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`
`The GSM System for
`
`Mobile Communications
`
`A comprehensive overview of the
`European Digital Cellular Systems
`
`

`
`TheGSM
`S ystem for
`Mobile
`
`Communications
`
`Michel MOULY
`
`Marie—Bernadette PAUTET
`
`

`
`This book is published by the author’s company, Cell & Sys.
`Correspondence,
`in particular for the orders, but also for comments,
`should be mailed to:
`
`CELL & SYS
`
`4, rue Elisée Reclus
`F-91 120 PALAISEAU
`FRANCE
`
`Telephone : +33 1 69 31 03 18
`Facsimile : +33 1 69 31 03 38
`
`Copyright © 1992, Michel MOULY and Marie-Bernadette PAUTET
`
`All rights reserved. No part of this book may be reproduced, translated, or
`utilised in any form or any means, electronic or mechanical, including
`photocopying, recording, or any information storage or retrieval system,
`without permission in writing from the authors.
`
`All drawings are original, and all corresponding rights reserved.
`
`The name and logo GSM (and “Global System for Mobile
`communications”) are registered.
`
`International Standard Book Number: 2-9507190-0-7
`
`

`
`ARCHITECTURE
`
`89
`
`The interface between the BSS and the mobile station is the already
`introduced radio interface, whereas the interface between the BSS and the
`
`NSS has been named the A interface in the Specifications. The MS, BSS
`and NSS form the operational part of the system, whereas the OSS
`provides means for the operator to control them. This model is shown in
`figure 2.4.
`
`the interactions between the subsystems can be
`On this scale,
`grouped in two main categories. The bottom part of
`the figure
`corresponds to a chain:
`
`external networks <i$ NSS <=I> BSS ¢> MS ¢:> users
`
`whose business as a whole is to provide transmission paths and
`means to establish them. This is the operational part of the system,
`handling the telecommunications traffic. Above it we find the control
`part, composed of the OSS and the operator, which interacts with the
`traffic handling part by observing and modifying it so as to maintain or
`improve its functioning.
`
`2.1. SUB-SYSTEMS
`
`This section will deal with each subsystem in turn. The main
`purpose is to introduce a number of terms, not to exhaust the subject. The
`level of detail here is just sufficient to get a general idea of the functional
`splits. More complete architectural considerations will be found in each
`of the other chapters of the book, where a more thorough description of
`the required functions will be found.
`
`2.1.1. THE MOBILE STATION (MS)
`
`
`
`The mobile station usually represents the only equipment
`the user ever sees from the whole system. Examples
`taken from the first types of GSM mobile stations to be
`on the market are shown in figures 2.5 and 2.6. Mobile
`station types include not only vehicle-mounted and
`portable equipment, but also handheld stations, which
`will probably make up most of the market.
`
`

`
`90
`
`THE GSM SYSTEM
`
`Figure 2.5 — A GSM portable mobile station (by courtesy of Orbitel)
`
`The first GSM mobile stations on the market were portable mobile stations,
`weighing around 2 kilograms, and also capable of being installed in a vehicle.
`
` ——————
`
`W 7-—~~
`
`

`
`ARCHITECTURE
`
`91
`
`Figure 2.6 — A GSM handheld mobile station (by courtesy of Alcatel Radiotéléphone)
`
`With their ever—decreasing weight and volume,
`handheld mobile stations represent a very attractive product for the user.
`
`But what does a mobile station involve? Beside generic radio and
`processing functions to access the network through the radio interface, a
`mobile station must offer either an interface to the human user (such as a
`microphone, loudspeaker, display and keyboard for the management of
`speech calls), or an interface to some other terminal equipment (such as
`an interface towards a personal computer or a facsimile machine), or both
`of them. An effort has been made to allow off-the-shelf terminal
`
`equipment to be connected to mobile stations (for instance group 3
`facsimile machines designed for connection to the telephone network),
`and specific terminal adaptation functions have been specified for this
`purpose. However, all implementation choices are possible and left open
`to manufacturers, enabling fully integrated compact mobile stations to
`coexist with mobile stations featuring standard interfaces.
`
`

`
`92
`
`THE GSM SYSTEM
`
`Figure 2.7 — Mobile station functional architecture
`
`The mobile station may be a standalone equipment for certain services
`or support the connection of external terminals,
`either directly or through relevant adaptation functions.
`
`This leads to the identification of three main functions, as shown in
`
`figure 2.7:
`
`0
`
`0
`
`the terminal equipment, carrying out functions specific to the
`service, without any GSM-specific functions: e.g., a fax
`machine;
`
`the mobile termination, carrying out, among others,
`functions related to transmission on the radio interface;
`
`all
`
`0 possibly a terminal adaptor, which acts as a gateway between
`the terminal and the mobile termination. A terminal adaptor is
`introduced when
`the
`external
`interface of
`the mobile
`
`terminal
`a
`ISDN standard for
`the
`termination follows
`installation, and the terminal equipment has a terminal-to-
`modem interface.
`
`
`
`

`
`ARCHITECTURE
`
`93
`
`The functional split between mobile termination, terminal adaptor
`and terminal equipment is very much related to the transmission needs of
`each service, and will be detailed in Chapter 3.
`
`
`
`Another, more significant, architectural aspect of the
`mobile station relates to the concept of subscriber
`module, or SIM (Subscriber Identity Module, a slightly
`restrictive name, as more than identity is involved). As
`described in Chapter 1, the SIM is basically a smart
`card (or a cut-out thereof), following ISO standards,
`containing all the subscriber-related information stored
`on the user’s side of the radio interface. Its functionalities, besides this
`information storage capability, relate also to the confidentiality area. The
`rest of the mobile station contains all
`the generic transmission and
`signalling means to access the network. The interface between the SIM
`and the rest of the equipment is fully specified in the Specifications, and
`is simply referred to as the “SIM-ME” interface (ME stands for “mobile
`equipment”). The functions of the SIM will be studied in detail
`in
`Chapter 7.
`
`In this book as well as in the Specifications, the term Mobile
`Station (MS) shall generally include a Mobile Equipment and a SIM,
`although a rare case exists where a mobile station could be operated
`without a SIM (i.e., reduced to the mobile equipment) for the handling of
`anonymous emergency calls when so permitted by the network.
`
`The concept of a removable storage device for subscriber data has
`far-reaching consequences. In previous cellular systems, except for the
`German C-network which introduced the smart card concept at the time
`when it was making its way in GSM committees, the personalisation of
`the mobile station required a non-trivial intervention, only possible for
`technical specialists and not for the operator’s administrative clerks. This
`situation lead to several drawbacks. A mobile station could only be sold
`by specialist dealers, able not only to install the equipment in a vehicle,
`but also to act as an intermediary between the user and the service
`provider to personalise the equipment. Should the mobile station fail
`(unfortunately not such a rare event), it was difficult to provide the user
`with a replacement during the repair period, and almost impossible to
`allow the user to keep the same directory number during this time.
`
`The removable SIM simplifies these issues, and also brings other
`benefits. A potential user may of course buy a mobile equipment, but he
`may also lease or borrow it for any period of time, and change it as he
`wishes without a lot of administration. All he needs is his own SIM,
`
`obtained through an operator or a service provider, independently of any
`
`

`
`94
`
`.
`
`THE GSM SYSTEM
`
`equipment choice. The last steps of the SIM personalisation can be done
`easily through a small computer and a simple adapter. Mobile equipment
`will be for sale on a much larger-scale than ever before, since their
`acquisition will not require the intervention of an operator or a service
`provider. Car phones will still require installation in the vehicle, but
`portables or handhelds will encourage users
`to buy their mobile
`equipment from any store.
`
`More advantages can be envisaged. For instance, rented cars could
`be equipped with a mobile equipment usable with any SIM, whether user-
`owned or also rented. The reverse situation may also bring benefit to
`subscribers, if not to operators: a subscriber may change his serving
`operator without replacing his ME. But most of all, as explained in
`Chapter 1, this personal chip secured in its plastic case and called SIM is
`the first brick in the building of a personal communication system
`enabling wide-ranging mobility between different telecommunications
`networks.
`
`2.1.2. THE BASE STATION SUB-SYSTEM (BSS)
`
`
`
`Largely speaking, the Base Station Subsystem groups
`the infrastructure machines which are specific to the
`radio cellular aspects of GSM. The BSS is in direct
`contact with mobile stations through the radio interface.
`As
`such,
`it
`includes
`the machines
`in charge of
`transmission and reception on the radio path, and the
`management thereof. On the other side, the BSS is in contact with the
`switches of the NSS. The role of the BSS can‘ be summarised as to
`
`connect the mobile station and the NSS, and hence the mobile station’s
`user with other telecommunications users. The BSS has to be controlled
`and is thus also in contact with the OSS. The external interfaces of the
`
`BSS are summarised in figure 2.8.
`
`According to the canonical GSM architecture, the BSS includes
`two types of machines: the BTS (Base Transceiver Station), in contact
`with the mobile stations through the radio interface, and the BSC (Base
`Station Controller), the latter being in contact with the switches of the
`NSS. The functional split is basically between a transmission equipment,
`the BTS, and a managing equipment, the BSC.
`
`

`
`ARCHITECTURE
`
`95
`
`control flow
`'I:__._...:
`\g-——\.
`
`user data flow ‘
`
`Figure 2.8 — The external environment of the BSS
`
`The BSS bridges the space between the mobile stations on one side
`(through the radio interface), and the switching functions on the other.
`It is controlled by the operator through the OSS.
`
`
`
`A BTS comprises radio transmission and reception
`devices, up to and including the antennas, and also all
`the signal processing specific to the radio interface.
`BTSs can be considered as complex radio modems, and
`have little other function. A typical first-generation
`BTS consists of a few racks (2m high and 80 cm wide)
`containing all electronic devices necessary for
`the
`transmission functions, as shown in figure 2.9 for a GSM900 BTS and
`figure 2.10 for a DCSl800 BTS. The antennas are usually a few tens of
`meters away, on a mast, and the racks are connected to it through a feeder
`cable. A one-rack first- generation BTS is typically able to handle three to
`five
`radio carriers,
`carrying between 20
`and 40
`simultaneous
`communications. Reducing the BTS volume is important to keep down
`the cost of the cell sites, and progress can be expected in this area.
`
`An important component of the BSS, which is considered in the
`canonical GSM architecture as a part of the BTS,
`is the TRAU, or
`Transcoder/Rate Adapter Unit. The TRAU is the equipment in which the
`GSM-specific speech encoding and decoding is carried out, as well as the
`rate adaptation in case of data. Although the Specifications consider the
`
`

`
`96
`
`THE GSM SYSTEM
`
`Figure 2.9 —~ A GSM BTS (by courtesy of Motorola)
`
`A one-rack BTS, such as the one shown, is typically able to handle up to 5 carriers.
`The picture shows the rack equipped for 3 carriers.
`
`TRAU as a sub-part of the BTS, it can be sited away from the BTS, and
`even more so since in many cases it is actually between the BSC and the
`MSC. Its remote position allows the advantage of more compressed
`transmission between BTS and TRAU, and its impact will be discussed in
`detail in Chapter 3.
`
`

`
`ARCHITECTURE
`
`97
`
`Figure 2.10 - A DCSl800 BTS (by courtesy of Nokia)
`
`The small rack shown is designed to be used outside,
`typically below the antenna mast.
`
`
`
`The internal structure of the BSS is represented in
`figure 2.11. On top of the BTS, it shows the second
`“canonical” component of the BSS, the BSC. The BSC
`is in charge of all
`the radio interface management
`through the remote command of the BTS and the MS,
`mainly the allocation and release of radio channels and
`the handover management. The BSC is connected, on
`one side, to several BTSS and on the other side, to the NSS (more exactly
`to an MSC).
`
`A BSC is in fact a small switch with a substantial computational
`capability. Its main roles are the management of the channels on the radio
`interface, and of the handovers. A typical BSC consists of one or two
`racks, as shown in figure 2.12 and can manage up to some tens of BTSS,
`depending on their traffic capacity.
`’
`
`

`
`A interface
`
`ii I
`
`i
`
`2 I E
`
`98
`
`THE GSM SYSTEM
`
`~~..sLs
`
`Figure 2.11 — BSS components and interfaces
`
`The Base Station Sub—system consists of BTSs, situated on the antenna sites,
`and of BSCs, each one in control of several BTSs.
`
`The concept of the interface between BSC and MSC, called the A
`interface, was introduced fairly early in the GSM standard elaboration
`process. Only later was it decided to also standardise the interface
`between BTS and BSC, and this interface therefore bears the (not any
`more meaningful than A!) name of “Abis” interface.
`
`In the GSM vocabulary, a BSS means the set of one BSC and all
`the BTSs under its control, not to be confused with the BSS as the sub-
`
`system including all the BSCs and BTSs.
`
`

`
`ARCHITECTURE
`
`99
`
`Ii2
`
`Figure 2.12 — A GSM BSC (by courtesy of Matra Communication)
`
`The BSC shown here consists of two cabinets:
`
`a control cabinet holding the duplicated central control and switching functions,
`and a cabinet handling the interfaces.