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`Gunnar Heine
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`Library of Congress Cataloging-in-Publication Data
`Heine, Gunnar.
`GPRS:gateway to third generation mobile networks / Gunnar Heine, Holger
`Sagkob.
`p. cm. — (Artech House mobile communications series)
`Includes bibliographical references and index.
`ISBN 1-58053-159-8 (alk. paper)
`1, Mobile computing.
`2. Mobile communication systems.
`I. Ticle.
`TE. Series.
`
`1. Sagkob, Holger.
`
`QA76.59 .H45
`004,6—de21
`
`2003
`
`2002043670
`
`British Library Cataloguing in Publication Data
`Heine, Gunnar
`GPRS: gateway to third generation mobile networks. — (Artech House mobile
`communicationsseries)
`1. General Packet Radio Service
`I. Title
`TH. Sagkob, Holger
`621.3'8456
`
`2. Global system for mobile communications
`
`ISBN 1-58053-159-8
`
`Cover design by Igor Valdman
`
`© 2003 ARTECH HOUSE, INC.
`685 Canton Street
`Norwood, MA 02062
`
`All rights reserved. Printed and boundin the United States of America. Nopart of this book
`may be reproduced or utilized in any form or by any means, electronic or mechanical,
`including photocopying,recording, or by any information storage andretrieval system, without
`permission in writing from the publisher.
`All terms mentioned in this book that are known to be trademarks or service marks have
`been appropriately capitalized. Artech House cannotattest to the accuracyofthis information.
`Use of a term in this book should not be regarded asaffecting the validity of any trademark
`or service mark,
`
`International Standard Book Number: 1-58053-159-8
`Library of Congress Catalog Card Number: 2002043670
`
`10987654321
`
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`
`
`The Basics: Principles of GSM and
`Influences on GPRS
`
`1.1. The Network Architecture of GSM
`
`As an overview, each GSM network can be subdivided into the basestation
`subsystem (BSS) and the network switching subsystem (NSS), as well as the
`mobile station. Please note that the introduction of GPRS can only expand,
`but must not change, the existing structure as presented in Figure 1.1, since
`both types of application—circuit switched and packet switched—should
`run via the mutual GSM/GPRSnetwork.
`
`4A///
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`
`Figure 1.1 GSM network architecture.
`
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`2 GPRS: Gateway to Third Generation Mobile Networks
`
`1.1.1 The BSS
`
`1.1.1.1.
`The Base Transceiver Station
`The BSS consists primarily of a larger number of base transceiver stations
`(BTSs) that enable wireless connection of the mobile stations to the network
`via the U,, or air interface (Figure 1.2). Apart from transcoding rate and
`adaption unit (TRAU) framing, the BTS assumesall layer 1 functions in
`communications between the network and the mobile station. These include,
`amongst others, channel coding, interleaving, ciphering (only GSM, not
`GPRS), and burst generating. Other functions include Gaussian minimum
`shift keying (GMSK) modulation and demodulation, which are carried out
`by the base station and will be discussed in detail later.
`
`1.1.1.2. The Base Station Controller
`
`All BTSs of a BSS are connected to the base station controller (BSC) via
`the Abis interface (Figure 1.3). The BSCis, by definition,a circuit switching
`exchangein addition to the mobile services switching center (MSC), which
`will be discussed later. The BSC was basically viewed as a further exchange
`in order to relieve the MSC from all wireless-related tasks. These include,
`in particular, the evaluation of the measurement results from the BTS and
`mobile station during a live connection and the handover and powercontrol
`adjustments resulting from this.
`These regulatory functions are generally performed in their entirety by
`the BSC,although the GSM standard expressly allows preliminary prepara-
`
`Air interface
`
`
`
`
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`
`
`Clock distribution O&M-Modules
`
`
`Operation and maintenance functions /
`
`Figure 1.2 Principal schematic diagram of the base transceiver station.
`
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`The Basics: Principles of GSM and Influences on GPRS
`
`3
`
`DB = Database
`TCE = Trunk contro! element
`TM = Transmission
`
`
`
`Trunk
`=|
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`control
`| Transmission
`Transmission: control
`1
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`| Central module
`
`
` OMC
`Central functions / clock distribution
`
`Figure 1.3 Principal circuit diagram of the BSC.
`
`tion of the measuring results in the BTS. Additional BSC functionsare the
`Peer function of the mobile station for the Radio Resource Management
`protocol (RR) and the resource administration on the Abis andair interface.
`The BSC, as a circuit switching network element,
`is a considerable
`hindrance to packet switched services (GPRS). Its exchange functions are
`almost unusablefor packet switchedservices, and the RRprotocolis extremely
`difficult to adjust to the requirements of packet switched services. Hence,
`if the BSS is to be used at all for GPRS, the BSC must either be modified
`accordingly or a new network element or an extension of the BSC will be
`necessary.
`
`1.1.1.3 The Transcoding Rate and Adaptation Unit
`The TRAUis the third BSS network element. The best-known task of the
`TRAU is speech compression from 64 Kbps to 16 Kbps (full-rate) or 8
`Kbps (half-rate). The TRAUalso carries out comfort noise generation while
`discontinuous transmission (DTX) is in operation.
`What is considerably more important for a basic understanding of
`signal processing within GSM is another TRAU function: the conversion
`ofall information coming from.the MSCinto so-called TRAU frames. This
`conyersionis carried out for fax, data, and speech. In other words,all payload
`transfer between mobile station and TRAUtakes place on the basis of TRAU
`frames. TRAUframes have a length of 320 bits. Every 20 ms a TRAU frame
`is transmitted or received. Consequently, there are channels of 16 Kbps.
`
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`GPRS: Gateway to Third Generation Mobile Networks
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`The numberofactual payloadbits will vary depending on the type of TRAU
`frame or application. For full-rate speech and enhanced full-rate speech, the
`TRAUframewill, for example, contain 260 bits of payload data, whereas
`the normal data TRAU frame contains 240 payload data bits (see Section
`L.7.1).
`As already implied, payload channels of 16 Kbps are used on account
`of the TRAU framing between the TRAU and the BTS,especially on the
`Abis interface. In other words, if more than 16 Kbps are transferred, there
`is a problem. This is, however, exactly what happens in data transfer via
`GPRS or EDGE. Most manufacturers will have to find new approaches in
`order to solve this. problem.
`Since the functions of the TRAU are specific layer 1 functions, the
`TRAUfunction should be assumedtobelocally situated in the BTS. Indeed,
`the GSM standard permits the integration of the TRAUinto the BTS. This
`possibility is illustrated in Figure 1.4. Most manufacturers, however, take
`a different course and use so-called remote TRAUs. The reason for this is
`the opportunity to save on connection costs. If the TRAUis installed on
`the MSC, then 16-Kbps channels can be used all the way from the MSC
`to the BTS, instead of 64-Kbps channels. In other words, a remote TRAU
`cuts connection costs by three-quarters. For the implementation of GPRS,
`the actual position of the TRAU is of somesignificance, since the packet
`switched GPRS data is fed into the existing GSM network at somepoint.
`We shall encounter this again in Chapter2.
`
`1.1.2 The Network Switching Subsystem
`As shown in Figure 1.1, the NSS consists of one or more homelocation
`registers (HLR) with the authentication center (AuC) and optionally with
`
` 16 Kbps
`
`TRAU
`
`Figure 1.4 Possible location of the TRAU.
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`The Basics: Principles of GSM and Influences on GPRS 5
`
`the equipmentidentity register (EIR) andvarious MSCs with a connected
`visitor location register (VLR). Please note that the NSS is also used by
`GPRS, at least in part.
`
`The Home Location Register and the Authentication Center
`1.1.2.1
`The HLRisastatic database in which information on hundreds of thousands
`of subscribers is stored. This information includes the telephone number(s)
`[i.e.,
`the mobile subscriber international service directory number (MS-
`ISDN)] of a subscriber as well as his service characteristics and service
`limitations. For mobility management (MM), whichis so important in GSM,
`the HLRholds the information as to which VLRarea a subscriberis currently
`registered. With the introduction of GPRS, the data on individualsubscribers
`in the HLR will be more comprehensive. This implies that for GPRS, the
`HLR must not only possess the information regarding the respective VLR
`but also that of the corresponding serving GPRS support node (SGSN).
`Other GPRS-specific data stored in the HLR are possible Packet Data
`Protocol (PDP) contexts and service characteristics and service limitations,
`only for GPRSthis time.
`The AuC,which is an integral part of the HLR,calculates the respective
`authentication results (SRES) and ciphering keys (Kc), using the algorithms
`A3 and A8 from RAND numbers (RAND = random number) and the
`subscriber keys Ki stored in the HLR. These processes are presented in
`diagrammatic form in Figures 1.5 and 1.6. Please note that the AuCpredeter-
`mines up to five so-called authentication triplets (RAND, SRES, Kc) for
`each subscriber and puts them at thedisposal of the VLR responsible, via
`
`
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`RAND
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`Figure 1.5 The determination of SRES from Ki and RAND.
`
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`6
`
`GPRS: Gateway to Third Generation Mobile Networks
`
`
`
`Figure 1.6 The determination of the ciphering key Ke from Ki and RAND.
`
`the HLR,for authentication purposes. A detailed description of the processes
`of GSM authentication and GSM ciphering can be found in [1, 2].
`The introduction of GPRS does notalter these GSM mechanisms.It
`should be noted, however, that in GPRS the authentication and activation
`of the GPRS ciphering are controlled by the SGSN. As a consequence, a
`mobile station can be authenticated twice, once by the VLR and once by
`the SGSN,each with a different RAND variable, of course. Accordingly,
`two different Ke values must be stored and ready for retrieval in the mobile
`station—one for GPRS and one for normal GSM. This poses a problem
`for older subscriber identity module (SIM) cards, which will be discussed
`in more detail later in the book together with ciphering in GPRS.
`
`1.1.2.2. The Mobile Services Switching Center and Visitor Location Register
`Before the introduction of GSM in the 1980s, MSC and VLRwere conceived
`as two independent network elements: the MSC as a network element for
`all call control (CC) functions and the VLRforthe greater part of the MM
`functions. Both protocols, CC and MM,are transparent for the BSS and
`are treated between the MSC and the VLR on the one hand, and by the
`mobile station on the other. A detailed presentation of both protocols and
`their functions can be foundin [1, 2]. In the early 1990s, after the introduc-
`tion of GSM,the physical independence of MSC and VLR disappeared,
`and by 1997 the MSC and VLR became the MSC/VLR. This did not,
`however, alter the protocol independence of MM and CC.
`It is important to understand about GSM that the MSCis essentially
`an ISDN exchange that has been modified for use as a GSM-MSC. ISDN
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`The Basics: Principles of GSM and Influences on GPRS
`
`
`
`exchanges, however, are circuit switched. Historically, this path was taken
`at the end ofthe 1980s because GSM wassupposedto be as ISDN compatible
`as possible. One ofthe problemsto be solved in this context was thatexisting
`exchanges, such as the Siemens EWSD System or the Alcatel System 12,
`could assume the RR functions, which are necessary for a mobile telephone
`system, only with great difficulty, at least not without drastic, and thus
`expensive, modifications. Therefore, an unusual way was taken with GSM
`and these RR functions were relocated to the BSC. As a consequence, circuit
`switched exchanges, which are unsuitable for a packet switched transfer
`process such as GPRS,are situated centrally in the form of MSCs in GSM
`networks. The consequences ofthis will be discussed in more detail in the
`next chapter.
`
`The Gateway MSC andthe Interworking Function
`In Figure 1.1, a typical Public Land Mobile Network (PLMN)with different
`MSCswaspresented. In total, only two of these MSCs have an interface to
`external networks. These special MSCs are described as gateway MSCs
`(G-MSCs) in GSM. The network operator has to decide whetherall or only
`selected MSCs should have this interface function.
`Ontheside facing away from the PLMN of a G-MSC,there is the
`so-called interworking function (IWE), which, amongst other things, takes
`care of the rate adaptation (RA) functions in connections to external data
`networks. For this reason, the IWF is also frequently called a modem base.
`GSM supports interworking with different types of external networks
`such as Circuit Switched Public Data Networks (CSPDNs), Packet Switched
`Public Data Networks (PSPDNs), the Public Switched Telephone Network
`(PSTN), and Integrated Services Digital Network (SDN).
`zs
`
`1.1.23 The Equipment Identity Register
`In contrast to the databases in GSM already described (ie., the VLR and.
`the HLR), the EIR does not administer subscriber data but the data of the
`mobile terminals themselves. Another difference from VLR and HLRis the
`fact that the EIR is an optional network element that has, for reasons of
`cost, only rarely been introduced by network operators.
`It is important to look at the historical development of EIR. In the
`standardizing phase of GSM in the 1980s, mobile devices and mobiletele-
`phoning were very expensive and the danger of theft and abuse was accord-
`ingly high. By definition, GSM opens up new doors for the black market
`since the subscriber’s identity [the international mobile subscriber identity
`(IMSI)] andall his data, such as the telephone number (the MS-ISDN), are
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`8 GPRS: Gateway to Third Generation Mobile Networks
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`separated from the identity of the device itself. In other words, theoretically,
`a stolen device could be used as early as the day ofits theft without anyone
`noticing whether a different SIM card is used. To counteract this danger,
`two measures were taken.First, every GSM device must be given an unchange-
`able and unmistakable identity number [the international mobile equipment
`identity (IMET)]. Second, the EIR, in which stolen or conspicuous IMEIs
`can be stored, was introduced. It became clear, however, on or shortly after
`the introduction of GSM in 1991 that the prices of GSM devices were going
`to fall and thus theft protection and mechanismsto counter the black market
`were no longer going to be ofprimary concern to the end user. In accordance
`with this, many network operators no longer placed orders for EIRs or
`cancelled existing orders.
`The GPRS core network, however, which we will introducelater, also
`has interfaces to the EIR for compatibility reasons.
`
`1.1.3 The GSM Mobile Station and the SIM
`The expression, “GSM Mobile Station and the Subscriber Identity Module”
`is in itself incorrect, because the GSM mobile station (MS) only arises
`through the physical connection of GSM mobile equipment (ME) with a
`SIM. To put it simply, ME + SIM = MS.Nevertheless, many specialists
`use the term “mobile station” as a synonym for the correct term, “mobile
`equipment,” which is why we shall not make any differentiation in the
`following unless it is necessary to do so.
`Let us return now to the GSM mobile device, which is an essential
`part of GSM’s success. Manycharacteristics of GSM are defined in terms
`of the mobile device:
`
`© The cellular network configuration with relatively small cell sizes
`enables low transmission energy consumption on the MSside, which
`is why the mobile station battery can be kept small and light.
`© The GMSK modulation used in GSM enables the use of low-
`cost power amplifiers; this is basically a simple modulation process.
`Production costs should also be accordingly low.
`° Theoriginal GSM standard did not provide for GSM mobile stations
`beingable to transmit andreceive simultaneously. Duplex operation
`was not envisaged. Consequently,a duplexeron the interface between
`transmitting/receiving path and antenna was not necessary. This
`factor also reduces the complexity and costs ofa GSM mobilestation.
`
`HEAAETRS
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`The Basics: Principles ofGSM and Influences on GPRS
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`g
`
`* As opposedto other network elements in the NSS and BSS, detailed
`requirements, for example, of the man-machine interface (MMJ),
`were defined for the mobile station.
`
`¢ The clear definition of compulsory and optional features of the
`GSM mobile station with regard to performance allows for hundreds
`of test cases. These are specially defined for mobile stations in the
`GSM standards (GSM 11.10). Every GSM mobile station must
`conform with these test cases before it is permitted to be retailed.
`Atfirst glance, this restriction may seem to be a hindrance, butit
`proves to be most advantageous in the long run because it ensures
`customerconfidence and reducessignificantly the numberof costly
`recall campaigns.
`
`~
`
`Despite these simplifications every GSM mobile station is a piece of
`top-rate technology. As illustrated in Figure 1.7, a GSM mobile station
`contains all layer 1 functions that can also be found in the BTS and the
`TRAU. Furthermore,
`the mobile station must support all MM and CC
`functions in conversation alongside the MSC and the VLR. There also have
`to be mechanical devices for the insertion and removal of the SIM. Finally,
`the different user interfaces have to be integrated. These include, in addition
`to the MMI,loudspeaker, microphone, andelectrical and/or optical interfaces
`for data connections.
`
`1.1.3.1
`
`The GPRS Mobile Station
`
`For the introduction of GPRS, the functions of the GSM mobile station
`must be diversified in many areas. Although we shall be examining this
`Process in moredetail later, the essential characteristics or differences between
`
`
`
`Channel decoding
`
`
`
`De-interleaving
`Reformating
`
`
`
`(i
`
`Channel enceding
`interleaving
`Burst generation
`
`
`
`SIM = subscriber identity module
`
`Figure 1.7 Circuit diagram of a GSM mobile station.
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`10 GPRS: Gateway to Third Generation Mobile Networks
`
`a GSM/GPRS mobile station as compared to a purely GSM mobile station
`should be outlined now:
`
`¢ New protocol stack: support of new protocols in the radio resource,
`mobility management, and session managementareas;
`° Support of new channel coding processes;
`¢ Multislot transmission: With higher multislot classes (type 2), even
`simultaneous transmitting and receiving is possible. Apart from the
`multifunctional capability itself, the increased demandson the bat-
`tery capacity must also be considered;
`° Data services require a new MMI. Forinstance, the touchscreen of
`GSM/GPRS-PDAs are used both as a keypad for telephoning and
`a display area for visual information;
`° Possibly the development of GPRS-only mobile stations, which are
`virtually wireless Internet sockets and no longer offer any speech
`services atall;
`the simultaneous support of channel and packet-
`° As an option,
`orientated services—for
`example,
`downloading e-mail while
`telephoning.
`
`1.2 The Multiple Access Processes: SDMA, FDMA, and
`TDMA
`
`Asasecond generation mobile communication network, GSM usesthethree
`classical multiple access processes, space division multiple access (SDMA),
`frequency division multiple access (FDMA), and time division multiple access
`(TDMA)in parallel and simultaneously. GPRS does notalter this nor many
`other basic GSM processes.
`
`1.2.1
`
`SDMA
`
`The entire geographical area is not supplied by a single transmitting station.
`The transmitting powerof the individual transmitting stationsis limited in
`order that a given frequency may be used again at a short distance away.
`Asillustrated in Figure 1.8, however, SOMA gives rise to a cellular network
`structure that has both advantages and disadvantages. The most important
`advantages are the high reusability rate of the frequencies used and,at least
`as important, the considerably lower demandson the transmitting powerof-
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`11
`
`& Frequency1
`
`@!
`
`Figure 1.8 The SDMA multiple access process givesrise to cellular network architecture.
`
`it is possible to produce small mobile
`the mobile stations. Accordingly,
`stations with low power requirements. On the other hand,
`the SDMA
`configuration automatically leads to a complex network structure, which is
`necessary to connectthe individual transmitters to each other and to enable
`standard functions such as roaming and handover.
`
`1.2.2
`FDMA
`Similarly to SDMA, FDMA is a multiple access process that is relatively
`easy to understand and in which the given frequency band is divided into
`individual frequency channels. Eachuseris allocated just one of these narrow
`channels. In this context it has to be considered that two frequency channels
`are necessary for a bidirectional connection: one for the transmission to the
`mobile station (downlink) and onefor the opposite direction from the mobile
`station to the base station (uplink). In GSM, a complete frequency channel
`thus requires 2 x 200 kHz. Here, the frequency distance between the uplink
`and downlink frequency channels is always determined precisely and only
`changes for the different GSM variants (see Figure 1.9). For example,
`in
`PCS1900, the GSM variant used in the United States, this distance between
`the uplink and downlink channels is exactly 80 MHz, whereas in P-GSM900,
`it is 45 MHz. A serious disadvantage of the usual FDMA systemsis the
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`GPRS: Gateway to Third Generation Mobile Networks
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`Duplex distance:
`P-GSM = 45 MHz
`
`
`
`PCS 1900 = 80 MHz
`
`Uplink frequency channel
`200 kHz
`
`Downlink frequency channel
`200 kHz
`
`Figure 1.9 There is a set distance between the uplink and downlink channels in the
`application of FDMA in GSM.
`
`necessity of so-called paired bands (i.e., two frequency bands that have to
`be provided at a fixed duplex distance form one another). Such systems are
`also described as frequencydivision duplex (FDD)systems. This requirement
`is of particular disadvantage because frequency is a rare resource, as the
`bidding for the UMTSlicenses has clearly demonstrated to the general
`public. If, for example, one wishes to operate GSM in a country or a region,
`it first has to be determined whether the uplink and downlink frequencies
`are even available.
`
`1.2.3 TDMA
`
`For FDMA,theavailable frequency rangeis divided into individual frequency
`channels. When there is an active connection, a subscriber receives this
`frequency channelexclusively for the entire duration of the call and no one
`else can use this part of the spectrum.
`With TDMA,a further step is taken. Each frequency channelis also
`subdivided temporally and each subscriber receives access rights to the fre-
`quency channel during a connection fora relatively short but repeated period
`of time. In a TDMA system these periodically repeated time intervals are
`called time slots (Figure 1.10). In order to give the impression of an uninter-
`rupted connection,sufficient information must be transmitted in these time
`slots per connection.
`In GSM, each frequency channel is divided into eight time slots (TS),
`as shown in Figure 1.11. Each time slot has a length of 576.9 ws (= 577 ys)
`or 156.25 bits and is repeated every 4.615 ms. Accordingto this definition, up
`to eight users in GSM can use one frequency channel almost simultaneously
`and independently from one another. It must be stressed again, however,
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`13
`
`u(t}
`
`
`
`
`TOMA
`Each frequency channel is divided
`
`
`
`i inte a numberoftime slots
`
`
`
`Frequency
`
`Time slots
` 8
`
`f
`
`FDMA
`The available frequency range
`is divided into different channels
`
`#7
`
`Figure 1.10 The combination of FOMA and TDMA.
`
`Frequency
`
`>! 577 us
`i
`
`
`
`sun. *
`4 © [iso[si[1s2[iss[184[18s[786[787|
`
`
`isd[ist[182[18s[1s4[15s[186[1ST]
`|
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`Piso[1st[182[13s[se[1s[188[187
`
`iso[Tt[se[13s[se[1Ss[186[187|
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`iso[is1[ts2[13[184[135[18[17|
`
` Time
`
`
`oo TDMA-frame>,
`4.615 ms
`
`
`
`Figure 1.11 The combination of FDMA and TDMAin GSM.
`
`that Figure 1.11 only represents one direction, but two frequencies are
`required for a bidirectional connection, in which the same timeslot is used.
`
`1.3 Chronological Sequence of Uplink and Downlink
`Transmission
`
`In GSM,thebase station always transmits three time slots before the mobile
`station. In other words, the transmission of a time slot in the downlink
`direction always takes place three time slots before the transmission of the
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`same time slot in the uplink direction. It could also be stated thus: time
`slot X in the downlink direction is three time slots before time slot X in
`the uplink direction (Figure 1.12). A mobile station that has synchronized
`itself to a base station andreceives information in time slot X will wait 3
`time slots, or 3 X 156.25 bits = 468.75 bits, before sending its data to the
`base station. This golden rule for GSM, which is only compromised in the
`transmission delay problem described in Section 1.4, does not change with
`the introduction of GPRS.
`Oneshould also consider Figure 1.12 from the point ofview ofmultislot
`transmission. If simultaneous transmission and reception ontheside of the
`mobile stations is to be avoided,there are only a few uplink/downlink time
`slot combinationsavailable that can avoid this problem. It is then actually
`impossible to provide an individual user with more than four timeslots in
`the downlink direction or four in the uplink direction. This applies in
`particular when transmissions are to take place in the opposite direction and
`the mobile station has to carry out neighboring cell measurements at the
`same time.
`
`1.4 Problems of Transmission Delay in TDMA Systems—
`Timing Advance Control
`
`In every TDMA system,data transmission in both directions necessarily
`takes place in the form of impulses. In GSM,theseimpulses are called bursts.
`One ofthe main problems of TDMA systems, which mustnotbe neglected,
`is the delay time thatit takes to transmit a burst from transmitter to receiver.
`In the ‘direction from the base station to the mobile station (downlink),
`there are no problemsin this respect as every mobile station can receiveits
`signal,
`its burst, independently from other mobile stations. In the other
`direction, however, [i-e., from the mobile station to the network (uplink)]
`there is the possibility of collisions with the bursts sent from various mobile
`stations. The cause of the unknown delay time is the unknown distances
`
`
`
`Figure 1.12 Synchronization of downlink and uplink transmission in GSM.
`
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`The Basics: Principles of GSM and Influences on GPRS 15
`
`between the mobile stations and thebase stations. Since mobile stations may
`also move within the network, these delay times vary during a connection.
`Accordingly, the various active mobile stations must constantly adjust the
`starting time of their transmission in order to reach their receiver window
`in the base station (Figure 1.13). The solution to this problem of delay time
`in the uplink direction is not only necessary for the beginning of a connection
`but is also necessary during alive connection. Otherwise, mobile stations
`would haveto be prohibited from movingatall during an active connection.
`In GSM delay time control is spoken of as timing advance (TA) control.
`
`1.4.1. Timing Advance Control When Accessing the Network
`Timing advance control appears particularly difficult when accessing the
`network. At this point, the mobile station can be almost any distance from
`the basesstation. In any case, this distance is unknown. One musttherefore
`ask:
`
`1. How does the mobile station inform the base station about its
`intention to access the network at this time?
`2. How can the collision of the signal from the mobile station with
`signals from other mobile stations due to the unknown delay time
`be avoided?
`
`In GSM, the mobile station uses the access burst for initial access to
`the network. This is much shorter than the normal burst and will thus
`
`Receive
`
`Start of transmission is put ahead
`according to the TA information
`
`Figure 1.13 The mobile station sets its start of transmission ahead accordingto the timing
`advance.
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`definitely fit into the base station’s receiver window, evenif it has been sent
`from a long distance (Figure 1.14). The mobile station always assumes the
`timing advance (iie., the distance from the network) to be zero when an
`access burst is transmitted (slotted Aloha; Chapter 2). The length of the
`access burst and the width of the receiver window on the BTSside are added
`to give the maximum radius of a base station of 35 km. According to the
`time of entry of the access burst at the respective receiver window, the base
`station estimates the distance to the mobile station and returns this value
`to the mobile station during the channel assignment. The mobile station
`then regulates its timing advance by the respective numberof bits and can
`then, from this time on, use normal bursts. Note that in GSM, without the
`so-called extended cell operation, the TA value varies, depending on the
`distance, between 0 and 634¢,.
`
`Example. The base station passes on a TA value of 26 to the mobile station.
`The mobile station then transmits not 486.75 bits, but 460.75 bits (486.75
`— 26) to the base station. There is then a 1:1 correspondence between the
`TA value and the delay time between receiving and transmitting on the
`mobilestation side.
`
`1.4.2 Timing Advance Control During a Connection
`
`During a connection,the base station receives a burst from the mobile station
`every 4.615 ms. Bursts are discussed in detail in Section 1.6.6. With the -
`help of the training sequence code (TSC) in normal burst (Figure 1.15 and
`
`Receiver window of a BTS
`
`Accessbursts
`
`Normal burst
`(fits exactly in one
`receiver window}
`
`Small, medium, maximum distance
`between BTS and mobile station
`(max. distance = 35 km}
`
`Figure 1.14 The short length of the access burst allows it to be sent from distances of
`up to 35 km.
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`The Basics: Principles of GSM and Influences on GPRS
`17
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`
`Training sequence code
`
`Normal burst (< TSC = 26 bit / variable bit pattern)
`
`Figure 1.15 The BTS measures the bit shifting of the known training sequence code in
`the uplink normal burst for determining the timing advance.
`
`Figure 1.34), familiar to both sides, the base station can use bit shifting.
`This arises due to varying distance within the training sequence code, for
`adjusting the timing advance. Note that the process presented is based on
`the periodic transmission of bursts in the uplink direction during an active
`connection. This condition also applies with an active DTX because even
`then, a burst is sent from the mobile station to the base station every 120
`ms, The question remains: How does timing advance control work in GPRS,
`which does not providefor this kind of regular transmitting? We will answer
`this question in a later chapter; our intention atthis stage is merely to point
`out the problem to the reader.
`
`1.5 Frame Hierarchy and Logical Channels in GSM
`
`As shown in Section 1.2, GSM uses TDMA as a multiple access process as
`well as SDMA and FDMA.Each frequency channel is subdividedinto eight
`independent time slots. However,a furtherstep is taken. As shown in Section
`1.2, each time slot is repeated every 4.615 ms. In order to be able to deal
`with all
`tasks,
`the different types of logical channel are placed onto the
`individual time slots. In other words, each timeslot is not occupied by the
`same logical channel type every time but is occupi