which have both circuit switched and packetswitched capabilities.

BSS

SMS-G/IW MSC

BSC

GGSN

MS

BTS

BTS

AUC

HLR

EXTERNALIP

NETWORK

BACKBONENETWORK

BTS

MSC/VLR

SGSN

TRAFFIC

GSM(existing)

Figure 3 GPRS architecture

SIGNALLING

MS

Gb

A Gs

Gd

Gr

EIRGf

GPRS(new)

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The HLR of GSM is modified to contain GPRS subscription data and routing informationand is accessible from the SGSN. It also maps each subscriber to one or more GGSNs.The HLR may be in a different PLMN than the current SGSN for roaming terminals.

The GSM interfaces are re-used except that they are enhanced to support GPRSnodes(see figure 4). The existing Abis interface transmission towards BSC is reused.In the GSM’s BTS new protocols supporting packet data for the air interface andfunctions for resource allocation for slot and channel allocation are implemented. GPRSuses the same pool of physical channels as speech. This way GPRS channels (PDCH)are mixed with circuit switched channels (TCH) in one cell. A TCH is allocated to onesingle user whereas several users can multiplex their traffic on one and the same PDCH.

GPRSBACKBONE

SS7NETWORK

DATA NETWORK(INTERNET)

DATA NETWORKIiNTERNET)

OTHER GPRSOPERATORS

MSC/VLR

ROUTER

LOCALAREA

NETWORK

BTS

FIREWALL

GGSN

GGSN

BORDERGATEWAY

SGSN

MS

EIR

SMS-GMSC

HLR

BSC

DATA NETWORKX.25

GrGf Gd

Gc

Gf

Gd

Gn,Gp

GsGb

FIREWALL

FIREWALL

FIREWALL Gp

Figure 4 GPRS infrastructure

GPRSINFRASTRUCTURE

PSTN

GSM

GPRS

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5. GSM and GPRS PROTOCOLS

This section compares the protocols stacks of both systems. This is done by brieflyintroducing the systems’ protocol stacks and then analyzing the differences and/orsimilarities.

The signaling protocol in GSM is structured into three general layers, depending on theinterface, as shown in Figure 5.

Applicat.TCPUDPIP | | |CM | | | CMMM | | | MM

GSMLayer 3

RR | RR | BSSMAP | BSSMAPLayer 2 LAPDm | LAPDm | SCCP | SCCPLayer 1 TDMA/FDMA | TDMA/FDMA | MTP | MTP

MS UM BTS Abis BSC A MSCTCP Transport Control Protocol RR Radio Resource ManagementUDP User Datagram Protocol LAPDm Link Access Protocol D-Channel

ModifiedIP Internet Protocol TDMA Time Division Multiple AccessCM Connection Management BSSMAP Base Station Subsystem Mobile Application PartMTP Message Transfer PartMM Mobility Management SCCP Signaling Connection Control Part

Figure 5 GSM Protocol Stack [12]

The protocol stack for GPRS,shown in Figure 6, provides transmission of user data andits associated signaling such as for flow control and error detection.

ApplicationNetwork(IP,X.25) RELAY

||

Network(IP,X.25)

||

NetworkLayer

SNDCP ||

||

SNDCP GTP ||

GTP ||

LLC |----|--- LLC RELAY

|-----|-----

LLC TCP,UDP ||

TCP,UDP ||

RLC ----|--- RLC BSSGP | BSSGP IP | IP |

DataLink

LayerMAC |

||

MAC |||

DataLink

Layer

|||

DataLink

Layer

|||

----|--- | | |Physicallayer

GSM RF|

GSM RF

FRAMERELAY

|

FRAMERELAY

Phy.Layer |

Phy.Layer |

|MS Um BSS Gb SGSN Gn GGSN Gi

IP Internet Protocol BSSGP BSS GPRS Application ProtocolLLC Logical Link Control GTP GPRS Tunneling ProtocolRLC Radio Link Control TCP Transmission Control ProtocolMAC Medium Access Control UDP User Datagram ProtocolPLL Physical link layer SNDCP Subnetwork Dependent Convergence ProtocolRFL Physical RF layer GSM RF GSM Radio Frequency ie. PLL and RFL

Figure 6. GPRS Protocol Stack[2]

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5.1 Physical layer

In both systems the physical layer between MS and BSS is divided into the twosublayers: the physical link layer (PLL) and the physical RF Layer (RFL). The PLLprovides a physical channel between the MS and the BSS. Its tasks include channelcoding (detection of transmission errors, forward error correction (FEC), indication ofuncorrectable codewords), interleaving, and detection of physical link congestion. TheRFL operates below the PLL. Among other things, it includes modulation anddemodulation.

GSM uses a combination of Frequency Division Multiple Access (FDMA) and TimeDivision Multiple Access (TDMA). The FDMA part involves the division by frequency ofthe maximum 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart.Each carrier frequency is further divided into 8 time slots, which make up a TDMA frame.A mobile station uses the same time slot in both the uplink and downlink. A group of 26TDMA frames are combined to form a 26-frame multiframe[12].GPRS is compatible with the standard TDMA scheme of GSM. With GPRS mobilestations can use more than one time slot within the same TDMA frame. It also uses a52-frame multiframe compared to the 26-frame multiframe used by GSM.

5.2 Link layer

On the air interfaces between the two systems there are a lot of different protocols.The GSM system uses LAPDm whereas GPRS uses LLC and RLC/MAC.The data link layer in GSM is formed by the LAPDm together with channel coding andburst formatting. The “m” stands for modified version of LAPD is an optimized version forthe GSM Air-Interface and was particularly tailored to deal with the limited resources andthe peculiarities of the radio link. LAPDm is responsible for the packaging of the data tobe transmitted which are then handed to the physical layer for transmission.All dispensable parts of the LAPD frame were removed to save resources. The LAPDmframe is particular, lacks the terminal endpoint identifier(TEI) , the frame checksequence(FCS ) and the flags at both ends[1]. The LAPDm frame does not need thoseparts, since their tasks is performed by other GSM protocols. The task of the FCS canbe performed by channel coding/decoding.

However on the GPRS, the LLC(between MS-SGSN) and RLC/MAC (between MS-BSS) layer that make up the data link between the MS and the network. The protocol ismainly an adapted version of the LAPDm protocol used in GSM.The LLC Protocolestablishes a logical link between MS and SGSN. Its functionality includes sequencecontrol, in-order delivery, flow control,detection of transmission errors, andretransmission (automatic repeat request (ARQ)). The data confidentiality is ensured byciphering functions. It operates either in an unacknowledged mode, not taking care ofpacket losses, or in an acknowledged mode, which applies retransmissions and flowcontrol to ensure a correct delivery of data.The RLC/MAC layer at the air interface includes two functions. The main purpose of theradio link control (RLC) layer is to establish a reliable link between the MS and the BSS.RLC is always operated in an acknowledged mode with a sliding window flow controlmechanism and a selective ARQ mode providing a reliable link between MS and BSS.This includes the segmentation and reassembly of LLC frames into RLC data blocks andARQ of uncorrectable codewords.

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This new medium access control (MAC) scheme was changed to meet the demands ofthe packet oriented data transmission. The RLC/MAC layer ensures the concurrentaccess to radio resources in a more flexible way compared to the unmodified TDMAstructure of GSM. It controls the access attempts of an MS on the radio channel sharedby several MSs. It employs algorithms for contention resolution, multiuser multiplexingon a PDTCH, and scheduling and prioritizing based on the negotiated QoS. Theflexibility is achieved by the introduction of a logical Packet Data Traffic Channel(PDTCH) which is multiplexed onto a physical data channel, the Packet Data Channel(PDCH), which corresponds to one timeslot (TS) in the GSM TDMA frame. Up to eight ofthese PDTCHs share one PDCH.

5.3 Network layer

Another difference is the Network Layer of the air interface of GSM. GSM uses threeprotocols named Connection Management (CM) , Mobility Management (MM) and RadioResources (RR) and GPRS uses the Subnetwork Dependent Convergence Protocol(SNDCP).In GSM:MM manages the location updating and registration procedures, as well as security andauthentication[12]. MM uses the channels that RR provides to transparently exchangedata between the MS and the NSS. From a hierarchical prospective, the MM lies abovethe RR, because MM data already are user data. The BSS does not, with a fewexceptions, process MM messages. A typical application of MM is location update[1].

CM handles general call control and manages Supplementary Services and the ShortMessage Service[12]. Like MM, CM uses the connection that RR provides forinformation exchange. In contrast to MM, which is use only to maintain the mobility of asubscriber, CC is a real application that at the same time provides an interface toISDN[1].

RR management controls the setup, maintenance, and termination of radio and fixedchannels, including handovers[12]. Messages in the area of RR are necessary tomanage the logical as well as the physical channels on the Air-Interface. Depending onthe message type, processing of RR messages is performed by the MS, in the BSS, oreven in the MSC. Involvement of the BSS distinguishes RR from MM and CC[1].

In GPRS, the SNDCP is used to transfer data packets between SGSN and MS. Itmultiplexes several connections of the network layer onto one virtual logical connectionof the underlying LLC layer. This comprises multiplexing of packets from differentprotocols, header compression (e.g. TCP/IP) and data compression (e.g. V42.bis), andsegmentation of packets larger than the maximum LLC packet data size.It also compresses and decompresses user data and redundant header information.

5.4 Signaling

In GSM signaling between the different entities in the fixed part of the network, such asbetween the HLR and VLR, is accomplished through the Mobile Application Part (MAP).MAP acts as a communication control between MAP and applications, and is a carrier ofsignaling data[1].

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MAP is built on top of the Transaction Capabilities Application Part (TCAP, the top layerof Signaling System Number 7). TCAP is built on top of SCCP. The SCCP analyzes thedata received from the MTP and forwards the data to the addressed subsystem, wherethe input data is associated with the various active transactions. The Message TransferPart (MTP) of SS7 is used. The Message Transfer Part provides all the functionality ofOSI Layer1 to 3 required to provide reliable transport of signaling data to the variousSS7 user parts, takes the necessary measures to ensure that the connection can bemaintained or prevents loss of data, like when switching to an alternative route.

GPRS uses the same protocols for signaling between the SGSN and the HLR, VLR,and EIR as used in GSM and extends them to GPRS functionality.The Gf interface between the SGSN and EIR, the Gr interface between the SGSN andthe HLR and the Gc interface between the GGSN and the HLR use the the same lowerlevels as used in GSM. That is the Physical layer, MTP, SCCP and TCAP. However anenhanced version of MAP denoted by MAP+, handles handovers, location updates ,routing information and user profiles.Like GSM a GGSN just send its information requests to any GSN connected to the SS7.

Another interface which is quite similar to GSM’s interfaces is the Gs interface betweenthe SGSN and the visited MSC with the VLR(see figure 3). In this case, only oneprotocol changes called BSSAP+ which is a subset of the base station subsystemapplication part (BSSAP) protocol used in GSM. BSSAP+ like BSSAP uses existingsignaling standards (SS7 and SCCP). This protocol was implemented to handlecombined GSM and GPRS services are requested.

The BSS GPRS Application Protocol (BSSGP) has also been derived from BSSMAPused in GSM. On the BSS it is used to deliver routing and QoS-related informationbetween BSS and SGSN.

Another difference between the GSM Protocol stack is one more new protocol at the Gninterface. The protocol called the GPRS Tunneling Protocol (GTP) tunnels mobileapplication part (MAP), IP, and x.25 messages between the GPRS support nodes(GSNs). The protocol is defined both between GSNs within one PLMN (Gn interface)and between GSNs of different PLMNs (Gp interface). GTP employs a tunnelmechanism to transfer user data packets specifying a tunnel control and managementprotocol. The signaling is used to create, modify, and delete tunnels. GTP packets carrythe IP or X.25 packets which were not supported by GSM. [13].

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6 Conclusion

This paper has briefly given an overview of GSM and GPRS:GSM is a wireless platform that uses radio frequencies. It has been designed for speechservices and uses circuit switched transmission. GSM standard circuit is a digital databearer service offering 9.6kb/s. This data transmission in these networks is regarded astoo slow and often too expensive for many applications. The cost is the total time thatthe user occupied that channel eventhough he may not be using the channel all the time.

GPRS is a separate packet data network which provides a packet base platform both forthe data transfer and signaling. It enables volume based charging in contrast to GSMlike charging of online time. It therefore allows users to stay constantly online while onlypaying for the occasional data transfer. GPRS increases the capacity of the system andreduces the idle periods of the radio channels. This is done by allowing for multiple usersper physical channel and using a channel only when it is needed, and releasing itimmediately after the transmission is complete.

This paper has also given a detail description and comparison of each systemarchitecture. GSM is composed of three broad parts. The MS which is carried by thesubscriber, the BSS which controls the radio link with the MS, and the NSS whichperforms the switching of calls between the mobile users, and between mobile and fixednetwork users. The GPRS uses the existing GSM network but adds two nodes theSSGN, and the GGSN to support packet-switching. As a result extra signaling interfaceswere required between the two added nodes and the GSM network.

Finally, the paper compared the GSM and GPRS protocols. The protocols stack at theGPRS MS was almost totally changed with the introduction of the SNDCP, LLC, andRLC/MAC. Signaling on the GPRS network was done using the existing SS7 and SCCPlayers but the upper layers were enhanced to support the functionality of GPRS. TheGTP protocol was also introduce to tunnel packets from data networks.

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Works Cited:

[1] Gunnar Heine, GSM Networks: Protocols, Terminology and Implementation,Artech House, Inc., Norwood, MA, 1999.

[2] Gotz Brasche , Bernhard Walke, “ Concepts, Services, and Protocols of the NewGSM Phase2+ General Packet Radio Service”, IEEE Communications Magazine,Vol. 35, No. 8, Aug. 1997, pp. 94-104.

[3] John Harris, “GSM Basics: An Introduction”, Microwave Journal, Vol. 39, No.10,Oct 1996, pp. 84-92.

[4] Fabri S. N. , Kondoz A., Tatesh S., Demetrescu C., “Proposed evolution of GPRSfor the support of voice services ” IEE Proceedings: Communications, Vol. 146,No. 5, October 1999, pp 325-330.

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[14] Simon Hoff, Michael Meyer, Jooachim Sachs, “Analysis of the General RadioService (GPRS) of GSM as Access to the Internet,” IEEE Communications,1998, p. 415-419.

[15] Performance Technology Incorporation, " SS7 Tutorial,"http://www.microlegend.com/sccp.shtml (Current April 23, 2000).

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AppendixA. ACRONYMS AND ABBREVIATIONS

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Appendix A

ACRONYMS AND ABBREVIATIONS

AMPS Advance Mobile Phone ServiceAUC Authentication CenterBG Border GatewayBGP Border Gateway ProtocolBSC Base Station ControllerBSS Base Station SystemBSSGP BSS GPRS ProtocolBTS or BS Base Transceiver StationDHCP Dynamic Host Configuration ProtocolDNS Domain Name SystemEIR Equipment Identity RegisterETSI European Telecommunications Standards InstituteGb Interface between an SGSN and a BSS.Gc Interface between a GGSN and an HLR.Gd Interface between a SMS-GMSC and an SGSN, and between a SMS-

IWMSC and an SGSN.Gf Interface between the SGSN and EIRGGSN Gateway GPRS Support NodeGi Reference point between GPRS and an external packet data network.GMSC Gateway MSCGn Interface between two GSNs within the same PLMN.Gp Interface between two GSNs in different PLMNs. The Gp interface

allows support of GPRS network services across areas served by theco-operating GPRS PLMNs

GPRS General Packet Radio ServiceGr Interface between an SGSN and an HLR.Gs Interface between an SGSN and an MSC/VLR.GSM Global System for Mobile CommunicationsGSN GPRS Support NodeGTP GPRS Tunnelling ProtocolGTP-id GTP IdentityHLR Home Location RegisterIETF Internet Engineering Task ForceIMSI International Mobile Station IdentityIP Internet ProtocolIPSEC IP Secure ProtocolISDN Integrated Services Digital NetworkISP Internet Service ProviderIWMSC Inter-working MSCLLC Logical Link ControlMAC Medium Access ControlMAP Mobile Application PartMIB Management Information BaseMS Mobile StationMSC Mobile Switching CenterMT Mobile Terminal

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MTP Level 1 Message Transfer Part is equivalent to the OSI Physical Layer, and isresponsible for the transfer of single bits[15].

MTP Level 2, Message Transfer Part 2 is equivalent to the OSI Data Link Layer,ensures accurate end-to-end transmission of a message across asignaling link. It also implements flow control, message sequencevalidation, and error checking. In addition, it defines the basic framestructure that is used by SS7 for all message types[15].

MTP Level 3 Message Transfer Part is equivalent to OSI Network layer, providesmessage routing between signaling points in the SS7 network. Itcontrols traffic during congestion and re-routes traffic away from failedlinks and signaling points.

O&M Operations & Maintenance

OMC Operations and Maintenance CenterPDCH Packet data channelPDN Packet Data NetworkPDP Packet Data ProtocolPLMN Public Land Mobile NetworkPTM-G Point-to-Multipoint Group CallPTM-M Point-to-Multipoint MulticastQoS Quality of ServiceRA Routing AreaRLC Radio link controlRSVP Resource reSerVation ProtocolRTP Real-time Transport ProtocolSCCP Signalling Connection Control Part

SCCP provides two connectionless and two connection-orientednetwork services. SCCP is used as the transport layer for TCAP-basedservices. The SCCP uses the layers MTP 1 through 3 of the SS7. Theservices of the SCCP are used by the BSSAP on the A-Interface andthe Mobile Application Part(MAP) on the various interfaces within theNSS. The SCCP offers end to end addressing, even across severalnetwork nodes and countries. The SCCP comes with its ownmanagement functions for administrative tasks, which are independentfrom those known from the SS7 functionality. It also provides featuresincluding mechanisms for error detection and an optional segmentationof the data to be transmitted[15].

S-CDR SGSN CDRSDM Site Data MediationSGSN Serving GPRS Support NodeSMG10 Special Mobile Group 10SMS Short Message ServiceSMSC Short Message Service CenterSNDCP Subnetwork Dependent Convergence ProtocolSNMP Simple Network Management ProtocolSS7 Signaling system 7

SS7 is a global standard that defines the procedures and protocol bywhich network elements in the public switched telephone network

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(PSTN) exchange information over a digital signaling network to effectwireless and wireline call-establishment, billing, routing and control.The SS7 network and protocol are used for different functions such asbasic call setup, management, and tear down ,wireless services suchas personal communications services, wireless roaming, mobilesubscriber authentication, and efficient and secure worldwidetelecommunications [15].SS7 signaling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-bandsignaling provides,faster call setup times ,more efficient use of voicecircuits [15].

TCH Traffic ChannelTCP Transmission Control ProtocolTMN Telecommunication Managed NetworkTRAU The Transcoding Rate and Adaptation Unit is located between the

BSC and the MSC. The purpose of the tRAu ist to compress ordecpmpresss speech between the MS and the TRAU. The methodused is calle dthe regular pulse excitation –long term prediction (RPE-LTP). It is able to compress speech from 64kbps to 16 kbps, in thecase of a fullrate channel and to 8Kbps in the case of a halfratechannel. The TRAU is not used for data connections.

UDP User Datagram ProtocolUm Interface between the mobile station (MS) and the GPRS fixed network

part. The Um interface is the GPRS network interface for providingpacket data services over the radio to the MS.

UMTS Universal Mobile Telecommunication SystemVLR Visitor Location RegisterVMSC Visitor MSCWAN Wide Access Network