© Copyright 2002 AIRCOM International LtdAll rights reservedAIRCOM Training is committed to providing our customers with q...
Table of Contents1. Introduction to Cellular Systems     1.1   Introduction .................................................
7. Mobility Management     7.1    Introduction ..............................................................................
Course Objectives and Structure                                        Course Objectives                  • Be familiar wi...
Intentional Blank Page                              GSM Technology for Engineersiv                             © AIRCOM In...
1. Introduction to Cellular Systems1. Introduction to CellularSystems_____________________________________________________...
1. Introduction to Cellular Systems_________________________________________________________________________________1.2 Ce...
1. Introduction to Cellular Systems                                               Cellular Generations                  Th...
1. Introduction to Cellular Systems               In early networks, the emphasis was to provide radio coverage with littl...
1. Introduction to Cellular Systems                                 The First Generation1.4 2nd Generation Cellular System...
1. Introduction to Cellular Systems               While first generation systems used a cellular structure and frequency r...
1. Introduction to Cellular Systems1.5 GSM Development and Characteristics              Originally GSM referred to the Eur...
1. Introduction to Cellular Systems1.6 Cellular Systems Subscriber Distribution                         Worldwide Mobile C...
1. Introduction to Cellular Systems1.7 2.5G Characteristics                                             2.5G Characteristi...
1. Introduction to Cellular Systems                                                      Summary                       Thi...
2. Architecture Overview2. Architecture Overview__________________________________________________________________________...
2. Architecture Overview________________________________________________________________________________2.2 GSM Identifier...
2. Architecture Overview              2.2.5 MSRN – MOBILE STATION ROAMING NUMBER              The MSRN is a temporary, loc...
2. Architecture Overview               A GSM network is made up of three subsystems:                  • The Mobile Station...
2. Architecture Overview              The two parts of the mobile station allow a distinction between the actual equipment...
2. Architecture Overview               The IMEI may be used to block certain types of equipment from accessing the network...
2. Architecture Overview                                           BSS Network Topologies                     • Chain: che...
2. Architecture Overview               The NSS combines the call routing switches (MSCs and GMSC) with database registers ...
2. Architecture Overview                                    Visitor Location Register (VLR)                       • Each M...
2. Architecture Overview                                    Home Location Register (HLR)                      • Stores det...
2. Architecture Overview              The data base can also contain other information such as:                  • teleser...
2. Architecture Overview               The GMSC routes calls out of the network and is the point of access for calls enter...
2. Architecture Overview________________________________________________________________________________2.7 GSM Interfaces...
2. Architecture Overview               This procedure occurs whenever an MS roams to another location area. Also, when a  ...
2. Architecture Overview              2.7.9 THE H (HLR-AuC) INTERFACE              When an HLR receives a request for auth...
2. Architecture Overview                           Intentional Blank Page                                                 ...
2. Architecture OverviewSection 2                  Self-Assessment ExercisesExercise 2.1 – GSM ArchitectureThe following e...
3. GSM Services 3. GSM Services_____________________________________________________________________3.1 Introduction      ...
3. GSM Services                                                      GSM Services                                         ...
3. GSM Services              GSM bearer services refer to those services provided to transport binary data across the GSM ...
3. GSM Services_____________________________________________________________________3.3 Teleservices             GSM defin...
3. GSM Services              The teleservices introduced as part of Phase 1 GSM included:                  • Full rate spe...
3. GSM Services             Supplementary Service Groups                        Supplementary Service Group             GS...
3. GSM Services                                           Circuit Switched Data Services                                  ...
3. GSM Services             SMS is classified as a GSM Teleservice and three SMS teleservices (TS21-23) have been         ...
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Aircom gsm training manual

  1. 1. © Copyright 2002 AIRCOM International LtdAll rights reservedAIRCOM Training is committed to providing our customers with quality instructor ledTelecommunications Training.This documentation is protected by copyright. No part of the contents of thisdocumentation may be reproduced in any form, or by any means, without the priorwritten consent of AIRCOM International.Document Number: P/TR/003/K011/4.4This manual prepared by: AIRCOM International Grosvenor House 65-71 London Road Redhill, Surrey RH1 1LQ ENGLAND Telephone: +44 (0) 1737 775700 Support Hotline: +44 (0) 1737 775777 Fax: +44 (0) 1737 775770 Web: http://www.aircom.co.uk GSM TECHNOLOGY FOR ENGINEERS
  2. 2. Table of Contents1. Introduction to Cellular Systems 1.1 Introduction ...........................................................................................................1 1.2 Cellular Radio ........................................................................................................2 st 1.3 1 Generation Cellular Systems..............................................................................3 nd 1.4 2 Generation Cellular Systems .............................................................................5 1.5 GSM Development and Characteristics...................................................................7 1.6 Cellular Systems Subscriber Distribution .................................................................8 1.7 2.5G Characteristics ..............................................................................................92. Architecture Overview 2.1 Introduction ......................................................................................................... 11 2.2 GSM Identifiers .................................................................................................... 12 2.3 General Architecture Overview ............................................................................. 13 2.4 The GSM Mobile Station (MS).............................................................................. 14 2.5 The Base Station Subsystem (BSS). ..................................................................... 16 2.6 The Network Switching Subsystem (NSS) ............................................................. 17 2.7 GSM Interfaces.................................................................................................... 22 Self-Assessment Exerecises ................................................................................ 273. GSM Services 3.1 Introduction ........................................................................................................ 29 3.2 GSM Bearer Services........................................................................................... 30 3.3 Teleservices ........................................................................................................ 32 3.4 Supplementary Services....................................................................................... 33 3.5 Circuit-Switched Data (CSD) Services ................................................................... 34 3.6 Short Message Service ....................................................................................... 35 3.7 USSD Data Services ............................................................................................ 37 Self-Assessment Exerecises ................................................................................ 414. The Air Interface 4.1 Introduction ......................................................................................................... 43 4.2 GSM Frequency Spectrum Allocation .................................................................... 44 4.3 GSM Multiple Access Techniques ......................................................................... 50 4.4 GSM Air Interface Channels ................................................................................. 52 4.5 Frames and Multiframes ....................................................................................... 56 Self-Assessment Exercises .................................................................................. 635. Protocols 5.1 Introduction ......................................................................................................... 65 5.2 The ISO 7-Layer OSI Model ................................................................................. 66 5.3 GSM Protocols Overview ..................................................................................... 67 5.4 GSM Transmission Protocols................................................................................ 69 5.5 GSM Signalling Protocols ..................................................................................... 71 5.6 GSM Air (Um) Interface Protocols ......................................................................... 73 5.7 GSM A-bis Interface Protocols .............................................................................. 75 5.8 The GSM A Interface Protocols............................................................................. 786. Speech and Channel Coding 6.1 Introduction ......................................................................................................... 79 6.2 Speech Coding Techniques .................................................................................. 80 6.3 GSM Speech Coding .......................................................................................... 81 6.4 Channel Coding Techniques ................................................................................. 83 6.5 GSM Channel Coding. ......................................................................................... 88 6.6 Interleaving.......................................................................................................... 89 6.7 Radio Burst Multiplexing…… .............. ………………………………………………..91 6.8 Summary of Coding Processes ………… ……………………………………………95 6.9 Radio Interface Modulation….. …………… ……………………………………………95GSM Technology for Engineers© AIRCOM International 2002 i
  3. 3. 7. Mobility Management 7.1 Introduction ......................................................................................................... 99 7.2 Mobility Management Procedures ....................................................................... 100 7.3 Network Areas ................................................................................................... 101 7.4 Mobility States ................................................................................................... 103 7.5 IMSI Attach/Detach............................................................................................ 104 7.6 Location Updating .............................................................................................. 107 7.7 Roaming ........................................................................................................... 109 7.8 TMSI Reallocation ............................................................................................. 1108. Radio Resource Management 8.1 Introduction ....................................................................................................... 113 8.2 Radio Resource Connection Setup ..................................................................... 114 8.3 Cell Selection and Reselection ........................................................................... 115 8.4 Handovers......................................................................................................... 1189. Call Management 9.1 Introduction ....................................................................................................... 123 9.2 Mobile Originated Calls ...................................................................................... 124 9.3 Mobile Terminated Calls..................................................................................... 125 9.4 Call Routing Examples ....................................................................................... 127 9.5 Echo Cancelling................................................................................................. 129 Self-Assessment Exercises ................................................................................ 13110. GSM Security 10.1 Introduction ..................................................................................................... 133 10.2 Purposes of GSM Security................................................................................ 134 10.3 User Identity Confidentiality .............................................................................. 134 10.4 Authentication.................................................................................................. 135 10.5 User Data Confidentiality - Encryption ............................................................... 138 10.6 Signalling Data Confidentiality........................................................................... 140 Self-Assessment Exercises .............................................................................. 14311. Billing Procedures Overview 11.1 Introduction ..................................................................................................... 145 11.2 Billing Principles............................................................................................... 146 11.3 Description of Call Components ........................................................................ 146 11.4 Charge Advice Information (CAI)....................................................................... 148 11.5 Advice of Charge (AoC) Calculations................................................................. 149 11.6 Call Detail Records .......................................................................................... 151 11.7 The Transferred Account Procedure (TAP) ........................................................ 15212. GSM Evolution 12.1 Introduction ..................................................................................................... 155 12.2 High Speed Circuit Switched Data (HSCSD) ...................................................... 156 12.3 General Packet Radio Service (GPRS).............................................................. 157 12.4 Enhanced Data for nGSM Evolution (EDGE) ..................................................... 159Appendix A - Solutions to Self Assessment ExercisesAppendix B - Glossary of Terms GSM Technology for Engineersii © AIRCOM International 2002
  4. 4. Course Objectives and Structure Course Objectives • Be familiar with the development of 2G mobile systems • Describe the architecture of a GSM network • Appreciate the main services provided within a GSM network • Understand the various facets of the GSM air interface including, Access structures, frequency allocations, physical and logical channels • Appreciate GSM transmission and signalling protocols • Describe the methods of speech and error coding on the air interface • Understand the principals mobility management • Understand the principles of radio resource management • Describe the processes involved in connection management • Appreciate the security measures implemented by GSM • Understand GSM mechanisms for customer billing • Be familiar with the future evolution of GSM including 2G+ technologies • Describe the evolution of GSM towards 3G systems Course Outline Day 1 Day 2 1. Introduction to Cellular Systems 7. Radio Resource Management 2. Architecture Overview 8. Mobility Management 3. Services 9. Connection Management 4. The Air Interface 10. Security 5. GSM Protocols 11. Billing Procedures Overview 6. Speech and Channel Coding 12. GSM EvolutionGSM Technology for Engineers© AIRCOM International 2002 iii
  5. 5. Intentional Blank Page GSM Technology for Engineersiv © AIRCOM International 2002
  6. 6. 1. Introduction to Cellular Systems1. Introduction to CellularSystems_________________________________________________________________________________1.1 Introduction This section reviews the characteristics of first and second generation cellular systems and is followed by an overview of the functional blocks of GSM architecture, and its functional entities. Topics covered include: • Cellular Radio • 1 st generation cellular characteristics and systems • 2 nd generation cellular characteristics and non-GSM systems • GSM development and characteristics • Worldwide roll-out of cellular systems and subscriber distributionGSM Technology for Engineers© AIRCOM International 2002 1
  7. 7. 1. Introduction to Cellular Systems_________________________________________________________________________________1.2 Cellular Radio Mobile Radio Networks Cellular • There are three major types of terrestrial mobile communications technologies: • Paging: Brief numeric, alphanumeric or voice messages PSTN are sent to the subscriber typically using simultaneous broadcasting. MSC • Cordless Communication: Users are provided limited mobility from a dedicated base station Cordless Paging Control Centre • Cellular: Users are provided wide area mobility from multiple base stations with handover permitted Paging Area Paging Area Cellular Radio Network Characteristics • Mobile Radio Network based on cellular structure for areas coverage • Frequency reuse required due to limited frequency availability (frequency re-use patterns) • Mobility between cell areas possible (handover) GSM Technology for Engineers2 © AIRCOM International 2002
  8. 8. 1. Introduction to Cellular Systems Cellular Generations The significant stages in the evolution of Data rate cellular radio systems is referred to in terms of generations: • 1st Generation or 1G • 2nd Generation or 2G • 2.5G • 3rd Generation or 3G • 4th Generation or 4G 1978 1992 2000 2002 ? time Progress of data rates with time and generation_________________________________________________________________________________1.3 1st Generation Cellular Systems 1st Generation Cellular Characteristics • Widespread Introduction in early 1980s • Analogue modulation • Frequency Division Multiple Access • Voice traffic only The 1st Generation of Cellular • No inter-network roaming possible Technology makes use of analogue modulation techniques such as FM • Insecure air interfaceGSM Technology for Engineers© AIRCOM International 2002 3
  9. 9. 1. Introduction to Cellular Systems In early networks, the emphasis was to provide radio coverage with little consideration for the number of calls to be carried. As the subscriber base grew, the need to provide greater traffic capacity had to be addressed. 1st Generation Standards • AMPS (Analogue Advanced Mobile Phone System) • North American Standard in cellular band (800MHz) • TACS (Total Access Communications System) • UK originated Standard based on AMPS in 900MHz band • NMT (Nordic Mobile Telephony System) • Scandinavian Standard in 450MHz and 900MHz bands • C-450 • German Standard in 450MHz band Analogue/AMPS • JTACS (Japanese Total Access Communications System) Nokia 252 • Japanese Standard in 900MHz band 1st Generation Planning • Macrocellular • High sites for coverage driven planning • Antennas above roof height • Frequency planning required F1 • For networks with more cells than frequencies these must be planned F2 F5 • Large cell size • Order 30km F4 • Hard handover F3 • Mobile only ever connected to a single cell • Hexagonal Grid Representation The above diagram shows how different frequencies are used in different cells in a cellular network GSM Technology for Engineers4 © AIRCOM International 2002
  10. 10. 1. Introduction to Cellular Systems The First Generation1.4 2nd Generation Cellular Systems Digital systems offer considerable advantages in terms of capacity and security and introduce new possibilities for data traffic. 2nd Generation Characteristics • Widespread Introduction in 1990’s • Uses digital modulation • Variety of multiple access strategies • More efficient use of radio spectrum 1 000 100111 • Voice and low rate circuit switched data 00 01 10 • International roaming capability 1 00 0 0 1 0100 010010 11100110010 110 00 1011 0 • Secure air interface 0 0 1110 11 0 • Compatibility with ISDNGSM Technology for Engineers© AIRCOM International 2002 5
  11. 11. 1. Introduction to Cellular Systems While first generation systems used a cellular structure and frequency re-use patterns, digital systems developed this concept to include multi-layer cellular patterns (microcells and macrocells). The greater immunity to interference inherent in digital transmission allowed tighter frequency re-use patterns to be implemented. cdmaOne Characteristics • First networks in 1996 • Derived from Qualcomm IS-95 air interface • Largely American subscriber base with some Asian networks • Code Division Multiple Access • The closest 2nd generation standard to many of the 3rd generation standards • ANSI-41 core network • Chip rate of 1.2288Mcps cdmaOne phones D-AMPS/PDC Characteristics • TDMA (D-AMPS) • PDC • North American TDMA/FDMA based • Japanese TDMA/FDMA based standard based upon AMPS standard • Predominantly used in North and • Predominantly used in Asia South America • Planning Similar to GSM • ANSI-41 Core Network • Planning Similar to GSM TDMA and PDC phones GSM Technology for Engineers6 © AIRCOM International 2002
  12. 12. 1. Introduction to Cellular Systems1.5 GSM Development and Characteristics Originally GSM referred to the European working party set up to establish a new standard (‘Groupe Speciale Mobile’) but was later amended to reflect a more global application (Global System for Mobile communications). Cellular Structure • The aim of a cellular system is to make best use of the available frequencies (spectrum) • The cellular structure allows the re-use of frequencies across the network • Planning the pattern of this frequency re-use is a key part of the system design • Hexagonal pattern best represents interlocking grid of cells GSM Planning GSM Planning • Key Network planning parameters • Coverage – getting a usable radio signal to all areas in the network • Capacity – handling the call traffic generated by the subscribers F1 • Quality – low interference, few calls dropped etc. F2 • Optional parameters requiring planning F5 • Hierarchical Cell Structures (macrocell/microcell) F4 • Frequency Hopping F3 • Discontinuous Transmission • Power Control • Subscriber/traffic analysis Cellular networks can use microcells to • Capacity limited by number of TRX’s provide additional capacityGSM Technology for Engineers© AIRCOM International 2002 7
  13. 13. 1. Introduction to Cellular Systems1.6 Cellular Systems Subscriber Distribution Worldwide Mobile Communications Systems Subscribers (x million) Second Generation - DAMPS 700 600 Second Generation - PDC 500 400 Second Generation - GSM 300 200 Second Generation - cdmaOne 100 0 01 First Generation - Analogue 91 93 95 97 99 20 19 19 19 19 19 Source:Wideband CDMA for 3rd Generation Mobile Communications, Artech House, 1998 Worldwide Mobile Subscriber Distribution Subscribers (x million) 2000 European Union Countries 1500 North America 1000 Asia Pacific 500 Rest of World 0 1995 2000 2005 2010 Source: Third Generation Mobile Communications Artech House, 2000 GSM Technology for Engineers8 © AIRCOM International 2002
  14. 14. 1. Introduction to Cellular Systems1.7 2.5G Characteristics 2.5G Characteristics • Available now... • Digital modulation • Voice and intermediate rate circuit/packet switched data • 2G technology roaming • Secure air interface • Based upon existing dominant 2G standards such as GSM and cdmaOne • Enhanced data ratesGSM Technology for Engineers© AIRCOM International 2002 9
  15. 15. 1. Introduction to Cellular Systems Summary This Section has covered: • Cellular Radio in context with other wireless systems • Cellular Radio Characteristics • 1st Generation Cellular Systems • 2nd Generation Cellular Systems • GSM-specific Characteristics • Cellular User Distribution • 2.5 G Characteristics GSM Technology for Engineers10 © AIRCOM International 2002
  16. 16. 2. Architecture Overview2. Architecture Overview_________________________________________________________________________________2.1 Introduction This section of the course provides an overview of the GSM network architecture. This includes a brief explanation of the different network subsystems and a description of the functionality of the elements within each of the se subsystems. Topics include: • A General architecture overview • The Mobile Station (MS) Subsystem and Elements • The Base Station Subsystem (BSS) and Elements • The Network Subsystem (NSS) and Elements • Introduction to network interfacesGSM Technology for Engineers© AIRCOM International 2002 11
  17. 17. 2. Architecture Overview________________________________________________________________________________2.2 GSM Identifiers GSM Identifiers • IMEI – International Mobile Equipment Identifier • IMSI – International Mobile Subscriber Identifier • TMSI –Temporary Mobile Subscriber Identity • MSISDN – Mobile Subscriber ISDN number • MSRN – Mobile Station Roaming Number • LAI – Location Area Identity • CI – Cell Identifier • BSIC – Base Station Identity Code 2.2.1 IMEI – INTERNATIONAL MOBILE EQUIPMENT IDENTIFIER. The IMEI is an internationally-unique serial number allocated to the MS hardware at the time of manufacture. It is registered by the network operator and (optionally) stored in the AuC for validation purposes. 2.2.2 IMSI – INTERNATIONAL MOBILE SUBSCRIBER IDENTIFIER When a subscriber registers with a network operator, a unique subscriber IMSI identifier is issued and stored in the SIM of the MS. An MS can only function fully if it is operated with a valid SIM inserted into an <MS with a valid IMEI. 2.2.3 TMSI –TEMPORARY MOBILE SUBSCRIBER IDENTITY A TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) update takes place. At the MS it is stored in the MS’s SIM. The issued TMSI only has validity within a specific LA. 2.2.4 MSISDN – MOBILE SUBSCRIBER ISDN NUMBER The MSISDN represents the ‘true’ or ‘dialled’ number associated with the subscriber. It is assigned to the subscriber by the network operator at registration and is stored in the SIM. It is possible for an MS to hold multiple MSISDNs, each associated with a different service. GSM Technology for Engineers12 © AIRCOM International 2002
  18. 18. 2. Architecture Overview 2.2.5 MSRN – MOBILE STATION ROAMING NUMBER The MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the MS. The MSRN is used by the VLR-associated MSC for call routing within the MSC/VLR service area. 2.2.6 LAI – LOCATION AREA IDENTITY Each Location Area within the PLMN has an associated internationally-unique identifier (LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH), thus uniquely identifying each cell with an associated LA. The purpose of LAs is covered later in this course. 2.2.7 CI – CELL IDENTIFIER The CI an identifier assigned to each cell within a network. However, the CI is only unique within a specific Location Area. When combined with the internationally unique LAI for its associated LA, the Global Cell Identity (GCI) is produced which is also internationally unique. 2.2.8 BSIC – BASE STATION IDENTITY CODE Each BTS is issued with a unique identity, the BSIC and is used to distinguish neighbouring BTSs._________________________________________________________________________________2.3 General Architecture Overview GSM Architecture Overview Air Interface Air Abis Interface Abis Interface A Interface A Interface (Um) (Um) OMC MS VLR VLR BSS HLR HLR MS TRX MSC AuC AuC BTS BTS BSC MS EIR NSS NSS PSTNGSM Technology for Engineers© AIRCOM International 2002 13
  19. 19. 2. Architecture Overview A GSM network is made up of three subsystems: • The Mobile Station (MS) • The Base Station Sub-system (BSS) – comprising a BSC and several BTSs • The Network and Switching Sub-system (NSS) – comprising an MSC and associated registers The interfaces defined between each of these sub systems include: • A interface between NSS and BSS • Abis interface between BSC and BTS (within the BSS) • Um air interface between the BSS and the MS Abbreviations: MSC – Mobile Switching Centre BSS – Base Station Sub-system BSC – Base Station Controller HLR – Home Location Register BTS – Base Transceiver Station VLR – Visitor Location Register TRX – Transceiver AuC – Authentication Centre MS – Mobile Station EIR – Equipment Identity Register OMC – Operations and Maintenance Centre PSTN – Public Switched Telephone Network_________________________________________________________________________________2.4 The GSM Mobile Station (MS) The Mobile Station (MS) consists of the physical equipment used by a PLMN subscriber to connect to the network. It comprises the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). The ME forms part of the Mobile Termination (MT) which, depending on the application and services, may also include various types of Terminal Equipment (TE) and associated Terminal Adapter (TA). GSM Mobile Terminal (MT) Reference Points R S Um m A Interface Interface Interface Interface TA Base Station GSM Core Subsystem Network SIM ME TE MS Mobile Terminal (MT) Mobile Terminal (MT) TE - Terminal Equipment TA - Terminal Adaptor MS - Mobile Station ME - Mobile Equipment SIM - Subscriber Identity Module GSM Technology for Engineers14 © AIRCOM International 2002
  20. 20. 2. Architecture Overview The two parts of the mobile station allow a distinction between the actual equipment and the subscriber who is using it. The IMSI identifies the subscriber within the GSM network while the MS ISDN is the actual telephone number a caller (possibly in another network) uses to reach that person. The Mobile Station (MS) • The mobile station consists of: • mobile equipment (ME) • subscriber identity module (SIM) • The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including: • The International Mobile Subscribers Identity (IMSI) • MS ISDN number of subscriber • Authentication key (Ki) and algorithms for authentication check • The mobile equipment has a unique International Mobile Equipment Identity (IMEI), which is used by the EIR Security is provided by the use of an authentication key (explained later in this section) and by the transmission of a temporary subscriber identity (TMSI) across the radio interface where possible to avoid using the permanent IMSI identity. Mobile Station Power Classes Full Rate MS Class Power mW dBm GSM class 2 960 39 (8W) GSM class 3 600 37 (5W) GSM class 4 240 33 (2W) GSM class 5 96 29 (0.8W) DCS class 1 120 30 (1W) DCS class 2 30 24 (.25W) DCS class 3 480 36 (4W) GSM class 1 – deleted under GSM Phase 2 Specification Power ( mW) = Nominal maximum mean power output (milliwatts) Power (dBm) = Maximum power output in dBm (+watts) Source: ETSI GSM 02.06 (Version 4.5.2)GSM Technology for Engineers© AIRCOM International 2002 15
  21. 21. 2. Architecture Overview The IMEI may be used to block certain types of equipment from accessing the network if they are unsuitable and also to check for stolen equipment. A number of GSM terminal types are defined within the GSM Specification. They are distinguished primarily by their power output rating. Mobile terminals are only specified for GSM classes 3 and 4 and DCS classes 1 and 2. The other classes are intended for static or vehicle-mounted installations_________________________________________________________________________________2.5 The Base Station Subsystem (BSS) The Base Station System (BSS) is the system of base station equipments (transceivers, controllers, etc) which is viewed by the MSC through a single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS may support one or more cells. A BSS may consist of one or more base stations, where an A-bis-interface is implemented. The BSS consists of one Base Station Controller (BSC) and one or more Base Transceiver Station (BTS). The Base Station Sub-System (BSS) Sub- • The BSS comprises: • Base Station Controller (BSC) • One or more Base Transceiver Stations (BTSs) BSS • The purpose of the BTS is to: • provide radio access to the mobile stations • manage the radio access aspects of the system BTS • BTS contains : • Radio Transmitter/Receiver (TRX) • Signal processing and control equipment BSC BSC • Antennas and feeder cables BTS • The BSC: • allocates a channel for the duration of a call BTS • maintains the call: • monitors quality BTS • controls the power transmitted by the BTS or MS • generates a handover to another cell when required • Siting of the BTS is crucial to the provision of acceptable radio coverage A Base Station Controller (BSC) is a network component in the PLMN with the functions for control of one or more BTS. A Base Transceiver Station (BTS) is a network component which serves one cell. GSM Technology for Engineers16 © AIRCOM International 2002
  22. 22. 2. Architecture Overview BSS Network Topologies • Chain: cheap, easy to implement • One link failure isolates several BTSs BSC • Ring: Redundancy gives some protection if a link fails • More difficult to roll-out and extend • ring must be closed BSC • Star: most popular configuration for first GSM systems • Expensive as each BTS has its own link BSC • One link failure always results in loss of BTS Base stations are linked to the parent BSC in one of several standard network topologies. The actual physical link may be microwave, optical fibre or cable. Planning of these links may be done using a tool such as Connect2.6 The Network Switching Subsystem (NSS) 2.6.1 NSS OVERVIEW Network Switching System (NSS) PSTN/ISDN • Key elements of the NSS: VLR MSC GMSC • Mobile Switching Centre (MSC) with: • Visitor Location Register (VLR) SS7 • Home Location Register (HLR) with: Network • Authentication Centre (AuC) AuC • Equipment Identity Register (EIR) EIR HLR • Gateway MSC (GMSC) • These elements are interconnected by means of an SS7 networkGSM Technology for Engineers© AIRCOM International 2002 17
  23. 23. 2. Architecture Overview The NSS combines the call routing switches (MSCs and GMSC) with database registers required to keep track of subscribers’ movements and use of the system. Call routing between MSCs is taken via existing PSTN or ISDN networks. Signalling between the registers uses Signalling System No. 7 protocol. 2.6.2 THE MOBILE SWITCHING CENTRE (MSC) The Mobile-services Switching Centre is an exchange which performs all the switching and signalling functions for mobile stations located in a geographical area designated as the MSC area. The main difference between a MSC and an exchange in a fixed network is that the MSC has to take into account the impact of the allocation of radio resources and the mobile nature of the subscribers and has to perform in addition, at least the following procedures: • procedures required for location registration (details in GSM 03.12); • procedures required for handover (details in GSM 03.09). Mobile Switching Centre (MSC) Functions of the MSC: • Switching calls, controlling calls and logging calls • Interface with PSTN, ISDN, PSPDN • Mobility management over the radio network and other networks • Radio Resource management - handovers between BSCs VLR • Billing Information MSC 2.6.3 THE VISITOR LOCATION REGISTER (VLR) A Visitor Location Register is a database serving temporary subscribers within an MSC area. Each MSC in the network has an associated VLR but a VLR may serve many MSCs. A mobile station roaming in an MSC area is controlled by the VLR associated with that MSC. When a Mobile Station (MS) enters a new location area it starts a registration procedure. The MSC in charge of that area notices this registration and transfers the identity of the location area where the MS is situated to the VLR. If this MS is no yet registered, the VLR and the HLR exchange information to allow the proper handling of calls involving the MS. GSM Technology for Engineers18 © AIRCOM International 2002
  24. 24. 2. Architecture Overview Visitor Location Register (VLR) • Each MSC has a VLR • VLR stores data temporarily for mobiles served by the MSC • Information stored includes: • IMSI VLR • MSISDN • MSRN MSC • TMSI • LAI • Supplementary service parameters The VLR contains also the information needed to handle the calls set-up or received by the MSs registered in its data base. The following elements are included: • the International Mobile Subscriber Identity (IMSI); • the Mobile Station International ISDN number (MSISDN); • the Mobile Station Roaming Number (MSRN) • the Temporary Mobile Station Identity (TMSI), if applicable; • the Local Mobile Station Identity (LMSI), if used; • the location area where the mobile station has been registered. This data item will be used to call the station. 2.6.4 THE HOME LOCATION REGISTER (HLR) The HLR is a database in charge of the management of mobile subscribers. A PLMN may contain one or several physical HLRs depending on the number of mobile subscribers, the capacity of the equipment and the organization of the network. However, even if the HLR comprises geographically separated hardware, it logically forms a single virtual database. Two kinds of information are stored there: • the subscription information; • location information enabling the charging and routing of calls towards the MSC where the MS is located (e.g. the MS Roaming Number, the VLR address, the MSC address, the Local MS Identity). Two types of number are attached to each mobile subscription and are stored in the HLR: • the International Mobile Station Identity (IMSI); • one or more Mobile Station International ISDN number(s) (MSISDN).GSM Technology for Engineers© AIRCOM International 2002 19
  25. 25. 2. Architecture Overview Home Location Register (HLR) • Stores details of all subscribers in the network , such as: • Subscription information • Location information: mobile station roaming number, VLR, MSC • International Mobile Subscriber Identity (IMSI) • MS ISDN number AuC • Tele-service and bearer service subscription information • Service restrictions • Supplementary services HLR • Together with the AuC, the HLR checks the validity and service profile of subscribers The IMSI or the MSISDN may be used as a key to access the information in the database for a mobile subscription. HLR Implementation • One HLR in a network • May be split regionally • Stores details of several thousand subscribers • Stand alone computer - no switching capabilities • May be located anywhere on the SS7 network AuC • Combined with AuC HLR GSM Technology for Engineers20 © AIRCOM International 2002
  26. 26. 2. Architecture Overview The data base can also contain other information such as: • teleservices and bearer services subscription information; • service restrictions (e.g. roaming limitation); • supplementary services; the HLR contains the parameters attached to these services. Supplementary services parameters need not all be stored in the HLR. However, it is considered safer to store all subscription parameters in the HLR even when some are stored in a subscriber card. The organization of the subscriber data is outlined in the ETSI GSM 03.08 Recommendation. Notice that the VLR stores the current Location Area of the subscriber, while the HLR stores the MSC/VLR they are currently under. This information is used to page the subscriber when they have an incoming call. 2.6.5 THE AUTHENTICATION CENTRE (AuC) The Authentication Centre (AuC) is associated with an HLR, and stores an identity key for each mobile subscriber registered with the associated HLR. This key is used to generate: • data which are used to authenticate the IMSI; • a key used to cipher communication over the radio path between the mobile station and the network. The procedures used for authentication and ciphering are described more fully in the security section of this course and in the ETSI GSM 03.20 recommendation. 2.6.6 THE GATEWAY MOBILE SWITCHING CENTRE (GMSC) Gateway Mobile Switching Centre (GMSC) • A Gateway Mobile Switching Centre (GMSC) is a device which routes traffic entering a mobile network to the correct destination • The GMSC accesses the network’s HLR to find the location of the required mobile subscriber • A particular MSC can be assigned to act as a GMSC • The operator may decide to assign more than one GMSC GMSCGSM Technology for Engineers© AIRCOM International 2002 21
  27. 27. 2. Architecture Overview The GMSC routes calls out of the network and is the point of access for calls entering the network from outside. If a network, delivering a call to the PLMN cannot interrogate the HLR directly, the call is routed to an MSC. This MSC will interrogate the appropriate HLR and then route the call to the MSC to which the mobile station is affiliated. The MSC which performs the routing function to the actual location of the MS is called the Gateway MSC (GMSC). The choice of which MSCs can act as Gateway MSCs is for the operator to decide (i.e. all MSCs or some designated MSCs). 2.6.7 THE EQUIPMENT IDENTITY REGISTER Equipment Identity Register (EIR) • EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each EIR item of mobile equipment • The EIR controls access to the network by returning the status of a mobile in response to an IMEI query • Possible status levels are: • White-listed The terminal is allowed to connect to the network. • Grey-listed The terminal is under observation by the network for possible problems. • Black-listed The terminal has either been reported stolen, or is not a type approved for a GSM network. The terminal is not allowed to connect to the network. The EIR contains one or several databases which store(s) the IMEIs used in the GSM system. The mobile equipment may be classified as "white listed", "grey listed" and "black listed" and therefore may be stored in three separate lists. An IMEI may also be unknown to the EIR. The EIR contains, as a minimum, a "white list" (Equipment classified as "white listed"). There is an optional implementation that may be used by the operator to control access to the network by certain types of equipment or to monitor lost or stolen handsets. GSM Technology for Engineers22 © AIRCOM International 2002
  28. 28. 2. Architecture Overview________________________________________________________________________________2.7 GSM Interfaces GSM Network Interfaces VLR VLR D HLR HLR MS Um B C H TRX MS AuC AuC A MSC BTS BTS Abis BSC BSS F MS EIR NSS 2.7.1 THE A (MSC-BSS) INTERFACE The interface between the MSC and its BSS is specified in the 08-series of GSM Technical Specifications. The BSS-MSC interface is used to carry information concerning: • BSS management; • call handling; • mobility management. 2.7.2 THE Abis (BSC-BTS) INTERFACE When the BSS consists of a Base Station Controller (BSC) and one or more Base Transceiver Stations (BTS), this interface is used between the BSC and BTS to support the services offered to the GSM users and subscribers. The interface also allows control of the radio equipment and radio frequency allocation in the BTS. This interface is specified in the 08.5x-series of GSM Technical Specifications. 2.7.3 THE B (MSC-VLR) INTERFACE The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently located in its area, it interrogates the VLR. When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information.GSM Technology for Engineers© AIRCOM International 2002 23
  29. 29. 2. Architecture Overview This procedure occurs whenever an MS roams to another location area. Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required. 2.7.4 THE C (MSC-HLR) INTERFACE The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing information for a call or a short message directed to that subscriber. 2.7.5 THE D (HLR-VLR) INTERFACE This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area. To support this, the location registers have to exchange data. The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station. The HLR sends to the VLR all the data needed to support the service to the mobile subscriber. The HLR then instructs the previous VLR to cancel the location registration of this subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means. 2.7.6 THE E (MSC-MSC) INTERFACE When a mobile station moves from one MSC area to another during a call, a handover procedure has to be performed in order to continue the communication. For that purpose the MSCs have to exchange data to initiate and then to realize the operation. After the handover operation has been completed, the MSCs will exchange information to transfer A-interface signalling as necessary. When a short message is to be transferred between a Mobile Station and Short Message Service Centre (SC), in either direction, this interface is used to transfer the message between the MSC serving the Mobile Station and the MSC which acts as the interface to the SC. 2.7.7 THE F (MSC-EIR) INTERFACE This interface is used between MSC and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station. 2.7.8 THE G (VLR-VLR) INTERFACE When a mobile subscriber moves from a VLR area to another Location Registration procedure will happen. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR. GSM Technology for Engineers24 © AIRCOM International 2002
  30. 30. 2. Architecture Overview 2.7.9 THE H (HLR-AuC) INTERFACE When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data, the HLR requests the data from the AuC. The protocol used to transfer the data over this interface is not standardized. 2.7.10 THE Um (MS-BTS) INTERFACE The interface between the MS and the BSS is specified in the 04- and 05-series of GSM Technical Specifications. Summary This section has covered: • General GSM Architecture Overview Air Interface (Um) Abis Interface Abis A Interface OMC OMC • GSM Network Components including MS • The Mobile Station BSS BSS HLR VLR • The Base Station Subsystem MS MS TRX TRX MSC BTS BSC BSC • The Network Subsystem AuC • GSM Network Interfaces Overview MS EIR PSTN • The Base Station Sub-system • The Network Switching SystemGSM Technology for Engineers© AIRCOM International 2002 25
  31. 31. 2. Architecture Overview Intentional Blank Page GSM Technology for Engineers26 © AIRCOM International 2002
  32. 32. 2. Architecture OverviewSection 2 Self-Assessment ExercisesExercise 2.1 – GSM ArchitectureThe following exercises tests your understanding of GSM architecture as applied to a smallnetwork.Here is a screen shot from Asset showing the site database of a small network:Sites 22 and 23 are connected in a star configuration to the BSC.Sites 25, 26 and 27 are connected in a chain.Draw a full architecture diagram for this network, showing all BSS and NSS elements andtheir connections.GSM Technology for Engineers© AIRCOM International 2002 27
  33. 33. 3. GSM Services 3. GSM Services_____________________________________________________________________3.1 Introduction The services offered by GSM are based on those of the fixed-line ISDN services and are therefore, as with ISDN, the GSM services are divided into three categories • Bearer Services • Teleservices • Supplementary Services In addition, this section will look at the current (2G) data services offered by GSM including: • Circuit Switched Data (CSD) • Unstructured Supplementary Service Data (USSD) • Short Message Service (SMS) Each of these services will be described in this section of the courseGSM Technology for Engineers© AIRCOM International 2002 29
  34. 34. 3. GSM Services GSM Services Teleservices Bearer Services PLMN Section Section outside PLMN TE TE IWF TE GSM Network Transit Network Terminating Network Bearer services provide the transmission capability between various elements of the overall transmission path. These bearer services carrying end-to-end teleservices. The Interworking Functions (IWF) are required to provide the mapping of GSM PLMN services to fixed (e.g. PSTN/ISDN) services. As teleservices are end-to-end, they are generally transparent to the IWF._____________________________________________________________________3.2 GSM Bearer Services GSM Bearer Services GSM Bearer Services • Bearer Services represent layers 1-3 of the OSI Model • Transparently transports application data between TEs • Bearer Services are uniquely numbered (BSxx) • Data transfer bearer services currently defined for GSM include: • Asynchronous circuit-switched data (BS 21-26) • Synchronous circuit-switched data (BS 31-34) • PAD access (BS 41-46) • Packet data (BS 51-53) • Alternate speech and data (BS 61) • Speech followed by Data (BS 81) GSM Technology for Engineers30 © AIRCOM International 2002
  35. 35. 3. GSM Services GSM bearer services refer to those services provided to transport binary data across the GSM network. They represent the functionality of layers 1 to 3 of the OSI 7-layer model. Both circuit switched (BS21-34) and packet switched (BS 41-53) data bearer services are supported. The packet data services are in the form of asynchronous access to PADs (BS41- 44) or as direct synchronous packet access (BS 51-53). Two modes of bearer services are offered; Transparent (T) or Non-Transparent (NT). The transparent mode services provide a circuit-switched connection between the TE and the IWF module. They are generally constant bit rate and are only forward error protected (FEC). Non-transparent mode services are protected by level 2 error protection over the air interface using a Radio Link Protocol (RLP). This protocol terminated at the MSC and uses backward (ARQ) error protection. Non-transparent mode operation generally offers a more reliable transmission path but at lower potential data rates than transparent mode services. GSM Bearer Service Characteristics • Each Bearer Service is uniquely identified by its characteristics which include: • Service Type (data, PAD, packet etc) • Structure (asynch, synch) • Bit Rate (300-14400 bps) • Mode (transparent, non-transparent) • Transmission (Unrestricted Data Information (UDI) or 3.1kHz) Bearer services 21-53 are further categorised into Unrestricted Digital Information (UDI) or 3.1kHz. The distinction is only important when operating outside the PLMN i.e. what type of Interworking functions needs to be implemented. The distinction is whether the data should be handled as the equivalent of 3.1kHz bandwidth audio signals over a modem or raw data over a digital link. The last two categories of bearer services shown in the diagram above refer to services which enable switching between voice and data during a session. BS 61 refers to the ability change between voice and data at will during a session (‘alternate voice and data’). BS 81 refers to the ability to initiate a call in voice and then switch to data (‘speech followed by data’)GSM Technology for Engineers© AIRCOM International 2002 31
  36. 36. 3. GSM Services_____________________________________________________________________3.3 Teleservices GSM defines a number of teleservices for use over the bearer services. These teleservices are generally end-to-end user applications and are therefore transparent to their transporting bearer services. GSM Teleservices • Teleservices are end-to-end subscriber services • Each teleservice is uniquely defined by a TS number (TS xx) • Teleservices currently defined for GSM include: • Speech (TS 11-12) • Short Message Service - SMS (TS 21-23) • Message Handling Systems - MHSs (TS 31) • Videotext (TS 41-43) • Teletext (TS 51) • Fax (TS 61-62) Speech Teleservices Speech Teleservices Two categories of Speech Teleservices: • Standard telephone services (TS 11) • Transmission of speech information and fixed network signalling tones • Transmission can be mobile originated as well as mobile terminated • Emergency Service (TS12) • Provides standard access to the emergency services irrespective of the country in which the call is made • Mandatory in GSM networks • May be initiated from a mobile without a SIM • Emergency calls can override any locked state the phone may be in • Uses a standard access to the emergency call (112) as well as the national emergency call code • If the national emergency code is used the SIM must be present GSM Technology for Engineers32 © AIRCOM International 2002
  37. 37. 3. GSM Services The teleservices introduced as part of Phase 1 GSM included: • Full rate speech • Emergency (speech) calls • SMS P-P and SMS Cell Broadcast (SMSCB) • Telefax • Voice/Fax mail Phase 2 GSM provided the following additional teleservices: • Half-rate speech • Improvements to SMS • Group 3 fax capability_____________________________________________________________________3.4 Supplementary Services A supplementary service modifies or supplements a basic teleservice. Consequently, it cannot be offered to a customer as a stand alone service, only in association with a basic teleservice. The same supplementary service may be applicable to a number of teleservices. Supplementary Services • Correspond to ISDN supplementary services • Are used only in connection with a teleservice • Examples of supplementary services include: • Call forwarding • Call Barring (incoming/outgoing calls) • Call hold - interrupting a call - normal telephony only • Call waiting - notification of new incoming call during another call • Multi-party service - simultaneous conversation between 3 - 6 subscribers • Calling line identification (CLI) - presentation of callers number • Closed user groups - group of users who can only call each other and certain specified numbers • Advice of charge - estimates of billing dataGSM Technology for Engineers© AIRCOM International 2002 33
  38. 38. 3. GSM Services Supplementary Service Groups Supplementary Service Group GSM Specification Call Deflection 02.72 Number Identifier 02.81 Call Offering 02.82 Call Completion 02.83 Multi-party 02.84 Commonality 02.85 User-to-user 02.87 Charging 02.86 Call Restriction 02.88 Precedence & Pre-emption 02.67 Call Transfer 02.91_____________________________________________________________________3.5 Circuit Switched Data (CSD) Service Circuit switched data falls within the Bearer Services category and more specifically to services (BS 21-26). Both asynchronous (BS 21-26) and synchronous (BS 31-34) options are available and each data rate (other than 1200bps synch) can operate in transparent or non - transparent mode (see Section 3.2 above for further explanation). Circuit Switched Data • Two modes defined: • Non-Transparent (error correction + flow control) • Transparent (no error protection and flow control) • PSTN access (V.32) • ISDN access (V.110) • Session-oriented • Limited to 9k6 (phase 1) or 14k4 (phase 2) per timeslot GSM Technology for Engineers34 © AIRCOM International 2002
  39. 39. 3. GSM Services Circuit Switched Data Services Structure BS no. Bit Rate Mode Asynch 21 300 T or NT 22 1200 T or NT 23 1200/75 T or NT 24 2400 T or NT 25 4800 T or NT 26 9600 T or NT Synch 31 1200 T 32 2400 T or NT 33 4800 T or NT 34 9600 T or NT T = Transparent Mode NT = Non-transparent Mode_____________________________________________________________________3.6 Short Message Service (SMS) SMS is a service that allows subscribers to send short messages (up to 160 characters) to other mobile subscribers. Rather than having to set up a call on a traffic channel, SMS uses spare capacity on the Standalone Dedicated Control Channel (SDCCH). Short Message Service (SMS) • Categorised under GSM as a Teleservice • Text-based messaging • Uses GSM signalling channels (SDCCH) . • 160-character messages • Uses store-and-forward packet switching protocol • Three SMS Teleservice types defined: • TS 21 – Mobile terminated point-to-point messaging • TS 22 – Mobile originated point-to-point messaging • TS 23 – Short Message cell broadcast (SMSCB)GSM Technology for Engineers© AIRCOM International 2002 35
  40. 40. 3. GSM Services SMS is classified as a GSM Teleservice and three SMS teleservices (TS21-23) have been defined: • TS 21 – Mobile terminated point-to-point messaging. A mobile can terminate an SMS message either from another MS or from the fixed network. • TS 22 – Mobile originated point-to-point messaging. A mobile can send a message either to another MS or into the fixed network (as an Email for example). • TS23 – SMS Cell Broadcast (SMSCB). A more recent variation of SMS is SMSCB. SMSCB messages are generally broadcast only in a specific network region. An MS cannot initiate such a message and does not acknowledge receipt of one. Only MSs in idle mode can receive SMSCB messages. These messages differ from standard SMS messages in that they are only 92 characters long. However, procedures exist to concatenate up to 15 SMSCB messages using a special reassembly mechanism. In order to implement SMS, a network operator must establish a SMS Service Centre which receives and processes SMS messages in a store-and-forward mode. Messages can be initiated in the fixed or mobile network and delivered to either the fixed or mobile network. SMS Architecture SME HLR HLR VLR VLR SME SMC SMSC GMSC SS7 Network Network MSC BSS SME SME Short Messaging Entities Short messaging entity (SME) is an entity which may receive or send short messages. The SME may be located in the fixed network, a mobile station, or another service centre. Short Message Service Centre Short message service centre (SMSC) is responsible for the relaying and store-and-forwarding of a short message between an SME and mobile station. GSM Technology for Engineers36 © AIRCOM International 2002

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