GSM Architecture

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GSM SubSystems

GSM architecture is mainly divided into three

Subsystems

1.Base Station Subsystem (BSS)

2.Network & Switching Subsystem (NSS)

3.Operations & Support Subsystem (OSS)

Mobile Station sometimes included in BSS

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GSM Network Architecture

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Um I/ F

MSBTS

BSC

T C MSC PSTN

VLR

HLR AUC

EIR

A-bisI/F

A-terI/F

A I / F

Simplified GSM Architecture

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Mobile Station (MS)

The MS consists of two parts

1.Mobile Equipment (ME)

2.Subscriber Identity module (SIM)

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+G S M

Global GSM MobilityCardThe Smart Card to use

SIM Card

+

BatteryHandset

battery

2Wjmhfodkgdjipj

f153454

Mobile Station

=

Mobile Station

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G S M

Global GSM MobilityCardThe Smart Card to use

+

SIM-Card

Handset

Subscriber knows- Called party number = MS-ISDN- PIN

Contains:- IMSI

= Calling line 0609225831

SIM-Card and GSM Mobile Equipment

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25 mm

15 mm

Microchip with storeduser information

Credit Card Size

Permanent data:- Unique mobile subscriber identitythrough IMSI number,

- Authentication parameter Ki,- Authentication algorithm A3,- Generating encryption key Kc algorithm A8.

Removable data:- Temporary Mobile Subscriber Number,- Location Area Identification.

µ SIM-Card

G S M

Global GSM MobilityCardThe Smart Card to use

The SIM-Card Functions

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NatureInternational Mobile Subscriber Identity

Conformity with E212

Mobile Station - Integrated Services Digital Network Nb

Similar to ISDN, Conformity with E164/E213

Nb. digits 3 2 max 10 1 to 3 2 to 4 total max 15

*This code does not identify a geographical areabut an operator

MS - ISDN

Format MCC MNC MSINH1 H2 x x x ......... x x x

CC NDC SNM1 M2 x x x x x x x x

MeaningMobile

CountryCode

MobileNetwork

Code

Mobile SubscriberIdent. Nb

H1 H2 = Identity of HLR within the home PLMN

CountryCode

(where subscription

has been made)

NationalDestination

Code *

Mobile Subscriber(national definition)

M1 M2 = nbr of logical HLR

IMSI

National Significant Mobile NumberIdentify a PLMN worldwide

Identify the subscriber of a PLMN

Subscriber Identification

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Type ApprovalCode

TAC FAC SNR SP

Final AssemblyCode

Serial number (SPare)

TYPE

APPROVED

Mobile Identification

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PocketHands-free Data

Booster2 W 5 W2 W 8 W

Fax OrganizerPC

Dual-band900-1800900-1900

Java

Trends in Mobile Station

Mobile Equipment (ME)

• The ME is the only part of the GSM network which the subscriber will really see.

• There are three main types of ME, these are listed below:1. Vehicle Mounted

2. Portable Mobile Unit

3. Hand portable Unit

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The SIM is a card which plugs into the ME. This card identifies the MS subscriber and also provides other

information regarding the services that subscriber should receive.

The SIM card, and the high degree of inbuilt system security, provides protection of the subscriber’s information and protection of networks against fraudulent access.

The SIM can be protected by use of Personal Identity Number (PIN) password, similar to bank/credit charge cards, to prevent unauthorized use of the card.

SIM cards are designed to be difficult to duplicate. By making a distinction between the subscriber identity and

the ME identity, GSM can route calls and perform billing based on the identity of the ‘subscriber’ rather than the equipment or its location.

Subscriber Identity module (SIM)

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The SIM contains several pieces of information:1. International Mobile Subscriber Identity (IMSI)

2. Temporary Mobile Subscriber Identity (TMSI)

3. Location Area Identity (LAI)

4. Subscriber Authentication Key (Ki)

5. Mobile Station Integrated Services Digital Network (MSISDN)

The SIM is capable of storing additional information

such as accumulated call charges.

The SIM also executes the Authentication Algorithm.

Subscriber Identity module Cntd….

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Subscriber Identity module Cntd….

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Base Station Subsystem (BSS)

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TCU

BSCOMC-R

MSCRadio

InterfaceA Interface

Ater Interface

Abis Interface

NSS

BSS

OMN Interface

Public Telephone Network

MS

MS

S2000H&LBTS

S8000Indoor

BTS

S8000Outdoor

BTS

Sun

StorEdge A5000

RadioInterface

BSS Architecture

Base Station Controller (BSC)

the BSC provides the control for the BSS.Any operational information required by the BTS will

be received via the BSC.Likewise any information required about the BTS (by

the OMC for example) will be obtained by the BSC.The BSC incorporates a digital switching matrix, which

it uses to connect the radio channels on the air interface with the terrestrial circuits from the MSC.

The BSC switching matrix also allows the BSC to perform “handovers” between radio channels on BTSs, under its control, without involving the MSC.

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BTS Abis interface A interface

- Radio Resource management for its BTSs

- Intercell hand-over

- Allocation of channels for communication

- Reallocation of frequencies among BTSs

- Time and frequency synchronization to BTSs

- Controls frequency hopping

O&M

To NetworkSubSystem

PCMcontroller

PCMcontroller

ProcessingUnit

Switchingmatrix

BSC

X.25controller

BSC General Architecture and Functions

Base Transceiver Station – BTS

• The BTS provides the air interface connection with the MS.

• It also has a limited amount of Control functionality which reduces the amount of traffic passing between the BTS and BSC.

• Where the BSC and BTS are both shown to control a function, the control is divided between the two, or may be located wholly at one.

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Transmission coupler

Reception coupler

Antenna

COUPLING SYSTEM

BCF(Base Common Functions)

BSC

TRX (Transceiver-Receiver)

Abisinterface

- Encodes, encrypts, modulates, feeds the RF signal to the antenna

- Decrypts and equalizes the signal then demodulates

- Mobile call detection

- Uplink channel measurements

- Timing advance

- Frequency hopping

- Multiplexes speech and user's data channels to BSC.- Multiplexes signaling channels to BSC.

Du

ple

xer

- Interface between Antennas and TRXs of each cell

BTS

BTS General Architecture and Functions

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BSS Configurations

• The maximum number of BTSs which may be controlled by one BSC is not specified by GSM.•The BTSs and BSC may either be located at the same cell site “co-located”, or located at different sites “Remote”.•Another BSS configuration is the daisy chain.•Problem- transmission delay through the chain.

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External PCMInterface

BSC MSC

Transcoder

TranscoderController

Ater

interface

A

interface

E1 trunkup to 120 user's

channels

T1 trunkup to 92 user's

andcontrol channels

Converts the 13 kbps GSM speech frame eitherinto a 64 kbps T1 PCM µ-law or into an E1 PCM A-law

E1 trunk = up to 31 user's channels

T1 trunk = up to 24 user's channels

Routes the users' data streamto suitable Inter-working function

TRAU

TRAU Architecture and Functions

Transcoder (XCDR)

• The 64 kbit/s PCM circuits from the MSC, if transmitted on the air interface without modification, would occupy an excessive amount of radio bandwidth.

• The Transcoder is to convert the speech or data output from the MSC into the form suitable for transmission over the air interface.

• The required bandwidth is therefore reduced by processing the 64 kbit/s circuits so that the amount of information required to transmit digitized voice falls to a gross rate of 16 kbit/s.

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• The transcoding function may be located at the MSC, BSC, or BTS.

• The content of the 16 kbit/s data depends on the coding algorithm used.

• There are two speech coding algorithms available.• The Full Rate speech algorithm is supported by all

mobiles and networks.• It produces 13 kbit/s of coded speech data plus 3 kbit/s

of control data known as TRAU data (Transcoder Rate Adaptation Unit).

• TRAU only used by BTS and discarded (not transmitted on air interface)

Transcoder (XCDR) Cntd….

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• the 13 kbit/s of speech data is processed at the BTS to form a gross rate of 22.8 kbit/s on the air interface which includes forward error correction.

• In the uplink direction the BTS adds in TRAU data which will be used by the transcoder.

• Enhanced Full Rate is an improved speech coding algorithm and is only supported by Phase 2+ mobiles.

• It produces 12.2 kbit/s from each 64 kbit/s PCM channel and TRAU data of 3.8 kbits/s

Transcoder (XCDR) Cntd….

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Network Switching System (NSS)

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MSC

AUC

GMSC

BSS

Other GSM,PSTN, ISDN

G-interface

IWF IWF

Site 2

C-interfaceA-interface A-interface

B-interface B-interface

E

F

E

F

H

DDBSS

Other GSM,PSTN, ISDN E-interface

Site 1

NSS Architecture

VLR

HLR

VLR

EIR

SMS-SC BillingServer

BillingServer

Network Switching System (NSS)

• The Network Switching System includes the main switching functions of the GSM network.

• It also contains the databases required for subscriber data and mobility management.

• The components of the Network Switching System are listed below:1. Mobile Services Switching Centre – MSC2. Home Location Register – HLR3. Visitor Location Register – VLR4. Equipment Identity Register – EIR5. Authentication Centre – AUC6. Interworking Function – IWF7. Echo Canceller – EC

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Mobile Switching Centre (MSC)

MSC is the heart of the system, controlling the Switching & Billing.

The MSC can carry out different functions depending upon its position in the network.

When provides interface between PSTN & BSS in GSM network then known as a Gateway MSC

provides service to MSs located within a defined geographic coverage area.

The network typically contains more than one MSC. One MSC is capable of supporting a regional capital

with approximately one million inhabitants.

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• Call Processing1. control of data/voice call setup2. inter-BSS and inter-MSC handovers 3. control of mobility management (subscriber validation

and location).• Operations and Maintenance Support

1. database management2. traffic metering and measurement3. A Man–machine interface.

• Internetwork Interworking1. Interface between the GSM network and the PSTN.

• Billing1. Collects call billing data.

MSC’s Functionalities

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SubscriberManagement

Center

HLR

Permanent records- MSISDN- IMSI- Subscriber's service provision

Temporary records- VLR address- Ciphering items (Kc, Sres, Rand)

Home Location Register

Home Location Register (HLR)

The HLR is the master database which contains each user’s service profile.

Various identification numbers and addresses are stored, as well as authentication parameters.

The data it contains is remotely accessed by all the MSCs and the VLRs in the network.

Although the network may contain more than one HLR, there is only one database record per subscriber .

The subscriber data may be accessed by either the IMSI or the MSISDN number.

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VLR

Permanent records- IMSI- Subscriber’s service provision

Temporary records- Ciphering items (Kc, Sres, Rand)

- LAI - TMSI

LA1

LA4

LA2LA3

Visitor Location Register

VLR is a temporary database for all user currently located in the system including roamers & non-roamers.

The data exists for only as long as the subscriber is “active” in the particular area covered by the VLR.

The VLR database will therefore contain some duplicate data as well as more precise data relevant to the subscriber.

This function eliminates the need for excessive and time-consuming references to the “home” HLR database.

Visitor Location Register (VLR)

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The additional data stored in the VLR is listed below:1. Mobile status (busy/free/no answer etc.).

2. Location Area Identity (LAI).

3. Temporary Mobile Subscriber Identity (TMSI).

4. Mobile Station Roaming Number (MSRN).

• MSC updates VLR with HLR information.

• Each MSC has VLR which resides with the MSC

& each G-MSC has a HLR which usually resides with the G-MSC

Visitor Location Register Cntd….

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EIR

Black list(barred ME)

Gray list(faulty ME)

White list(valid ME)

MobileEquipment

IMEI

Equipment Identity Register

• The EIR contains a centralized database for validating the IMEI.

• This database is concerned solely with MS equipment and not with the subscriber who is using it to make or receive a call.

• The EIR database consists of lists of IMEIs (or ranges of IMEIs) organized as follows:1. White List

2. Black List

3. Grey List

Equipment Identity Register (EIR)

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Equipment Identity Register Cntd….

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AUC

SecurityA3, A8 algorithms

Ki 5

SRES, Kc, RAND

RAND

AUC provides

Ciphering Triplets

IMSI

HLR Request

Authentication Center

Authentication Centre (AuC)

• The AuC is a processor system that performs the “authentication” function.

• It is normally co-located with the HLR as it will be required to continuously access and update, as necessary, the system subscriber records.

• The authentication process will usually take place each time the subscriber “initializes” on.

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MobileSwitching

Center

IWF

MS

BSS PSTN

DTE

Land-DTE

Rateadaptation

DTEsignaling

Modem

Modem

Data + DTE signals

InterWorking Function

Interworking Function (IWF)

• IWF provides the function to enable the GSM system to interface with the various forms of public and private data networks.

• The basic features of the IWF are 1. Data rate adaptation.

2. Protocol conversion.

• Some systems require more IWF capability than others, this depends upon the network to which it is being connected.

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Interworking Function (IWF) Cntd…

• The IWF also incorporates a ‘‘modem bank”, which may be used when

• for example, the GSM Data Terminal Equipment (DTE) exchanges data with a land DTE connected via an analogue modem.

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Echo Canceler

MobileSwitching

Center

BaseStation

SubSystem

EchoCanceler

Switch

Land telephone

4w to 2wtransformer

GSM network 4 wire circuit PSTN

Localloop

Talker Echo

Two wire circuit

4 wirecircuit

Tal

ker

Ec

ho

4 wirecircuit(PCM)

Echo Canceller (EC)

• An EC is used on the PSTN side of the MSC for all voice circuits.

• Echo control is required at the switch because the inherent GSM system delay(180 ms approx) can cause an unacceptable echo condition.

• This would not be apparent to the MS subscriber, but for the inclusion of a 2-wire to 4-wire hybrid transformer in the circuit.

• The transformer causes the echo. This does not affect the land subscriber.

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Echo Canceller (EC) cntd….

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• During a normal PSTN land to land call, no echo is apparent because the delay is too short.

• without the EC, the effect would be very irritating to the MS subscriber.

• disrupting speech and concentration.

• EC provides cancellation of up to 68 ms on the “tail circuit”.

• the tail circuit is the connection between the output of the EC and the land telephone.

Echo Canceller (EC) cntd….

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Operations & Support System (OSS)

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Operations & Support System (OSS)

• The OSS provides the capability to manage the GSM network remotely.

• This area of the GSM network is not currently tightly specified by the GSM specifications.

• It is left to the network provider to decide what capabilities they wish it to have.

• The Operations and Maintenance System comprises of two parts:1. Network Management Centre (NMC)2. Operations and Maintenance Centre (OMC)

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Operations & Support System Cntd....

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Operations & Support System Cntd....

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Network Management Centre (NMC)

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Operations and Maintenance Centre (OMC)

The OMC provides a central point from which to control and monitor the other network entities (i.e. base stations, switches, database, etc).

It also monitors the quality of service being provided by the network.

There are two types of OMC these are:1. OMC-R OMC controls specifically the Base Station System.2. OMC-S OMC controls specifically the Network Switching System.

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The OMC should support the following

functions as per ITS–TS recommendations:

1. Event/Alarm Management.

2. Fault Management.

3. Performance Management.

4. Configuration Management.

5. Security Management.

Operations and Maintenance Centre Cntd….

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Operations and Maintenance Centre Cntd….

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Network in Reality

• In reality a GSM network is much more complicated than we have seen.

• The diagram in the next slide illustrates how multiple BSS and Network Switching System components will be connected within a network

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Practical Example; London City

• Typical city for example, London will have approximately the following number of network components:

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• A typical network (for example, UK) will have approximately the following number of network components.

Mobile Network in U.K

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