GSM System Introduction 1

7/27/2019 GSM System Introduction 1

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GSM System introduction

By

Muhammad Farhan Shafique


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Introduction

Global System for Mobile Communication (GSM)

Anybody 500 million users (may 2001)

Anywhere 168 countries (may 2001)

Any media voice, messaging, data, multimedia


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GSM Architecture

OMC

Home Location Register

AuC

Equipment ID

Network

Management

Center

B

TS

B

TS

B

TS

ME

ME

ME

SubscriberIdentity

Module

SubscriberIdentityModule

SubscriberIdentityModule

Base station

controller

PSTN

Mobile

switchingcenter

Data

communication

network

BTS = Base Transceiver Station

AuC = Authentication Center

OMC = Operation and Maintenance Center

PSTN = Public Switched Telephone Network

ME = Mobile Equipment Visitor LocationRegister

Base Station Subsystem


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Mobile Station (MS)

TEMPORARY DATA PERMANENT DATA

- Temporary Subscriber Identity Permanent Subscriber Identity

- Current Location Key/Algorithm for Authentication.

- Ciphering Data

Provides access to the GSM n/w

Consists of

Mobile equipment (ME)

Subscriber Identity Module (SIM)


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Block diagram of GSM MS


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SIM (Subscriber Identity Module)

Billions of Calls

Millions of Subscribers

Thousand of Different Types of Telephones

Hundreds of Countries

Dozens of Manufacturers.

And only one Card: The SIM

Administrative data

Security data

Subscriber data

Roaming data

PLMN (Public Land Mobile Network


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SIM

The real identity of GSM MS and its subscriberis the data stored in SIM.

ME+SIM=MS

Consists of ROM and EEPROM SIM memory is divided in to different directories

like hard disk.

SIM communicate directly with VLR andindirectly with HLR.

It consists of IMSI,MSISDN,LAI and many otherinformation like ciphering key, location updatestatus,


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IMEI

International Mobile Station Equipment

Identity

a serial number which uniquely identifies the

mobile station internationally.

usually requested from the MS by the network at

registration and comprises:

Type App rova l Code (TAC): 6 decimal digits

F ina l Assemb l y Code (FAC): 6 decimal digits

Ser ia l Num ber (SNR): 6 decimal digits

Spare(SP): 1 decimal digit

IMEI = TAC + FAC + SNR + SP (19 digits)


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IMSI

International Mobile Subscriber Identity

t h eunique identifier stored on the SIM

a MS will only operate if a SIM with a valid IMSI is

inserted into equipment with a valid IMEI as this is

the only way to bill the subscriber

comprises:

Mobile Country Code (MCC): 3 decimal digits

Mobile Network Code (MNC): 2 decimal digits

Mobile Subscriber Identification Number (MSIN): 10decimal digits

IMSI = MCC + MNC + MSIN (15 digits)


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MSISDN

Mobile Subscriber ISDN Number

the real telephone number of the subscriber / MS

stored on the HLR and associated with the IMSI

protects the confidentiality of the IMSI making

fraud more difficult

comprises:

Coun t r y Code (CC): up to 3 decimal places

Nat ion a l Des t i na t ion Cod e (NDC): typically 2 or 3 decimalplaces

Subsc r i be r Num be r (SN): maximum 10 decimal places

MSISDN = CC + NDC + SN


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LAI

Location Area Identity

Each location area of a PLMN has its own identifier

Location Area ID (LAI)

internationally unique comprising: Country Code (CC): 3 decimal digits

Mobile Network Code (MNC): 2 decimal digits

Location Area Code (LAC): maximum of 5 decimal digits

LAI = CC + MNC + LAC

Broadcast regularly on the Broadcast Control Channel (BCCH)

Mobile recognises LAI stored on SIM and change of LA

gives rise to location update request


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Base Station Subsystem (BSS)

With air interface BSS provides MS and

NSS. It consists of following elements.

One or more BTS base (transceiverstation)

One BSC (Base station controller)

One TRAU (transcoding rate and

adaptation unit)


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Base transceiver station

(BTS) provides GSM radio coverage within

a cell. It comprises radio transmitting and

receiving equipment (including antennas)

and associated signal processing. Today

the size of BTS is just as compared to mail

box.


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Implementation

TO NSS

O&M Module Operation and maintenance functionality/clock distribution

Digital signalprocessing (NF

functionality)

HF Transmitter

(HF-TX)

Input filter

Output filter

HF Receiver(HF-RX)

Slowf

reque

ncy

hopping

Transmission

system

TRX

Block Diagram of a BTS with one TRX


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Functionality of BTS 1

Transmitter/receiver module:

This is the most important part of BTS from

signal processing point of view.

It consist of Low frequency parts for DSP and

HF part for GMSK modulation/demodulation.

All other parts of BTS are associated with TRX

and performs auxiliary or administrative


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Operation and maintenance module.

It consist of at least one central unit which

administrate all other parts of BTS. That central unit contain system and

operation software.

It also provides Human machine interface

for local control of BTS.


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Clock Module Module of clock generation and distribution is also a part

of O&M.

Although the trend is to derive the reference clock from

PCM signal but it is necessary when BTS is tested instandalone environment with out link to BSC.

When analyzing errors clock plays major role especiallywhen handover takes place minor deviation can createbig errors.

The accuracy of signal has to be 0.05 part per million.For example a clock generator that derives clock from10MHz signal has to provide the clock with accuracy of10MHz0.5Hz.


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Input and output filters.

Both input and output filters are used to limit the

bandwidth of received and transmitted.

Input filters are generally non adjustable filters

which lets pass all GSM 900, DCS 1800, PCS

1900 in uplink direction.

Output filters are controllable by O&M centerthat limits the bandwidth of 200KHz in down link

direction.


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BTS configuration

Standard configuration

Umbrella cell configuration.

Sectorized BTS configuration.


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Standard configuration


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Umbrella cell configuration


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Sectorized configuration

GSM System Introduction 1

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