Overview GSM giới thiệu về mạng GSM

giới thiệu về mạng GSM, quá trình phát triển của mạng thông tin di động trên thế giới.In early networks, the emphasis was to provide radio coverage with little consideration for the number of calls to be carried

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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

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GSM TECHNOLOGY FOR ENGINEERS

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Table of Contents

1 Introduction to Cellular Systems

1.1 Introduction 1

1.2 Cellular Radio 2

1.3 1st Generation Cellular Systems 3

1.4 2nd Generation Cellular Systems 5

1.5 GSM Development and Characteristics 7

1.6 Cellular Systems Subscriber Distribution 8

1.7 2.5G Characteristics 9

2 Architecture Overview 2.1 Introduction 11

2.2 GSM Identifiers 12

2.3 General Architecture Overview 13

2.4 The GSM Mobile Station (MS) 14

2.5 The Base Station Subsystem (BSS) 16

2.6 The Network Switching Subsystem (NSS) 17

2.7 GSM Interfaces 22

Self-Assessment Exerecises 27

3 GSM Services 3.1 Introduction 29

3.2 GSM Bearer Services 30

3.3 Teleservices 32

3.4 Supplementary Services 33

3.5 Circuit-Switched Data (CSD) Services 34

3.6 Short Message Service 35

3.7 USSD Data Services 37

Self-Assessment Exerecises 41

4 The Air Interface 4.1 Introduction 43

4.2 GSM Frequency Spectrum Allocation 44

4.3 GSM Multiple Access Techniques 50

4.4 GSM Air Interface Channels 52

4.5 Frames and Multiframes 56

Self-Assessment Exercises 63

5 Protocols 5.1 Introduction 65

5.2 The ISO 7-Layer OSI Model 66

5.3 GSM Protocols Overview 67

5.4 GSM Transmission Protocols 69

5.5 GSM Signalling Protocols 71

5.6 GSM Air (Um) Interface Protocols 73

5.7 GSM A-bis Interface Protocols 75

5.8 The GSM A Interface Protocols 78

6 Speech and Channel Coding 6.1 Introduction 79

6.2 Speech Coding Techniques 80

6.3 GSM Speech Coding 81

6.4 Channel Coding Techniques 83

6.5 GSM Channel Coding 88

6.6 Interleaving 89

6.7 Radio Burst Multiplexing…… ……… 91

6.8 Summary of Coding Processes ………… ………95

6.9 Radio Interface Modulation… ……… ………95

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7 Mobility Management

7.1 Introduction 99

7.2 Mobility Management Procedures 100

7.3 Network Areas 101

7.4 Mobility States 103

7.5 IMSI Attach/Detach 104

7.6 Location Updating 107

7.7 Roaming 109

7.8 TMSI Reallocation 110

8 Radio Resource Management 8.1 Introduction 113

8.2 Radio Resource Connection Setup 114

8.3 Cell Selection and Reselection 115

8.4 Handovers 118

9 Call Management 9.1 Introduction 123

9.2 Mobile Originated Calls 124

9.3 Mobile Terminated Calls 125

9.4 Call Routing Examples 127

9.5 Echo Cancelling 129

10 GSM Security 10.1 Introduction 133

10.2 Purposes of GSM Security 134

10.3 User Identity Confidentiality 134

10.4 Authentication 135

10.5 User Data Confidentiality - Encryption 138

10.6 Signalling Data Confidentiality 140

11 Billing Procedures Overview 11.1 Introduction 145

11.2 Billing Principles 146

11.3 Description of Call Components 146

11.4 Charge Advice Information (CAI) 148

11.5 Advice of Charge (AoC) Calculations 149

11.6 Call Detail Records 151

11.7 The Transferred Account Procedure (TAP) 152

12 GSM Evolution 12.1 Introduction 155

12.2 High Speed Circuit Switched Data (HSCSD) 156

12.3 General Packet Radio Service (GPRS) 157

12.4 Enhanced Data for nGSM Evolution (EDGE) 159

Appendix A - Solutions to Self Assessment Exercises

Appendix B - Glossary of Terms

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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

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Intentional Blank Page

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1 Introduction to Cellular Systems

• Cellular Radio

• 1st generation cellular characteristics and systems

• 2nd generation cellular characteristics and non-GSM systems

• GSM development and characteristics

• Worldwide roll-out of cellular systems and subscriber distribution

1.1 Introduction

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_

Paging Control Centre

Paging Area

Mobile Radio Networks

• There are three major types of terrestrial mobile communications technologies:

•Cellular:

Users are provided wide area mobility from multiple base stations with handover permitted

MSC

Cellular

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)

1.2 Cellular Radio

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Cellular Generations

The significant stages in the evolution of cellular radio systems is referred to in terms of generations:

Progress of data rates with time and generation

1978 1992 2000 2002 ?

_

1st Generation Cellular Characteristics

• Widespread Introduction in early 1980s

• Analogue modulation

• Frequency Division Multiple Access

• Voice traffic only

• No inter-network roaming possible

• Insecure air interface

The 1st Generation of Cellular Technology makes use of analogue modulation techniques such as FM

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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

• JTACS (Japanese Total Access Communications System)

• Japanese Standard in 900MHz band

Analogue/AMPS Nokia 252

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

• Large cell size

• Order 30km

• Hard handover

• 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

F2

F1

F4 F5

F3

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The First Generation

• Problems with the analog systems included:

• Limited capacity – could not cope with increase in subscribers

• Bulky equipment

• Poor reliability

• Lack of security – air interface analogue signals could be intercepted

• Incompatibility between systems in different countries - no roaming

• To improve on the analog systems, the European Conference of Posts and Telecommunications Administrations (CEPT) established ‘Groupe SpecialeMobile’ (GSM) to set a new standard

• The system developed became the Global System for Mobile Communications (also GSM)

Digital systems offer considerable advantages in terms of capacity and security and introduce new possibilities for data traffic

2nd Generation Characteristics

• Widespread Introduction in 1990’s

• Uses digital modulation

• Variety of multiple access strategies

• More efficient use of radio spectrum

• Voice and low rate circuit switched data

• International roaming capability

• Secure air interface

• Compatibility with ISDN

000110100110

111001001111 000

001001011110 0

111100010110000010010 0

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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

• Code Division Multiple Access

many of the 3rd generation standards

• ANSI-41 core network

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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)

• 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)

F3

1.5 GSM Development and Characteristics

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Worldwide Mobile Communications Systems

Subscribers (x million)

Source:Wideband CDMA for 3rd Generation Mobile Communications, Artech House, 1998

0 100 200 300 400 500 600 700

Second Generation - DAMPS Second Generation - PDC Second Generation - GSM Second Generation - cdmaOne First Generation - Analogue

Worldwide Mobile Subscriber Distribution

Source: Third Generation Mobile Communications Artech House, 2000

Subscribers (x million)

0 500 1000 1500 2000

European Union Countries North America

Asia Pacific Rest of World1.6 Cellular Systems Subscriber Distribution

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• Secure air interface

• Based upon existing dominant 2G standards such as GSM and cdmaOne

• Enhanced data rates

1.7 2.5G Characteristics

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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

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• 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

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2 Architecture Overview

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

2.2 GSM Identifiers

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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

2.2.8 BSIC – BASE STATION IDENTITY CODE

Each BTS is issued with a unique identity, the BSIC and is used to distinguish neighbouring BTSs

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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

_

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)

Mobile Terminal (MT) Mobile Terminal (MT)

TE - Terminal Equipment

TA - Terminal Adaptor

MS - Mobile Station

ME - Mobile Equipment SIM - Subscriber Identity Module

Base Station Subsystem

T A

Reference Points

R 2.4 The GSM Mobile Station (MS)

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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

36 (4W) 480

DCS class 3

24 (.25W) 30

DCS class 2

30 (1W) 120

DCS class 1

29 (0.8W) 96

GSM class 5

33 (2W) 240

GSM class 4

37 (5W) 600

GSM class 3

39 (8W) 960

GSM class 2

dBm Power mW

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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

• The BSS comprises:

• Base Station Controller (BSC)

• One or more Base Transceiver Stations (BTSs)

• 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

• Siting of the BTS is crucial to the provision of acceptable radio coverage

BSC

BTS

BTS BTS

BSS

BTS

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

2.5 The Base Station Subsystem (BSS)

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BSS Network Topologies

• Chain: cheap, easy to implement

• One link failure isolates several BTSs

• Ring: Redundancy gives some protection if a

link fails

• More difficult to roll-out and extend

• ring must be closed

• Star: most popular configuration for first GSM

systems

• Expensive as each BTS has its own link

• One link failure always results in loss of BTS

2.6.1 NSS OVERVIEW

Network Switching System (NSS)

• Key elements of the NSS:

• Mobile Switching Centre (MSC) with:

• Visitor Location Register (VLR)

• Home Location Register (HLR) with:

• Authentication Centre (AuC)

• Equipment Identity Register (EIR)

MSC VLR

HLR AuC GMSC

2.6 The Network Switching Subsystem (NSS)

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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

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

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Visitor Location Register (VLR)

• Each MSC has a VLR

• VLR stores data temporarily for mobiles served by the MSC

• Information stored includes:

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)

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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)

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

• Combined with AuC

HLR AuC

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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

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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

• 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

EIR

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

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H B

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

2.7 GSM Interfaces

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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

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

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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

TRX

Air Interface (Um)

BSS BSS

OMC OMC

A Interface Abis

This section has covered:

• General GSM Architecture Overview

• GSM Network Components including

• The Mobile Station

• The Base Station Subsystem

• The Network Subsystem

• GSM Network Interfaces Overview

• The Base Station Sub-system

• The Network Switching System

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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

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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

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_

GSM Bearer Services

• Bearer Services represent layers 1-3 of the OSI Model

• Transparently transports application data between TEs

• Bearer Services are uniquely numbered (BSxx)

• Data transfer bearer services currently defined for GSM include:

• Asynchronous circuit-switched data (BS 21-26)

• Synchronous circuit-switched data (BS 31-34)

• PAD access (BS 41-46)

• Packet data (BS 51-53)

• Alternate speech and data (BS 61)

• Speech followed by Data (BS 81)

3.2 GSM Bearer Services

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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’)

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3 GSM Services

_

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)

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

3.3 Teleservices

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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)

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

3.4 Supplementary Services

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3 GSM Services

Supplementary Service Groups

Supplementary Service Group GSM Specification

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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

3.5 Circuit Switched Data (CSD) Service

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