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`Library of Congress Cataloging—in-I’ublication Data
`Heine, Gunnar.
`
`QA76.59 .H45
`004.6—dc21
`
`2003
`
`2002043670
`
`I. Sagkob, Holger.
`
`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.
`II. Title.
`III. Series.
`
`10987654321
`
`British Library Cataloguing in Publication Data
`Heine, Gunnar
`
`GPRS: gateway to third generation mobile networks. —— (Artech House mobile
`communications series)
`1. General Packet Radio Service
`I. Title
`II. Sagkob, Holger
`62138456
`
`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 resLerved. Printed and bound in the United States of America. No part 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 and retrieval 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 cannot attest to the accuracy of this information.
`Use of a term in this book should not be regarded as affecting the validity of any trademark
`or service mark.
`
`International Standard Book Number: 1—58053—159—8
`
`Library of Congress Catalog Card Number: 2002043670
`
`2 of 49
`
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`

`external networks
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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 base station
`subsystem (B55) 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 applicationwcitcuit switched and packet switched—should
`run via the mutual GSM/GPRS network.
`
`G ateway to
`external networks /
`
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`Figure 1.1 GSM network architecture.
`
`3 of 49
`
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`

`

`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 Um or air interface (Figure 1.2). Apart from transcoding rate and
`adaption unit (TRAU) framing, the BTS assumes all 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 BSC is, by definition, a circuit switching.
`exchange in 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 power control
`adjustments resulting from this.
`These regulatory functions are generally performed in their entirety by
`the BSC, although the GSM standard expressly allows preliminary prepara—
`
`Figure1.2 Principal schematic diagram of the base transceiver station.
`
`Airinterface
`
`RF-transmit
`
`RF-receive
`(RF‘RXl
`
`TRX
`_
`g“; Digitalsignal
`processing
`2} (low frequency)
`
`Transmission
`
`Ahis
`interface ,
`
`_
`0&M MDdUIeS
`
`Operation and maintenance functions/
`Clock distribution
`
`4 of 49
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`The Basics: Principles of GSM and Influences on GPRS
`
`I
`
`-
`
`l
`' Trunk
`TransmISSIOH‘; control
`1 element
`
`DB = Database
`TCE = Trunk control element
`TM =Transmission
`I
`'
`Trunk
`control ETransmission
`element
`g
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`l
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`
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`
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`
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`
`is transmitted or received. Consequently, there are channels of 16 Kbps.
`
`TRAU is speech compression from 64 Kbps to 16 Kbps (full-rate) or 8
`Kbps (half—rate). The TRAU also 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
`Of all information coming from the MSC into so—called TRAU frames. This
`Conversion is carried out for fax, data, and speech. In other words, all payload
`transfer between mobile station and TRAU takes place on the basis of TRAU
`frames. TRAU frames have a length of 320 bits. Every 20 ms a TRAU frame
`
`
`
`Abisinterface
`
`: Trunk
`Transmission: control
`l element
`: Trunk
`I
`I
`Transmissron : control
`i element
`.l
`
`T
`
`I Central module I
`
`Central functions/clock distribution
`
`Figure 1.3 Principal circuit diagram of the BSC.
`
`tion of the measuring results in the BTS. Additional BSC functions are the
`Peer function of the mobile station for the Radio Resource Management
`protocol (RR) and the resource administration on the Abis and air interface.
`The BSC, as a circuit switching network element,
`is a considerable
`hindrance to packet switched services (GPRS). Its exchange functions are
`almost unusable for packet switched services, and the RR protocol is extremely
`difficult to adjust to the requirements of packet switched services. Hence,
`if the B85 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 TRAU is the third BSS network element. The best—known task of the
`
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`GPRS: Gateway to Third Generation Mobile Networks
`
`the opportunity to save on connection costs. If the TRAU is 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 some significance, since the packet
`switched GPRS data is fed into the existing GSM network at some point.
`We shall encounter this again in Chapter 2.
`
`1.1.2 The Network Switching Subsystem
`
`As shown in Figure 1.1, the NSS consists of one or more home location
`registers (HLR) with the authentication center (AuC) and optionally with
`
`The number of actual payload bits will vary depending on the type of TRAU
`frame or application. For full-rate speech and enhanced full—rate speech, the
`TRAU frame will, for example, contain 260 bits of payload data, Whereas
`the normal data TRAU frame contains 240 payload data bits (see Section
`1.7.1).
`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
`TRAU function should be assumed to be locally situated in the BTS. Indeed,
`the GSM standard permits the integration of the TRAU into 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
`
`Figure 1.4 Possible location of the TRAU.
`
`
`
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`
`The Basics: Principles of GSM and Influences on GPRS
`
`the equipment identity register (EIR) andvarious MSCs with a connected
`visitor location register (VLR). Please note that the N55 is also used by
`GPRS, at least in part.
`
`The Home Location Register and the Authentication Center
`1.1.2.1
`The HLR is a static database in which information on hundreds of thousands
`
`
`
`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 AuC predeter-
`mines up to five so—called authentication triplets (RAND, SRES, Kc) for
`each subscriber and puts them at the disposal of the VLR responsible, Via
`
`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), which is so important in GSM,
`the HLR holds the information as to which VLR area a subscriber is currently
`registered. With the introduction of GPRS, the data on individual subscribers
`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 GPRS this time.
`The AuC, which is an integral part of the HLR, calculates the respective
`authentication results (SRES) and ciphering‘ keys
`using the algorithms
`A3 and A8 from RAND numbers (RAND = random number) and the
`
`Figure 1.5 The determination of SRES from Ki and RAND.
`
`
`
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`GPRS: Gateway to Third Generation Mobile Networks
`
`Figure 1.6 The determination of the ciphering key Kc 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 not alter 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 Kc 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
`
`an ISDN exchange that has been modified for use as a GSM-MSC. ISDN
`
`Before the introduction of GSM in the 19805, MSC and VLR were conceived
`as two independent network elements: the MSC as a network element for
`all call control (CC) functions and the VLR for the greater part of the MM
`functions. Both protocols, CC and MM, are transparent for the B88 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 found in [1, 2]. In the early 19903, 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 undersrand about GSM that the MSC is essentially
`
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`The Basics: Principles 0f GSM and Influences on GPRS
`
`exchanges, however, are circuit switched. Historically, this path was taken
`at the end of the 19805 because GSM was supposed to be as ISDN compatible
`as possible. One of the problems to be solved in this context was that existing
`exchanges, such as the Siemens EWSD System or the Alcatel System12,
`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 of this will be discussed in more detail in the
`next chapter.
`
`(IMSI)] and all his data, such as the telephone number (the MS—ISDN), are
`
`The Gateway MSC and the Inten/vorking Function
`In Figure 1.1, a typical Public Land Mobile Network (PLMN) with different
`MSCs was presented. 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 Whether all or only
`selected MSCs should have this interface function.
`On the side facing away from the PLMN of a G—MSC, there is the
`so-called interworking function (TWP), which, amongst other things, takes
`care of the rate adaptation
`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 (ISDN).
`/
`
`1.1.2.3 The Equipment Identity Register
`In contrast to the databases in GSM already described (i.e., 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 HLR is 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 19803, mobile devices and mobile tele—
`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
`
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`GPRS: Gateway to Third Generation Mobile Networks
`
`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. Many characteristics of GSM are defined in terms
`of the mobile device:
`
`separated from the identity of the device itself. In other words, theoretically,
`a stolen device could be used as early as the day of its 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 (IMEI)]. 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 mechanisms to counter the black market
`were no longer going to be of primary 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 introduce later, also
`has interfaces to the EIR for compatibility reasons.
`
`factor also reduces the complexity and costs of a GSM mobile station.
`
`° The cellular network configuration with relatively small cell sizes
`enables low transmission energy consumption on the MS side, 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.
`
`The original GSM standard did not provide for GSM mobile stations
`being able to transmit and receive simultaneously. Duplex operation
`was not envisaged. Consequently, a duplexer on the interface between
`transmitting/receiving path and antenna was not necessary. This
`
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`The Basics: Principles of GSM and Influence: on GPRS
`
`' As opposed to other network elements in the N55 and B55, detailed
`requirements, for example, of the man—machine interface (MMI),
`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.
`At first glance, this restriction may seem to be a hindrance, but it
`proves to be most advantageous in the long run because it ensures
`customer confidence and reduces significantly the number of costly
`recall campaigns.
`
`‘
`
`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 more detail later, the essential characteristics or differences between
`
`Figure1.7 Circuit diagram of a GSM mobile station.
`
`Despite thesesimplifications 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, and electrical and/or optical interfaces
`for data connections.
`
`Channel decoding
`Die-interleaving
`
`A
`Vorce
`decoding
`Channel encoding
`lnterleaving
`Burst generation
`
`SIM = subscriber identity module
`
`11 of 49
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`GPRS: Gateway to Third Generation Mobile Networks
`
`a GSM/GPRS mobile station as compared to a purely GSM mobile station
`should be outlined now:
`
`as important, the considerably lower demands on the transmitting power of '
`
`The entire geographical area is not supplied by a single transmitting station.
`The transmitting power of the individual transmitting stations is limited in
`order that a given frequency may be used again at a short distance away.
`As illustrated in Figure 1.8, however, SDMA 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
`
`° New protocol stack: support of new protocols in the radio resource,
`mobility management, and session management areas;
`
`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 demands on the bat—
`tery capacity must also be considered;
`
`Data services require a new MMI. For instance, 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 oner any speech
`services at all;
`
`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
`
`As a second generation mobile communication network, GSM uses the three
`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 not alter this not many
`other basic GSM processes.
`
`1.2.1
`
`SDMA
`
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`The Basics: Principles of GSM and Influence: on GPRS
`
`fill
`.‘ Frequency]
`
`it is 45 MHz. A serious disadvantage of the usual FDlVLA systems is the
`
`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. Each user is 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 one for 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
`PC51900, the GSM variant used in the United States, this distance between
`the uplink and downlink channels is exactly 80 MHZ, whereas in P—GSM900,
`
`Figure 1.8 The SDMA multiple access process gives rise 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 connect the individual transmitters to each other and to enable
`standard functions such as roaming and handover.
`
`1.2.2
`
`FDMA
`
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`GPRS: Gateway to Third Generation Mobile Networks
`
`Duplex distance:
`
`P-GSM = 45 MHZ
`
`PCS 1900 = 80 MHz
`
`and independently from one another. It must be stressed again, however,
`
`frequency channel exclusively 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 channel is also
`subdivided temporally and each subscriber receives access rights to the fre-
`quency channel during a connection for a 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 thesetime
`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 0f576.9 ,us (= 577 M5)
`or 156.25 bits and is repeated every 4.615 ms. According to this definition, up
`to eight users in GSM can use one frequency channel almost simultaneously
`
`Uplinkfrequency channel
`200 kHz
`
`Downlink frequency channel
`200 kHz
`
`Figure1.9 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 frequency division duplex (FDD) systems. This requirement
`is of particular disadvantage because frequency is a rare resource, as the
`bidding for the UMTS licenses 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 F DMA, the available frequency range is divided into individual frequency
`channels. When there is an active connection, a subscriber receives this
`
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`

`r—'———_————"
`
`The Basics: Principles of GSM and Influences on GPRS
`
`Ultl
`
`TDMA
`Each frequency channel is divided
`
`/ into a number oftime slots
`
`Frequency
`Time slots
`
`f4
`
`f5
`
`
`
`In GSM, the base 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
`
`f6
`
`FDMA
`The available frequency range
`is divided into different channels
`
`Figure 1.10 The combination of FDMA and TDMA.
`
`+i 577 risk-
`
`Frequency
`
`TSU
`
`m
`
`T87
`
`T37
`
`T85
`
`T86
`
`:‘_______ TDMA-frame _______._.>:
`4.615 ms
`
`Figure 1.11 The combination of FDMA and TDMA in GSM.
`
`that Figure 1.11 only represents one direction, but two frequencies are
`required for a bidirectional connection, in which the same time slot is used.
`
`1.3 Chronological Sequence of 'Uplink and Downlink
`Transmission
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`14
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`GPRS: Gateway to Third Generation Mobile Networks
`
`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, these. impulses are called bursts.
`One of the main problems of TDMA systems, which must not be neglected,
`is the delay time that it takes to transmit a burst from transmitter to receiver.
`In the direction from the base station to the mobile station (downlink),
`there are no problems in this respect as every mobile station can receive its
`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
`
`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 and receives 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.
`One should also consider Figure 1 . 12 from the point ofview ofmultislot
`transmission. If simultaneous transmission and reception on the side of the
`mobile stations is to be avoided, there are only a few uplink/downlink time
`slot combinations available that can avoid this problem. It is then actually
`impossible to provide an individual user with more than four time slots 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.
`
`Figure 1.12 Synchronization of downlink and uplink transmission in GSM.
`
`Downlink
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`The Basics: Principle: of GSM and Influence: on GPRS
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`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 bases station. In any case, this distance is unknown. One must therefore
`ask:
`
`. How does the mobile station inform the base station about its
`intention to access the network at this time?
`
`. 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 bum: and will thus
`
`between the mobile stations and the base 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 a live connection. Otherwise, mobile stations
`would have to be prohibited from moving at all during an active connection.
`In GSM delay time control is spoken of as timing advance (TA) control.
`
`advance.
`
`Receive
`
`T5 5
`
`Start of transmission is put ahead
`according to the TA information
`
`Figure 1.13 The mobile station sets its start of transmission ahead according to the timing
`
`
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`GPRS: Gateway to Third Generation Mobile Networks
`
`definitely fit into the base station’s receiver window, even if it has been sent
`from a long distance (Figure 1.14). The mobile station always assumes the
`timing advance (i.e., 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 BTS side 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
`
`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
`mobile station 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
`
`to the mobile station during the channel assignment. The mobile station
`then regulates its timing advance by the respective number of 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 63dez.
`
`up to 35 km.
`
`Access bursts
`
`Normal burst
`(fits exactly In one
`receiver wmdow)
`
`Small, medium, maximum distance
`between BTS and mobile station
`(max. distance 2 35 km)
`
`Figure 1.14 The short length of the access burst allows it to be sent from distances of
`
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`The Basics: Principles of GSM and Influence: on GPRS
`17
`
`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 provide for this kind of regular transmitting? We Will answer
`this question in a later chapter; our intention at this stage is merely to point
`out the problem to the reader.
`
`1.5 Frame Hierarchy and Logical Channels in GSM
`
`the so-called Carrier 0 (C0; BCCH carrier):
`
`As shown in Section 1.2, GSM uses TDMA as a multiple access process as
`well as SDMA and FDMA. Each frequency channel is subdivided into eight
`independent time slots. However, a further step 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 time slot is not occupied by the
`same logical channel type every time but is occupied by different logical
`Channel types in sequence. This principle especially applies to the different
`signaling channels, and also to the traffic channels and their associated c