© Copyright 2002 AIRCOM International Ltd
All rights reserved

AIRCOM Training is committed to providing our customers wit...
Table of Contents
1. Introduction to Cellular Systems
1.1
1.2
1.3
1.4
1.5
1.6
1.7

Introduction .............................
7. Mobility Management
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8

Introduction ........................................................
Course Objectives and Structure
Course Objectives
• Be familiar with the development of 2G mobile systems
• Describe the a...
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GSM Technology for Engineers
© AIRCOM International 2002
1. Introduction to Cellular Systems

1. Introduction to Cellular
Systems
_________________________________________________...
1. Introduction to Cellular Systems

_________________________________________________________________________________

1....
1. Introduction to Cellular Systems

Cellular Generations
The significant stages in the evolution of
cellular radio system...
1. Introduction to Cellular Systems

In early networks, the emphasis was to provide radio coverage with little considerati...
1. Introduction to Cellular Systems

The First Generation

1.4 2nd Generation Cellular Systems
Digital systems offer consi...
1. Introduction to Cellular Systems

While first generation systems used a cellular structure and frequency re-use pattern...
1. Introduction to Cellular Systems

1.5 GSM Development and Characteristics
Originally GSM referred to the European worki...
1. Introduction to Cellular Systems

1.6 Cellular Systems Subscriber Distribution

Worldwide Mobile Communications Systems...
1. Introduction to Cellular Systems

1.7 2.5G Characteristics

2.5G Characteristics
• Available now...
• Digital modulatio...
1. Introduction to Cellular Systems

Summary
This Section has covered:
• Cellular Radio in context with other
wireless sys...
2. Architecture Overview

2. Architecture Overview
_______________________________________________________________________...
2. Architecture Overview

________________________________________________________________________________

2.2 GSM Identi...
2. Architecture Overview

2.2.5 MSRN – MOBILE STATION ROAMING NUMBER
The MSRN is a temporary, location-dependant ISDN numb...
2. Architecture Overview

A GSM network is made up of three subsystems:
• The Mobile Station (MS)
• The Base Station Sub-s...
2. Architecture Overview
The two parts of the mobile station allow a distinction between the actual equipment and the
subs...
2. Architecture Overview

The IMEI may be used to block certain types of equipment from accessing the network if they
are ...
2. Architecture Overview

BSS Network Topologies

• Chain: cheap, easy to implement

BSC

• One link failure isolates seve...
2. Architecture Overview

The NSS combines the call routing switches (MSCs and GMSC) with database registers
required to k...
2. Architecture Overview

Visitor Location Register (VLR)
• Each MSC has a VLR
• VLR stores data temporarily for mobiles s...
2. Architecture Overview

Home Location Register (HLR)
• Stores details of all subscribers in the network , such as:
• Sub...
2. Architecture Overview
The data base can also contain other information such as:
• teleservices and bearer services subs...
2. Architecture Overview

The GMSC routes calls out of the network and is the point of access for calls entering the
netwo...
2. Architecture Overview

________________________________________________________________________________

2.7 GSM Interf...
2. Architecture Overview
This procedure occurs whenever an MS roams to another location area. Also, when a
subscriber acti...
2. Architecture Overview

2.7.9 THE H (HLR-AuC) INTERFACE
When an HLR receives a request for authentication and ciphering ...
2. Architecture Overview

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2. Architecture Overview

Section 2

Self-Assessment Exercises

Exercise 2.1 – GSM Architecture
The following exercises te...
2. Architecture Overview

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GSM Technology for Engineers
© AIRCOM International 2002
3. GSM Services

3. GSM Services
_____________________________________________________________________

3.1 Introduction
T...
3. GSM Services

GSM Services
Teleservices

Bearer Services
PLMN Section

Section outside PLMN
TE

TE

IWF
TE
GSM Network
...
3. GSM Services

GSM bearer services refer to those services provided to transport binary data across the GSM
network. The...
3. GSM Services

_____________________________________________________________________

3.3 Teleservices
GSM defines a num...
3. GSM Services

The teleservices introduced as part of Phase 1 GSM included:
• Full rate speech
• Emergency (speech) call...
3. GSM Services

Supplementary Service Groups

Supplementary Service Group
Call Deflection
Number Identifier
Call Offering...
3. GSM Services

Circuit Switched Data Services
Structure

BS no.

Bit Rate

Mode

Asynch

21

300

T or NT

22

1200

T o...
3. GSM Services
SMS is classified as a GSM Teleservice and three SMS teleservices (TS21-23) have been
defined:
•

TS 21 – ...
3. GSM Services
SMS Gateway Mobile Switching Centre
The SMS–gateway mobile switching centre (SMS–GMSC) is an MSC capable o...
3. GSM Services
USSD is defined within the GSM standard in the documents GSM 02.90 (USSD Stage 1) and
GSM 03.90 (USSD Stag...
3. GSM Services

Inadequacies Of Current Data Services

• Slow data rates 9.6Kbps
• Large bills for passing data due to co...
3. GSM Services

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GSM Technology for Engineers
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3. GSM Services

Section 3

Self-Assessment Exercises

Exercise 3.1 – SMS vs USSD
Describe the advantages and disadvantage...
3. GSM Services

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GSM Technology for Engineers
© AIRCOM International 2002
4. The Air Interface

4. The Air Interface
_____________________________________________________________________

4.1 Intr...
4. The Air Interface

_____________________________________________________________________

4.2 GSM Frequency Spectrum Al...
4. The Air Interface

A major initial financial outlay for network operators is to acquire a licence to use a particular
b...
4. The Air Interface

Up and downlink channel frequencies can be calculated as follows:
Uplink frequencies: Fu(n) = 890 + ...
4. The Air Interface

4.2.3 DCS-1800 SPECTRUM

DCS - 1800 Spectrum
1710

1785

1805

Uplink

1880

MHz

Downlink
Duplex sp...
4. The Air Interface

1800 MHz Utilization in UK
The present distribution of frequencies among UK operator is:
1710

1721....
4. The Air Interface

Personal Communication System (PCS) – 1900 MHz is used in USA and Central America to
provide a servi...
4. The Air Interface

GSM – 450 (Extended) Spectrum
478.8

486

488.8

Uplink

496

MHz

Downlink
Duplex spacing = 10 MHz
...
4. The Air Interface
Multiple access techniques are essential to allow more efficient use of the radio spectrum. 1 st
gene...
4. The Air Interface
TDMA became possible with digital systems such as GSM in which the data stream could be
divided into ...
4. The Air Interface

One burst of data (0.577 ms or 156.25 bit periods) is a physical channel. This is used via
multifram...
4. The Air Interface
The purpose of these channels is outlined in the following slides.

Traffic Channels (TCH)
• One phys...
4. The Air Interface

Broadcast Channels (BCH)
BCH channels are all downlink and are allocated to
timeslot zero. BCH chann...
4. The Air Interface

Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink / downlink)
poi...
4. The Air Interface

The term ‘logical’ channel is used because the traffic and signalling channels do not have
exclusive...
4. The Air Interface

4.5.3.1 TCH Multiframe Time Duration
Notice that a multiframe always refers to a set of instances of...
4. The Air Interface

4.5.4.2 Control Channel Multiframe Structures

Control Channel Multiframe
• The control channel mult...
4. The Air Interface

Multiple Signalling Channel Configurations
• In a non combined multiframe, up to 7 of the 9 blocks m...
4. The Air Interface

4.5.5.1 Superframes
The primary purpose of the superframe layer is to provide a point at which both ...
4. The Air Interface

Summary
In this section we have looked at:
• GSM Frequency Allocation
• Multiple Access Techniques
•...
4. The Air Interface

Section 4

Self-Assessment Exercises

Exercise 4.1 – Radio Spectrum Allocation
1. The diagram shows ...
4. The Air Interface

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5. GSM Protocols

5. GSM Protocols
_____________________________________________________________________

5.1 Introduction...
5. GSM Protocols

_____________________________________________________________________

5.2 The ISO 7-Layer OSI Model
Dev...
5. GSM Protocols

Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations,
exchanges, a...
5. GSM Protocols

As GSM is a transport network, it is primarily only the lower 3 layers of the OSI Model that
are defined...
5. GSM Protocols

•

Connection management (CM) is used to set up, maintain and clear call connections.
It comprises three...
5. GSM Protocols

At some stage the 13kbps channels must be converted into 64kbps for transportation across
the PSTN. This...
5. GSM Protocols

G.703
The TRAU converts the GSM channels into ISDN D-channels in accordance with the G.703
Recommendatio...
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  1. 1. © Copyright 2002 AIRCOM International Ltd All rights reserved AIRCOM Training is committed to providing our customers with quality instructor led Telecommunications Training. This documentation is protected by copyright. No part of the contents of this documentation may be reproduced in any form, or by any means, without the prior written consent of AIRCOM International. Document Number: P/TR/003/K011/4.4 This manual prepared by: AIRCOM International Grosvenor House 65-71 London Road Redhill, Surrey RH1 1LQ ENGLAND Telephone: Support Hotline: Fax: Web: +44 (0) 1737 775700 +44 (0) 1737 775777 +44 (0) 1737 775770 http://www.aircom.co.uk GSM TECHNOLOGY FOR ENGINEERS
  2. 2. Table of Contents 1. Introduction to Cellular Systems 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Introduction ...........................................................................................................1 Cellular Radio ........................................................................................................2 st 1 Generation Cellular Systems..............................................................................3 nd 2 Generation Cellular Systems .............................................................................5 GSM Development and Characteristics...................................................................7 Cellular Systems Subscriber Distribution .................................................................8 2.5G Characteristics ..............................................................................................9 2. Architecture Overview 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Introduction ......................................................................................................... 11 GSM Identifiers .................................................................................................... 12 General Architecture Overview ............................................................................. 13 The GSM Mobile Station (MS).............................................................................. 14 The Base Station Subsystem (BSS). ..................................................................... 16 The Network Switching Subsystem (NSS) ............................................................. 17 GSM Interfaces.................................................................................................... 22 Self-Assessment Exerecises ................................................................................ 27 3. GSM Services 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Introduction ........................................................................................................ 29 GSM Bearer Services........................................................................................... 30 Teleservices ........................................................................................................ 32 Supplementary Services....................................................................................... 33 Circuit-Switched Data (CSD) Services ................................................................... 34 Short Message Service ....................................................................................... 35 USSD Data Services ............................................................................................ 37 Self-Assessment Exerecises ................................................................................ 41 4. The Air Interface 4.1 4.2 4.3 4.4 4.5 Introduction ......................................................................................................... 43 GSM Frequency Spectrum Allocation .................................................................... 44 GSM Multiple Access Techniques ......................................................................... 50 GSM Air Interface Channels ................................................................................. 52 Frames and Multiframes ....................................................................................... 56 Self-Assessment Exercises .................................................................................. 63 5. Protocols 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Introduction ......................................................................................................... 65 The ISO 7-Layer OSI Model ................................................................................. 66 GSM Protocols Overview ..................................................................................... 67 GSM Transmission Protocols................................................................................ 69 GSM Signalling Protocols ..................................................................................... 71 GSM Air (Um) Interface Protocols ......................................................................... 73 GSM A-bis Interface Protocols .............................................................................. 75 The GSM A Interface Protocols............................................................................. 78 6. Speech and Channel Coding 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Introduction ......................................................................................................... 79 Speech Coding Techniques .................................................................................. 80 GSM Speech Coding .......................................................................................... 81 Channel Coding Techniques ................................................................................. 83 GSM Channel Coding. ......................................................................................... 88 Interleaving.......................................................................................................... 89 Radio Burst Multiplexing…… .............. ………………………………………………..91 Summary of Coding Processes ………… ……………………………………………95 Radio Interface Modulation….. …………… ……………………………………………95 GSM Technology for Engineers © AIRCOM International 2002 i
  3. 3. 7. Mobility Management 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Introduction ......................................................................................................... 99 Mobility Management Procedures ....................................................................... 100 Network Areas ................................................................................................... 101 Mobility States ................................................................................................... 103 IMSI Attach/Detach............................................................................................ 104 Location Updating .............................................................................................. 107 Roaming ........................................................................................................... 109 TMSI Reallocation ............................................................................................. 110 8. Radio Resource Management 8.1 8.2 8.3 8.4 Introduction ....................................................................................................... 113 Radio Resource Connection Setup ..................................................................... 114 Cell Selection and Reselection ........................................................................... 115 Handovers......................................................................................................... 118 9. Call Management 9.1 9.2 9.3 9.4 9.5 Introduction ....................................................................................................... 123 Mobile Originated Calls ...................................................................................... 124 Mobile Terminated Calls..................................................................................... 125 Call Routing Examples ....................................................................................... 127 Echo Cancelling................................................................................................. 129 Self-Assessment Exercises ................................................................................ 131 10. GSM Security 10.1 10.2 10.3 10.4 10.5 10.6 Introduction ..................................................................................................... 133 Purposes of GSM Security................................................................................ 134 User Identity Confidentiality .............................................................................. 134 Authentication.................................................................................................. 135 User Data Confidentiality - Encryption ............................................................... 138 Signalling Data Confidentiality........................................................................... 140 Self-Assessment Exercises .............................................................................. 143 11. Billing Procedures Overview 11.1 11.2 11.3 11.4 11.5 11.6 11.7 Introduction ..................................................................................................... 145 Billing Principles............................................................................................... 146 Description of Call Components ........................................................................ 146 Charge Advice Information (CAI)....................................................................... 148 Advice of Charge (AoC) Calculations................................................................. 149 Call Detail Records .......................................................................................... 151 The Transferred Account Procedure (TAP) ........................................................ 152 12. GSM Evolution 12.1 12.2 12.3 12.4 Introduction ..................................................................................................... 155 High Speed Circuit Switched Data (HSCSD) ...................................................... 156 General Packet Radio Service (GPRS).............................................................. 157 Enhanced Data for nGSM Evolution (EDGE) ..................................................... 159 Appendix A - Solutions to Self Assessment Exercises Appendix B - Glossary of Terms ii GSM Technology for Engineers © 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 Evolution GSM Technology for Engineers © AIRCOM International 2002 iii
  5. 5. Intentional Blank Page iv GSM Technology for Engineers © AIRCOM International 2002
  6. 6. 1. Introduction to Cellular Systems 1. Introduction to Cellular Systems _________________________________________________________________________________ 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 distribution GSM 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: PSTN Brief numeric, alphanumeric or voice messages 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) 2 GSM Technology for Engineers © AIRCOM International 2002
  8. 8. 1. Introduction to Cellular Systems Cellular Generations The significant stages in the evolution of cellular radio systems is referred to in terms of generations: Data rate • 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 • No inter-network roaming possible The 1st Generation of Cellular Technology makes use of analogue modulation techniques such as FM • Insecure air interface GSM 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 • For networks with more cells than frequencies these must be planned F1 F2 F5 • Large cell size F4 • Order 30km • 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 4 GSM Technology for Engineers © AIRCOM International 2002
  10. 10. 1. Introduction to Cellular Systems The First Generation 1.4 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 • More efficient use of radio spectrum • Voice and low rate circuit switched data • International roaming capability • Secure air interface • Compatibility with ISDN GSM Technology for Engineers © AIRCOM International 2002 1110 0 000 100111 1 • Variety of multiple access strategies 00 01 10 00 00 0 0 1 0100 010010 11100110010 110 1 1011 0 0 11 0 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 • PDC • TDMA (D-AMPS) • North American TDMA/FDMA based standard based upon AMPS • Japanese TDMA/FDMA based standard • Predominantly used in North and South America • Predominantly used in Asia • Planning Similar to GSM • ANSI-41 Core Network • Planning Similar to GSM TDMA and PDC phones 6 GSM Technology for Engineers © AIRCOM International 2002
  12. 12. 1. Introduction to Cellular Systems 1.5 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 • Quality – low interference, few calls dropped etc. • Optional parameters requiring planning • • • • Hierarchical Cell Structures (macrocell/microcell) Frequency Hopping F1 F2 F5 F4 F3 Discontinuous Transmission Power Control • Subscriber/traffic analysis • Capacity limited by number of TRX’s GSM Technology for Engineers © AIRCOM International 2002 Cellular networks can use microcells to provide additional capacity 7
  13. 13. 1. Introduction to Cellular Systems 1.6 Cellular Systems Subscriber Distribution Worldwide Mobile Communications Systems Subscribers (x million) Second Generation - DAMPS 700 600 500 400 300 200 100 0 Second Generation - PDC Second Generation - GSM Second Generation - cdmaOne 20 01 19 99 19 97 19 95 19 93 19 91 First Generation - Analogue 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 8 GSM Technology for Engineers © AIRCOM International 2002
  14. 14. 1. Introduction to Cellular Systems 1.7 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 rates GSM 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 10 GSM Technology for Engineers © AIRCOM International 2002
  16. 16. 2. Architecture Overview 2. 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 interfaces GSM 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. 12 GSM Technology for Engineers © 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 (Um) (Um) Abis Interface Abis Interface A Interface A Interface OMC MS VLR VLR BSS MS HLR HLR TRX BTS BTS MSC BSC AuC AuC MS EIR NSS NSS PSTN GSM 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 S R A Um m Interface Interface Interface Interface Base Station Subsystem TA SIM TE GSM Core Network ME MS Mobile Terminal (MT) Mobile Terminal (MT) TE - Terminal Equipment TA - Terminal Adaptor MS - Mobile Station ME - Mobile Equipment SIM - Subscriber Identity Module 14 GSM Technology for Engineers © 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 contains : • Radio Transmitter/Receiver (TRX) • Signal processing and control equipment • Antennas and feeder cables • The BSC: • allocates a channel for the duration of a call • maintains the call: • • • monitors quality controls the power transmitted by the BTS or MS generates a handover to another cell when required BTS BSC BSC BTS BTS BTS • 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. 16 GSM Technology for Engineers © AIRCOM International 2002
  22. 22. 2. Architecture Overview BSS Network Topologies • Chain: cheap, easy to implement BSC • One link failure isolates several BTSs • Ring: Redundancy gives some protection if a link fails • More difficult to roll-out and extend BSC • ring must be closed • Star: most popular configuration for first GSM systems BSC • Expensive as each BTS has its own link • 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 Connect 2.6 The Network Switching Subsystem (NSS) 2.6.1 NSS OVERVIEW Network Switching System (NSS) • Key elements of the NSS: VLR PSTN/ISDN MSC GMSC • Mobile Switching Centre (MSC) with: • Visitor Location Register (VLR) • Home Location Register (HLR) with: • Authentication Centre (AuC) • Equipment Identity Register (EIR) SS7 Network AuC EIR • Gateway MSC (GMSC) HLR • These elements are interconnected by means of an SS7 network GSM 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. 18 GSM Technology for Engineers © 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 • TMSI MSC • 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 HLR • Supplementary services • 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 20 GSM Technology for Engineers © 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 GMSC GSM 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 item of mobile equipment EIR • 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. 22 GSM Technology for Engineers © AIRCOM International 2002
  28. 28. 2. Architecture Overview ________________________________________________________________________________ 2.7 GSM Interfaces GSM Network Interfaces D VLR VLR HLR HLR MS Um MS C B TRX BTS BTS Abis BSC A BSS H AuC AuC MSC 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. 24 GSM Technology for Engineers © 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 • GSM Network Components including Air Interface (Um) • The Mobile Station • The Base Station Subsystem Abis Interface Abis A Interface OMC OMC MS HLR BSS BSS VLR MS MS TRX TRX BTS • The Network Subsystem MSC BSC BSC AuC • GSM Network Interfaces Overview • The Base Station Sub-system MS PSTN EIR • The Network Switching System GSM Technology for Engineers © AIRCOM International 2002 25
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  32. 32. 2. Architecture Overview Section 2 Self-Assessment Exercises Exercise 2.1 – GSM Architecture The following exercises tests your understanding of GSM architecture as applied to a small network. 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 and their connections. GSM Technology for Engineers © AIRCOM International 2002 27
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  34. 34. 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 course GSM Technology for Engineers © AIRCOM International 2002 29
  35. 35. 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: • • • • • • 30 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 Engineers © AIRCOM International 2002
  36. 36. 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 (BS4144) 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
  37. 37. 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 32 GSM Technology for Engineers © AIRCOM International 2002
  38. 38. 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 data GSM Technology for Engineers © AIRCOM International 2002 33
  39. 39. 3. GSM Services Supplementary Service Groups Supplementary Service Group Call Deflection Number Identifier Call Offering Call Completion Multi-party Commonality User-to-user Charging Call Restriction Precedence & Pre-emption Call Transfer GSM Specification 02.72 02.81 02.82 02.83 02.84 02.85 02.87 02.86 02.88 02.67 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 34 GSM Technology for Engineers © AIRCOM International 2002
  40. 40. 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 31 1200 T 32 2400 T or NT 33 4800 T or NT 34 9600 T or NT Synch 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
  41. 41. 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 SMSC SMC 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. 36 GSM Technology for Engineers © AIRCOM International 2002
  42. 42. 3. GSM Services SMS Gateway Mobile Switching Centre The SMS–gateway mobile switching centre (SMS–GMSC) is an MSC capable of receiving a short message from an SMSC, interrogating a home location register (HLR) for routing information, and delivering the short message to the visited MSC of the recipient mobile station. The SMS interworking MSC (SMS–IWMSC) is an MSC capable of receiving a short message from the mobile network and submitting it to the appropriate SMSC. The SMS– GMSC/SMS–IWMSC are typically integrated with the SMSC. Home Location Register The HLR is a database used for permanent storage and management of subscriptions and service profiles. Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. The HLR also informs the SMSC, which has previously initiated unsuccessful short message delivery attempts to a specific mobile station, that the mobile station is now recognized by the mobile network to be accessible. Mobile Switching Centre The MSC performs the switching functions of the system and controls calls to and from other telephone and data systems. Visitor Location Register The visitor location register (VLR) is a database that contains temporary information about subscribers. This information is needed by the MSC to service visiting subscribers. The Base Station System All radio-related functions are performed in the base-station system (BSS). The BSS consists of base-station controllers (BSCs) and the base-transceiver stations (BTSs), and its primary responsibility is to transmit voice and data traffic between the mobile stations. The Mobile Station The mobile station (MS) is the wireless terminal capable of receiving and originating short messages as well as voice calls. The wireless network signalling infrastructure is based on signalling system 7 (SS7). SMS makes use of the mobile application part (MAP), which defines the methods and mechanisms of communication in wireless networks, and uses the services of the SS7 transaction capabilities application part (TCAP). An SMS service layer makes use of the MAP signalling capabilities and enables the transfer of short messages between the peer entities. _____________________________________________________________________ 3.7 USSD Data Services Unstructured Supplementary Services Data (USSD) is a means of transmitting information or instructions over a GSM network. USSD has some similarities with SMS since both use the GSM network's signalling path (the SDCCH). Unlike SMS, USSD is not a store and forward service and is session-oriented such that when a user accesses a USSD service, a session is established and the radio connection stays open until the user, application, or time out releases it. This has more in common with Data than SMS. USSD text messages can be up to 182 characters in length. GSM Technology for Engineers © AIRCOM International 2002 37
  43. 43. 3. GSM Services USSD is defined within the GSM standard in the documents GSM 02.90 (USSD Stage 1) and GSM 03.90 (USSD Stage 2). In USSD Stage 1, the interactions are initiated by the mobile phone. In USSD Stage 2, the application can also initiate USSD-based transactions. Unstructured Supplementary Service Data (USSD) • Text-based messaging Service • Uses GSM Signalling channels (SDCCH) . • 182-character messages • Session-oriented. • No Store-and-Forward Comparison of USSD and SMS • USSD Advantages: • Up to seven times faster than SMS • Longer character messages than SMS (182 compared to 160) • Works on all existing GSM terminals • Supported by SIM Toolkit and WAP • Stage 2 can provide WAP-like features (on existing handsets) • USSD Disadvantages: • No Store-and-Forward. • Additional loading on signalling channels • Session-oriented (Traffic loading and costs compared to SMS). • Stage 1 implementation not intuitive 38 GSM Technology for Engineers © AIRCOM International 2002
  44. 44. 3. GSM Services Inadequacies Of Current Data Services • Slow data rates 9.6Kbps • Large bills for passing data due to continuous call connection (CSD) • Limited size of SMS/USSD messages Summary This section has covered the following topics: • Bearer Services • Teleservices • Supplementary Services • Data Services (CSD, USSD, SMS) GSM Technology for Engineers © AIRCOM International 2002 39
  45. 45. 3. GSM Services Intentional Blank Page 40 GSM Technology for Engineers © AIRCOM International 2002
  46. 46. 3. GSM Services Section 3 Self-Assessment Exercises Exercise 3.1 – SMS vs USSD Describe the advantages and disadvantages when comparing SMS and USSD data capabilities. Exercise 3.2 – GSM Services 1. Describe briefly the three categories of services offered by GSM. 2. Describe briefly the basic data services offered by GSM Phase 1. GSM Technology for Engineers © AIRCOM International 2002 41
  47. 47. 3. GSM Services Intentional Blank Page 42 GSM Technology for Engineers © AIRCOM International 2002
  48. 48. 4. The Air Interface 4. The Air Interface _____________________________________________________________________ 4.1 Introduction The ‘Air Interface’ refers to the interface between a mobile terminal and the base station to which it is affiliated. This interface takes the form of a radio path through the air, hence the term ‘Air’ interface. In this section of the course the following air-interface-related topics are covered: • • • GSM Frequency Spectrum Allocation Radio Access Methods GSM Air Interface channels and frames GSM Technology for Engineers © AIRCOM International 2002 43
  49. 49. 4. The Air Interface _____________________________________________________________________ 4.2 GSM Frequency Spectrum Allocation Spectrum for Mobile Communication • Radio Spectrum is a limited resource • For mobile communication we are using following ranges: SHF 3 GHz - 30 GHz UHF 300 MHz - 3 GHz VHF 30 MHz - 300 MHz The whole radio spectrum is divided for convenience into bands such as VHF, UHF and so on. The range of the spectrum used by GSM is in the UHF band. Frequency Allocation • Authority to use a frequency is given under certain conditions such as: • Location • Power levels • Modulation types • Bandwidth • Regulatory bodies deal with this allocation in different parts of the world: • International Telecommunications Union (ITU) • European Telecommunications Standards Institute (ETSI) • Radiocommunications Agency (RA) in the UK 44 GSM Technology for Engineers © AIRCOM International 2002
  50. 50. 4. The Air Interface A major initial financial outlay for network operators is to acquire a licence to use a particular bandwidth of radio spectrum. The method of allocation differs from country to country, but may be by auction or direct choice of operators by the government organisation responsible. 4.2.1 PRIMARY GSM SPECTRUM P-GSM Spectrum (Primary GSM) 890 915 935 Uplink 960 MHz Downlink Duplex spacing = 45 MHz Range of ARFCN: 1 - 124 Fu(n) 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing GSM uses Frequency Division Duplexing (FDD) where the uplink and downlink of each channel operates on a different frequency. Therefore, two frequency bands were allocated to GSM, 20 MHz apart. The following frequency bands were initially allocated to GSM (now known as Primary GSM): Uplink sub band: 890 MHz to 915 MHz Downlink sub band: 935 MHz to 960 MHz Each band is divided into a number of carriers, with each carrier having a 200kHz Bandwidth. Therefore 124 carriers were available within each of the up and down link bands (allowing for guard bands). The channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 45Mhz apart Each frequency pair is identified by an ‘Absolute Radio Frequency Carrier Number ‘ (ARFCN)’. GSM Technology for Engineers © AIRCOM International 2002 45
  51. 51. 4. The Air Interface Up and downlink channel frequencies can be calculated as follows: Uplink frequencies: Fu(n) = 890 + 0.2 n (1 <= n <= 124) Downlink frequencies: Fd(n) = Fu(n) + 45 4.2.2 EXTENDED PRIMARY GSM (E-GSM) SPECTRUM E-GSM Spectrum (Extended GSM) 880 915 925 Uplink 960 MHz Downlink Duplex spacing = 45 MHz Range of ARFCN: 1 – 124 975 - 1023 Fu(n) 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing E-GSM allocated an additional 10MHz of bandwidth at the bottom end of each of the up and down link frequency bands. The new frequency bands are: Uplink sub band: 880 MHz to 915 MHz Downlink sub band: 925 MHz to 960 MHz This resulted in the following: • An extra 50 carrier pairs (10MHz / 200kHz channels) • A reduction in bandwidth isolation between up and down link frequency bands from 20MHz to 10MHz • An increase in size of up and down link frequency bands from 25Mhz to 35MHz • Assignment of new ARFCN in the range 975-1023 • The P-GSM ARFCN 0 now reverts from a guard band to a valid ARFCN E-GSM up and downlink channel frequencies can be calculated as follows: Uplink frequencies: Fu(n) = 890 + 0.2 n (0 <= n <= 124) Fu(n) = 890 + 0.2 (n – 1024) (975 <= n <= 1023) Downlink frequencies: Fd(n) = Fu(n) + 45 46 GSM Technology for Engineers © AIRCOM International 2002
  52. 52. 4. The Air Interface 4.2.3 DCS-1800 SPECTRUM DCS - 1800 Spectrum 1710 1785 1805 Uplink 1880 MHz Downlink Duplex spacing = 95 MHz Fu(n) Range of ARFCN: 512 - 885 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing Digital Communication System (DCS) – 1800 introduced a further 1800 MHz spectrum range for GSM. The characteristics of radio frequencies in this range are such that DCS-1800 is typically used for smaller microcells overlaid over existing GSM-900 macrocells. The up and down link bands are 75MHz each and have a 20MHz separation in the following ranges: Uplink sub band: 1710 MHz to 1785 MHz Downlink sub band: 1805 MHz to 1880 MHz Each band is divided into 200kHz carriers, as with GSM -900. Therefore 374 carriers are available within each of the up and down link bands (allowing for guard bands). Channel numbers are in the range 512-885 (ARFCNs). The channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 95Mhz apart. DCS-1800 up and downlink channel frequencies can be calculated as follows: Uplink frequencies: Fu(n) = 1710.2 + 0.2 (n – 512) (512 <= n <= 885) Downlink frequencies: Fd(n) = Fu(n) + 95 GSM Technology for Engineers © AIRCOM International 2002 47
  53. 53. 4. The Air Interface 1800 MHz Utilization in UK The present distribution of frequencies among UK operator is: 1710 1721.5 1781.5 1785 1751.5 Vodafone/ Cellnet One 2 One DECT Uplink MHz Orange Downlink 1805 1816.5 1846.5 One 2 One 1876.5 1880 MHz DECT: Digital Enhanced Cordless Telecommunications This spectrum diagram shows the way in which the 1800 MHz band is currently distributed among operators in the UK. Note the uplink and downlink sub bands are shown on the one diagram. 4.2.4 PCS-1900 SPECTRUM PCS - 1900 Spectrum 1850 1910 1930 Uplink 1990 MHz Downlink Duplex spacing = 80 MHz Range of ARFCN: 512 - 810 Guard Band 100 kHz wide Fu(n) 1 2 3 4 n Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing 48 GSM Technology for Engineers © AIRCOM International 2002
  54. 54. 4. The Air Interface Personal Communication System (PCS) – 1900 MHz is used in USA and Central America to provide a service similar to GSM. The 1900MHz frequency band was selected due to a shortage of available 1800 band frequencies in the USA. The up and down link bands are 60MHz each and have a 20MHz separation in the following ranges: PCS-1900 frequency bands are: Uplink sub band: 1850 MHz to 1910 MHz Downlink sub band: 1930 MHz to 1990 MHz Each band is divided into 200kHz carriers, as with GSM. Therefore 299 carriers are available within each of the up and down link bands (allowing for guard bands). Channel numbers are in the range 512-810 (ARFCNs). The PCs-1900 channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 80Mhz apart. PCS-1900 up and downlink channel frequencies can be calculated as follows: Uplink frequencies: Fu(n) = 1850.2 + 0.2 (n – 512) (512 <= n <= 810) Downlink frequencies: Fd(n) = Fu(n) + 80 4.2.5 GSM-450 SPECTRUM The GSM-450 standard has grown from a study undertaken to evaluate a digital standard to replace the widespread analogue NMT-450 systems. The 450MHz band has a number of advantages over existing GSM bands, not least of which is the increased coverage per cell (up to 120km) and hence a lower cell count. It can also provide valuable additional capacity GSM - 450 Spectrum 450.4 457.6 460.4 Uplink 467.6 MHz Downlink Duplex spacing = 10 MHz Fu(n) 1 2 3 4 Guard Band 100 kHz wide n Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing GSM Technology for Engineers © AIRCOM International 2002 49
  55. 55. 4. The Air Interface GSM – 450 (Extended) Spectrum 478.8 486 488.8 Uplink 496 MHz Downlink Duplex spacing = 10 MHz Fu(n) 1 2 3 4 n Guard Band 100 kHz wide Guard Band 100 kHz wide Channel Numbers (n) (ARFCN) 200 kHz spacing _____________________________________________________________________ 4.3 GSM Multiple Access Techniques Multiple Access Techniques • Purpose: to allow several users to share the resources of the air interface in one cell • Methods: • FDMA - Frequency Division Multiple Access • TDMA - Time Division Multiple Access • CDMA - Code Division Multiple Access 50 GSM Technology for Engineers © AIRCOM International 2002
  56. 56. 4. The Air Interface Multiple access techniques are essential to allow more efficient use of the radio spectrum. 1 st generation systems used only FDMA so that a complete radio carrier was allocated to a user throughout their call. This made poor use of the spectrum, but was all that was possible with an analogue system. Frequency Division Multiple Access (FDMA) • Divide available frequency spectrum into channels each of the same bandwidth Frequency • Channel separation achieved by filters: • Good selectivity • Guard bands between channels User 1 User 2 User 3 • Signalling channel required to allocate a traffic channel to a user User 4 User 5 • Only one user per frequency channel at any time Time • Used in analog systems, such as AMPS, TACS • Limitations on: channel bandwidth • frequency re-use • number of subscribers per area Time Division Multiple Access (TDMA) • Access to available spectrum is limited to timeslots • User is allocated the spectrum for the duration of one timeslot Frame GSM Technology for Engineers © AIRCOM International 2002 User 7 User 6 User 5 User 4 User 3 User 2 User 1 Signalling User 7 User 6 User 5 User 4 User 3 User 2 User 1 Signalling Frequency • Timeslots are repeated in frames Time Timeslot 51
  57. 57. 4. The Air Interface TDMA became possible with digital systems such as GSM in which the data stream could be divided into bursts and allocated to a timeslot. By sharing access to the spectrum, the traffic capacity of the system is enhanced. GSM uses both FDMA to provide carriers and TDMA to share access to the carriers. _____________________________________________________________________ 4.4 GSM Air Interface Channels GSM Channels GSM defines two fundamental channel types: • Physical Channels: • the individual channels carried by a radio frequency carrier • Each carrier comprises 8 time-separated channels • Logical Channels: • time-dependant virtual channels carried on a single physical channel • one physical channel may support one or multiple logical channels GSM Physical Channels • GSM employs both FDMA and TDMA at the Air Interface • Each BTS may comprise a number of TRXs, with the carrier of each TRX operating on a different frequency (FDM) • Each GSM carrier supports 8 time-separated physical channels (TDMA) • Each physical channel is allocated to a specific timeslot on the carrier • A group of 8 timeslots on a carrier is known as a TDMA frame 1 frame period 4.615 ms 0 1 2 3 4 5 6 7 timeslot = 0.577 ms 52 GSM Technology for Engineers © AIRCOM International 2002
  58. 58. 4. The Air Interface One burst of data (0.577 ms or 156.25 bit periods) is a physical channel. This is used via multiframe structures to provide all the logical channels required. GSM Logical Channels • • Two types of logical channel are defined; traffic and control channels Each is further sub-divided as shown: Traffic Traffic Control Control TCH TCH BCH BCH CCCH CCCH DCCH DCCH TCH/H TCH/H FCCH FCCH PCH PCH SDCCH SDCCH SCH SCH RACH RACH SACCH SACCH BCCH BCCH TCH/F TCH/F AGCH AGCH FACCH FACCH CBCH CBCH NCH NCH The naming of the GSM logical channels is as follows: TCH Traffic Channels TCH/F Traffic Channel (full rate) (U/D) TCH/H Traffic Channel (half rate) (U/D) BCH Broadcast Channels FCCH Frequency Correction Channel (D) SCH Synchronisation Channel (D) BCCH Broadcast Control Channel (D) CCCH PCH RACH AGCH CBCH NCH Common Control Channels Paging Channel (D) Random Access Channel (U) Access Grant Channel (D) Cell Broadcast Channel (D) Notification Channel (D) DCCH Dedicated Control Channels SDCCH Stand alone Dedicated Control Channel (U/D) SACCH Slow Associated Control Channel (U/D) FACCH Fast Associated Control Channel (U/D) U = Uplink GSM Technology for Engineers © AIRCOM International 2002 D = Downlink 53
  59. 59. 4. The Air Interface The purpose of these channels is outlined in the following slides. Traffic Channels (TCH) • One physical channel (1 timeslot) can support: • 1 TCH/F or 2 TCH/H • TCH/F: 13 kb/s voice or 9.6 kb/s data • TCH/H: 6.5 kb/s voice or 4.8 kb/s data Uplink / Downlink Synchronisation The MS transmit burst is delayed by 3 timeslots after the BTS burst.. This delay allows enables: BTS transmits: 0 1 2 3 4 5 6 7 0 1 2 3 4 • Use of the same UL and DL timeslot number in TDMA frame • Avoids simultaneous Tx/Rx requirement • Allows for timing advance (TA) • Allows time to switch between Tx and Rx MS transmits: 5 6 7 Full Rate traffic channels (TCH/F) and Enhanced Full Rate (EFR) traffic channels are allocated dedicated physical channels in form of one timeslot allocated per TDMA frame. If Half Rate Traffic Channels are implemented, two traffic channels are allocated to single physical channel with a proportional reduction in available bit rate per traffic channel as shown above. A three timeslot delay exists between downlink and uplink TDMA frames. The purpose of this delay is: • • • 54 To enable the same timeslot to be used for the both up and down link within the same TDMA frame without the requirement for simultaneous transmit and receive. Allows Timing Advance to be implemented which can reduce the 3-timeslot delay. To provide time for the MS to switch between transmit and receive modes. GSM Technology for Engineers © AIRCOM International 2002
  60. 60. 4. The Air Interface Broadcast Channels (BCH) BCH channels are all downlink and are allocated to timeslot zero. BCH channels include: • FCCH: Frequency control channel sends the MS a burst of all ‘0’ bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time-synchronise. • SCH: Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN), together with the BTS’s BSIC • BCCH: Broadcast Control Channel sends network-specific information such as radio resource management and control messages, Location Area Code etc. Common Control Channels (CCCH) CCCH contains all point to multi-point downlink channels (BTS to several MSs) and the uplink Random Access Channel: • RACH: Random Access Channel is sent by the MS to request a resources from the network e.g. an SDCCH channel for call setup. • AGCH: Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobile. • PCH: Paging Channel sends paging signal to inform mobile of a call. • CBCH: Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages, for example road traffic reports or network engineering messages. • NCH: Used for GSM Phase II voice services such as Voice Broadcast Service (VBS) or Voice Group Calling Service (VGCS). GSM Technology for Engineers © AIRCOM International 2002 55
  61. 61. 4. The Air Interface Dedicated Control Channels (DCCH) DCCH comprise the following bi-directional (uplink / downlink) point to point control channels: • SDCCH: Standalone Dedicated Channel is used for call set up, location updating and also SMS • SACCH: Slow Associated Control Channel is used for link measurements and signalling during a call • FACCH: Fast Associated Control Channel is used (when needed) for signalling during a call, mainly for delivering handover messages and for acknowledgement when a TCH is assigned _____________________________________________________________________ 4.5 Frames and Multiframes 4.5.1 LOGICAL CHANNEL CONCEPT Logical Channels • Multiframes provide a way of mapping the logical channels on to the physical channels (timeslots) • A logical channel is a series of consecutive instances of a particular timeslot Time TDMA Frame 0 1 2 3 4 TDMA Frame 5 6 7 0 1 2 Logical Channel 3 4 5 TDMA Frame 6 7 1 0 1 1 1 2 3 4 5 6 7 1 • A multiframe is a repeating combination of logical channels 56 GSM Technology for Engineers © AIRCOM International 2002
  62. 62. 4. The Air Interface The term ‘logical’ channel is used because the traffic and signalling channels do not have exclusive use of a physical resource i.e. the carrier frequency (unlike 1 st Generation cellular systems). Multiframes allow one timeslot allocation (physical channel) to be used for a variety of purposes (logical channels) by multiplexing the logical channels onto the timeslot. 4.5.2 MULTIFRAME CONCEPT The term ‘multiframe ’ is used to describe a repeating pattern of TDMA frame timeslots transmitted in sequence on a physical channel. When a user is allocated a timeslot within a TDMA frame for a voice traffic call, he generally has exclusive use of that timeslot (full rate traffic) for the duration of the call. Therefore, every 8 th timeslot (one TS per TDMA frame) the user transmits a burst of in data. However, after 12 bursts have been sent, a SACCH logical channel burst is inserted and after another 12 traffic burst have been sent an idle burst is inserted. This pattern then repeats itself over the next 26 bursts and continues to do so until the call is terminated 4.5.3 TRAFFIC CHANNEL(TCH) MULTIFRAMES This repeating pattern of 26 traffic channel bursts is known as a traffic channel ‘multiframe’ as illustrated below: Traffic Channel Multiframe • The TCH multiframe consists of 26 timeslots. • This multiframe maps the following logical channels: •TCH •SACCH •FACCH • TCH Multiframe structure: T T T T T T T T T T T T S T T T T T T T T T T T T 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 I T = TCH S = SACCH I = Idle FACCH is not allocated slots in the multiframe. It steals TCH slots when required. GSM Technology for Engineers © AIRCOM International 2002 57
  63. 63. 4. The Air Interface 4.5.3.1 TCH Multiframe Time Duration Notice that a multiframe always refers to a set of instances of the same timeslot within a TDMA frame, therefore each ov the above timeslots occur 4.615mS apart (the duration of a TDMA frame). Therefore, the time duration for a multiframe is calculated as the length of the multiframe (in timeslots) x 4.615mS. In addition to TCH and SACCH timeslots, the traffic channel can also carry FACCH information. The FACCH is unique amongst logical channels in that it does not have a dedicated timeslot for transmission. 4.5.3.2 TCH Multiframe SACCH Function During a call the MS is continually monitoring power levels from neighbouring base stations. It does this in the time intervals between its allocated transmission timeslot. Once, during each traffic channel multiframe, there is an uplink SACCH burst which is used to send a report on these measurements to the current serving base station. The BTS uses the downlink SACCH burst to send power control and other call-control signals to the mobile. 4.5.3.3 TCH Multiframe Idle Channel Function The idle slot (TS 25 in the multiframe) occurs to allow for half rate TCH/H operation in which two mobiles would share the multiframe and sets of reports would need to be sent to the base station. Slot 25 would then be a second SACCH burst. 4.5.3.4 TCH Multiframe FACCH Function The FACCH is used for purposes that require instant access such as a handover command message from the base station. When this is needed, FACCH uses a TCH burst and sets a ‘stealing flag’ in the burst to show that it is not a traffic channel burst. 4.5.4 CONTROL CHANNEL MULTIFRAMES 4.5.4.1 BCCH Carriers Although a cell can contain multiple carriers (frequencies), every cell must have at least one TS of one of its carriers dedicated to control functions. This physical control channel transports a number of logical signalling channels multiplexed together. The most important of these logical signalling channels is the BCCH as it carries network configuration information. It is for this reason that the carrier containing this control channel (and therefore the BCCH logical channel) is often referred to as the ‘BCCH Carrier’. 58 GSM Technology for Engineers © AIRCOM International 2002
  64. 64. 4. The Air Interface 4.5.4.2 Control Channel Multiframe Structures Control Channel Multiframe • The control channel multiframe is formed of 51 timeslots • CCH multiframe maps the following logical channels: Downlink: Uplink: • RACH • FCCH • SCH • BCCH • CCCH (combination of PCH and AGCH) Downlink F = FCCH F S BCCH CCCH F S CCCH CCCH F 0 1 2- 5 6-9 10 11 12- 15 16-19 20 21 S S = SCH I = Idle CCCH CCCH F S CCCH CCCH F S CCCH CCCH I 22-25 26- 29 30 31 32- 35 36-39 40 41 42- 45 46-49 50 RACH Uplink Control channel multiframes are inherently more complex than TCH multiframes as they carry multiple logical signalling channels multiplexed onto a single physical channel (TS0). The logical channels that are transported on the control, channel are organised such that the control channel sequence repeats every 51 occurrences of TS0 in consecutive TDMA frames as shown above. 4.5.4.3 Combined and Non-Combined CC Multiframes The CCCH are general signalling blocks (each of 4 TSs) that are allocated to specific signalling channels depending on the signalling capacity requirements. This includes SDCCH, SACCH AGCH and PCH allocation. When the signalling capacity requirements are calculated, it may be determined that the capacity available on a single control channel is not sufficient. In such cases, additional physical channels are allocated to signalling. When multiple physical signalling channels are required they are always allocated on the BCCH carrier using TS 2 4 or 6 (in addition to TS0) . Where this is the case, the multiple signalling channels are arranged in either combined or non -combined formats as shown below. Therefore the structure of the signalling channel multiframe may vary depending on the signalling capacity requirements. GSM Technology for Engineers © AIRCOM International 2002 59
  65. 65. 4. The Air Interface Multiple Signalling Channel Configurations • In a non combined multiframe, up to 7 of the 9 blocks may be reserved for AGCH: F S BCCH F CCCH S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH I CCCH • In a combined multiframe, up to 2 of the 3 blocks may be reserved for AGCH: F S BCCH CCCH F S CCCH CCCH F S SDCCH 0 SDCCH 1 F S SDCCH 2 SDCCH 3 F S SACCH 0 SACCH 1 I • Additional CCCH capacity can be provided on other timeslots (TS 2,4 or 6) of the BCCH carrier if required • The number of AGCH blocks reserved is indicated to the MS in the system information messages that the MS reads on the BCCH 4.5.5 GSM MULTIFRAME HIERARCHICAL STRUCTURE Frame Hierarchy 1 timeslot = 0.577 ms 0 Multiframe: Superframe: Hyperframe: 60 1 2 3 4 5 6 7 1 frame = 8 timeslots = 4.615 ms = 26 TCH Frames (= 120 ms) or 51 BCCH Frames (= 235 ms) = 26 BCCH Multiframes (= 6.12s) or 51 TCH Multiframes (= 6.12s) = 2048 Superframes (= 3 hr 28 min 53.76 s) GSM Technology for Engineers © AIRCOM International 2002
  66. 66. 4. The Air Interface 4.5.5.1 Superframes The primary purpose of the superframe layer is to provide a point at which both TCH and CC multiframes are synchronised. Therefore, 51 TCH multiframes of 26 TS each are grouped together and 25 CC multiframes of 51 TS are grouped together. IN both cases the time duration of a superframe is 26x51 TS = 6.12 seconds. 4.5.5.2 Hyperframes The synchronisation channel (SCH) transmits a TDMA frame number (FN) which enables a mobile to synchronise with the base station at TDMA-frame level. The FN is a 22 bit number which resets after each hyperframe, i.e. after 2048 x 26 x 51 = 2715648 TDMA frames. GSM Technology for Engineers © AIRCOM International 2002 61
  67. 67. 4. The Air Interface Summary In this section we have looked at: • GSM Frequency Allocation • Multiple Access Techniques • Air Interface Channels: • Physical Channels • Logical Channels • Frames and Multiframes 62 GSM Technology for Engineers © AIRCOM International 2002
  68. 68. 4. The Air Interface Section 4 Self-Assessment Exercises Exercise 4.1 – Radio Spectrum Allocation 1. The diagram shows the spectrum for E-GSM 880 915 925 960 Calculate the up and down link frequencies for ARFCNs 0, 124, 975 and 1023. Mark these carriers on the diagram. 2. An operator using DCS-1800 is allocated ARFCNs 601 to 625 inclusive. Calculate the highest and lowest frequencies used for the uplink. GSM Technology for Engineers © AIRCOM International 2002 63
  69. 69. 4. The Air Interface Intentional Blank Page 64 GSM Technology for Engineers © AIRCOM International 2002
  70. 70. 5. GSM Protocols 5. GSM Protocols _____________________________________________________________________ 5.1 Introduction Protocols • Protocols are needed whenever systems pass information from one to another • A protocol is just a set of rules that both sides agree on so that meaningful communication can take place ? GSM Technology for Engineers © AIRCOM International 2002 65
  71. 71. 5. GSM Protocols _____________________________________________________________________ 5.2 The ISO 7-Layer OSI Model Development of the Open Standards Interconnection (OSI) reference model was started in 1983 by an number of major computer and telecommunications companies. It was eventually adopted as an international standard by the International Standards Organisation (ISO) and is currently embodied within the ITU-TS X.200 Recommendation. The model comprises 7 layers which define various functions involved in establishing and servicing end-to-end communications circuits across a network. These 7 layers are generally viewed in two blocks; • Application Functional Layers. These are layers 4-7 of the OSI Model and relate to the end-to-end functions between two or more users at the periphery of a network. • Network Functional Layers. These are layers 1-3 of the OSI Model and refer to the functions required to transport data across a network. ISO 7-Layer OSI reference Model 7 APPLICATION 6 PRESENTATION 5 SESSION 4 TRANSPORT 3 NETWORK Network routing, addressing, call management Network management 2 DATA LINK Data link control (framing, error control) Data link control (framing, error control) 1 PHYSICAL Mechanical and electrical interfacing interfacing management File transfer, access management Syntax and data representation management data representation management Application entity dialogue and synchronisation End -to-end message transfer End-to- end transfer Layer 7: The application layer...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.) Layer 6: The presentation layer...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). This layer is sometimes called the syntax layer. 66 GSM Technology for Engineers © AIRCOM International 2002
  72. 72. 5. GSM Protocols Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. Layer 4: The transport layer...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. Layer 3: The network layer...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Layer 2: The data-link layer...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management. Layer 1: The physical layer...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier. _____________________________________________________________________ 5.3 GSM Protocols Overview Within a GSM network, different protocols are needed to enable the flow of data and signalling between different GSM subsystems. The following diagram shows the interfaces that link the different GSM subsystems and the protocols used to communicate on each interface. GSM Protocol Layers • GSM protocols are basically divided into three layers: • Layer 1: Physical layer • • • • Enables physical transmission (TDMA, FDMA, etc.) Assessment of channel quality Definition of physical links (e.g radio, PCM30 ISDN etc) Error detection (based on line coding) • Layer 2: Data link layer • • • • • Multiplexing of one or more layer 2 connections on control/signalling channels Error detection (based on HDLC) Flow control Transmission quality assurance Routing • Layer 3: Network Layer • • • • GSM Technology for Engineers © AIRCOM International 2002 Connection management Management of location data Subscriber identification Management of Services 67
  73. 73. 5. GSM Protocols As GSM is a transport network, it is primarily only the lower 3 layers of the OSI Model that are defined in the GSM Recommendations. As GSM is predominantly a transport network it is less concerned with the end-to-end user application layer (layers 4-7). Therefore this section of the course notes looks specifically at the protocols used within GSM at layers 1-3. 5.3.1 LAYER 1 SERVICES The Physical Layer (Layer 1) contains all the functions necessary for the transmission of bit streams over the physical medium. It provides a transport service for the GSM logical channels. Services offered at Layer 1 include: • • • Access Capabilities. Layer 1 carries out the cell selection functions for MSs in idle mode, in cooperation with the Layer 3 Radio Resource (RR) functions. Error Detection . Forward and backward error correction is implemented at layer 1 (see section on speech coding for details). Errored frames are not passed to Level 2 for processing. Encryption. Data encryption is also implemented at Layer 1 (see section on GSM security for details). 5.3.2 LAYER 2 SERVICES Here, the LAPDm protocol is used (similar to ISDN LAPD). LAPDm has the following functions: • • • Connectionless transfer on point-to-point and point-to-multipoint signalling channels, Setup and take-down of layer 2 connections on point-to-point signalling channels, Connection-oriented transfer with retention of the transmission sequence, error detection and error correction. 5.3.3 LAYER 3 SERVICES Layer 3 contains the following sublayers which control signalling channel functions (BCH, CCCH and DCCH): • • 68 Radio resource management (RR). The role of the RR management layer is to establish and release stable connection between mobile stations (MS) and an MSC for the duration of a call, and to maintain it despite user movements. The following functions are performed by the MSC: • Cell selection, • Handover, • Allocation and take-down of point-to-point channels, • Monitoring and forwarding of radio connections, • Introduction of encryption, • Change in transmission mode. Mobility management (MM). Mobility Management handles the control functions required for mobility including: • Authentication • Assignment of TMSI • Management of subscriber location. GSM Technology for Engineers © AIRCOM International 2002
  74. 74. 5. GSM Protocols • Connection management (CM) is used to set up, maintain and clear call connections. It comprises three subgroups: • Call control (CC) - manages call connections, • Supplementary service support (SS) - handles special services, • Short message service support (SMS) - transfers brief texts. Neither the BTS nor the BSC interpret CM and MM messages. They are simply exchanged between the MSC and the MS using the Direct Transfer Application Part (DTAP) protocol on the A interface (see below). RR messages are mapped to or from the Base Station System Application Part (BSSAP) for exchange with the MS. _____________________________________________________________________ 5.4 GSM Transmission Protocols Transmission protocols relate to the physical transportation of raw data across the GSM network and the various protocols associated with this function GSM Transmission Protocol Stack (TRAU at MSC) Abis Um A BTS BSC BTS GSM Core Network GSM VOICE GSM Voice Sub- channel FEC Cipher TRAU PSTN ISDN Voice channel FEC Cipher MSC TDMA FDMA 13kbps TDMA D-ch D-ch 16kbps D-ch D-ch 16kbps Local Interface G.703 64kbps FDMA At the transmission level, the physical layer produces user channels each comprising 13kbps of user data. However, in order to adapt top the frailties of the radio interface additional overheads are included for forward error detection and correction (FEC). In addition the transmission layer control encryption as well as the air interface FDMA/TDMA access procedures (see Section 4 of these course notes for more details). GSM Technology for Engineers © AIRCOM International 2002 69
  75. 75. 5. GSM Protocols At some stage the 13kbps channels must be converted into 64kbps for transportation across the PSTN. This function is carried out by a network element known as the Transcoder and Rate Adaption Unit (TRAU). The primary function of the TRAU is to convert 16kps (inc signalling) GSM speech channels to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction. The reason this process is necessary is because MSCs only switch at the 64kbps channel level. Therefore, when making a MS-to-MS call, the originating channel has to be converted from 16kbps to 64kbps, switched by the MSC and then converted back to 16kbps for onward transmission to the destination MS. The rate conversion is carried out using A-law coding. Technically, the TRAU can be physically located in the BTS, BSC or MSC and hence leads to a variety of installation configurations. TRAU Configurations Um Abis BTS Site A BSC Site MSC Site CCU A TRAU CCU 64kbps 16kbps BTS Site BTS Site 64kbps 64kbps MSC Site MSC Site BSC Site CCU B TRAU CCU 16kbps 16kbps BTS Site 16kbps 16kbps 64kbps BSC Site BSC Site MSC Site CCU C TRAU CCU 16kbps 16kbps CCU Channel Coding Unit Channel 16kbps MSC Node 64kbps 64kbps BSC Node If the TRAU is installed at the BTS, each 16kbps GSM channel would need to be mapped to its own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface. However, if the TRAU is placed at the MSC, as is generally the case in current networks, a multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary, thereby maximising the efficient usage of the transmission medium by increasing the GSM channel throughput per PCM 2048 bearer from 30 to 120 channels. Also, by centralising the TRAU function at the MSC, the number of TRAUs deployed across the network is significantly reduced. 70 GSM Technology for Engineers © AIRCOM International 2002
  76. 76. 5. GSM Protocols G.703 The TRAU converts the GSM channels into ISDN D-channels in accordance with the G.703 Recommendation. This Recommendation specifies physical and electrical characteristics of the interfaces at hierarchical bit rates that are described in Recommendation G.702. The interfaces are defined in terms of general characteristics, specifications at the output ports and input ports and/or cross-connect points, earthing of outer conductor or screen and coding rules etc. _____________________________________________________________________ 5.5 GSM Signalling Protocols GSM Signalling Protocol Stack Abis Um A BTS GSM Core Network Network BSC MSC PSTN BTS CM CM MM MM RR DTAP BSSMAP DTAP BSSMAP SCCP SCCP MTP’ MTP’ RR RR LAPDm TDMA MS BTSM BTSM LAPDm LAPD LAPD TDMA BTS G.703 Layer 3 Layer 2 G.703 BSC Layer 1 MSC The GSM Signalling Protocol Stack (Um / A-bis / A Interfaces) contains the following protocols: • Layer 1 Protocol TDMA – Time Division Multiple Access G.703 – ITU PCM frame structure MTP – Message Transfer Part • Layer 2 Protocols LAPDm Link Access Protocol D-channel-mobile LAPD Link Access Protocol D-Channel GSM Technology for Engineers © AIRCOM International 2002 71

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