GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATION)

By

Ank

ur

Kum

ar

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Contents• GSM-Introduction

• Business Objectives of GSM

• Technical Aspects

• History of GSM

• GSM in India

• GSM Services

• Architecture

• GSM Entities

• Mobile Station

• Base Station Subsystem

• Network Switching Subsystem

• Operation Support Subsystem

•

• Call Routing

• Incoming call

• Outgoing call

• Handover

• Mobile Management

• Security

• Authentication

• Key Generation and encryption

• Characteristics and features

• Advantages

• Applications

• Future in GSM

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GSM-INTRODUCTION

• Global System for Mobile (GSM) is a second generation cellular standard developed to cater voice services and data delivery using digital modulation

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GSM: Business Objectives • Support for international roaming• Good speech quality• Ability to support handheld terminals• Low terminal and service cost• Spectral efficiency• Support for a range of new services and facilities• ISDN compatibility

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GSM: Technical aspects• GSM uses combination of FDMA, TDMA & SDMA

• GSM has an allocation of 50 MHz (890-915 MHz & 935-950 MHz) bandwidth in 900 MHz frequency band

• using FDMA this band is divided into 124 channels (125, 1 not used) each with 200 KHz bandwidth

• Using TDMA, each of these channels divided into 8 time slots, realizing 992 channels

• To serve hundreds of thousands users, frequency must be reused using SDMA (cells)

• regular repetition of frequencies in cells results in clustering of cells.

• Number of cells (k) in each cluster defines the size of that cluster

• A cluster consumes whole frequency band

• GSM operates in GSM 900 MHz, GSM 1800 MHz and GSM 1900 MHz frequency ranges

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GSM: History

Year Event

1982 Conference of European Post and Telecommunication (CEPT) formed a study group called Group Special Mobile to study and develop Global System for Mobile Communication(GSM)

1987 Essential elements of wireless transmission specified

1989 Responsibility of GSM transferred to European Telecommunication Standard Institute (ETSI)

1991 First GSM network launched (phase 1)

1993 First roaming agreement came into effect

1994 Data transmission capability launched

1995 Fax & SMS roaming services offered (phase 2)

2002 SMS volume crosses 24 billion/year, 750 million subscribers

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GSM in India

Figures: March 2005

Bharti27%

BSNL22%

Spice 4%

IDEA13%

Hutch19%

BPL6%

Aircel4%

Reliance3%

MTNL2%

Bharti

BSNL

Hutch

IDEA

BPL

Aircel

Spice

Reliance

MTNL

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GSM SERVICESGSM provides integrated services for voice and data. It delivers following three kind of services: Tele-services: These services enables voice communication via mobile

phones. Offered services are Mobile telephony, Emergency calling etc.

Supplementary services: GSM also offers following call related

supplementary services:• Call Waiting- Notification of an incoming call while on the handset• Call Hold- Put a caller on hold to take another call• Call Barring- All calls, outgoing calls, or incoming calls• Call Forwarding- Calls can be sent to various numbers defined by the user• Multi Party Call Conferencing - Link multiple calls together

Bearer or Data Services: Include various data services for information

transfer between GSM and other networks like PSTN, ISDN etc. these

are short Message Service (SMS) , Unified Messaging Services(UMS),

Voice mailbox, Electronic mail etc.

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GSM Architecture• It consists minimum one administrative region (PLMN:

public land mobile network) assigned to one Mobile Switching Center (MSC)

• Each administrative region is divided into one or many Location Area (LA)

• One LA consists of many cell groups • Each cell group is assigned to one Base Switching Center

(BSC)• Cells are formed by radio areas covered by Base

Transceiver Station (BTS) • Several BTSs are controlled by one BSC• Traffic from Mobile Station (MS) is routed through MSC,

Gateway MSC, ISDN/ Public Switched Telephone Network (PSTN)/ Packet Data Network (PDN), internet.

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GSM System Architecture

BSC

BSC

MSC

MS

MS

MS BTS

BTS

BTS

GMSC

PSTNISDNPDN

EIRAUC

HLR

VLR

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GSM EntitiesGSM network can be divided into five main groups:• The Mobile Station (MS): it includes Mobile Equipment (ME) & Subscriber Identity Module (SIM)• The Base Station Subsystem (BSS): It includes BTS & BSC• The Network and Switching Subsystem (NSS): It includes

– Mobile Switching Center (MSC)– Home Location Register (HLR)– Visitor Location Register (VLR)– Equipment Identity Register (EIR)– Authentication Center (AUC)

•The Operation Support Subsystem (OSS): it includes Operation and Maintenance Center (OMC)

•The data infrastructure that includes Public Switched Telephone Network (PSTN), Integrated System Digital Network(ISDN), Public Data Network (PDN)

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GSM Entities

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Mobile Station (MS): Mobile Equipment

Mobile Equipment (ME) or the mobile device or a phone without SIM

• Portable, vehicle mounted, hand held device • Uniquely identified by an IMEI (International Mobile Equipment Identity)• Voice and data transmission • Monitoring power and signal quality of surrounding cells for optimum

handover• Power level : 0.8W – 20 W• Other features are weight, volume, battery life etc.

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Mobile Station (MS): Subscriber Identity Module (SIM)

• Smart processor with small memory• Smart card contains the International Mobile Subscriber Identity (IMSI)• Allows user to send and receive calls and receive other subscribed

services• SIM Application Toolkit (SAT) is used to develop and store the

applications into the SIM• Protected by a password or PIN• Can be moved from phone to phone – contains key information to

activate the phone

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Base Station Subsystem (BSS)

Base Transceiver Station (BTS):• Encodes, encrypts, multiplexes, modulates and feeds the RF signals

to the antenna.• Communicates with Mobile station and BSC

Base Station Controller (BSC)• Manages Radio resources for BTS• Assigns Frequency and time slots for all MS’s in its area• Handles call set up• Handover for each MS• Radio Power control • It communicates with MSC and BTS

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Network Switching Subsystem(NSS)

Mobile Switching Center (MSC)

• Heart of the network equivalent of an exchange in a fixed network• Manages communication between GSM and other networks• Call setup function and basic switching• Call routing by using HLR/VLR.• Billing information and collection• Mobility management

- Registration- Location Updating- Inter BSC and inter MSC call handoff

• MSC does gateway function while its customer roams to other network by using HLR/VLR.

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Network Switching Subsystem(NSS)

• Home Location Registers (HLR)

• permanent database that stores all mobile subscribers in a large service area (generally one per GSM network operator)

• database contains • International Mobile Subscriber Identity (IMSI), • Mobile Subscriber ISDN number (MSISDN),• Subscribed services ( Call forwarding, call routing, prepaid /

postpaid, roaming restrictions, GPRS, supplementary services etc)

• Visitor Location Registers (VLR)

• Temporary database which updates whenever new MS enters in its area, by HLR database

• Controls those mobiles roaming in its area • Reduces number of queries to HLR • Some VLR are capable to manage upto one million customers

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Network Switching Subsystem(NSS)

• Equipment Identity Register (EIR)- Database of handsets using the IMEI (International Mobile Equipment

Identity) registered for this network- Made up of three sub-classes:

- The White List (list of valid device IMEIs), - The Black List (list of locked/stolen device IMEIs) and - the Gray List (list of malfunctioning device IMEIs)

• Authentication Center (AUC) - Protects against intruders in air interface- Maintains authentication keys and algorithms and provides security- Generally associated with HLR

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Operation Support Subsystem (OSS)

• Connected to different components of the NSS & BSS• Operation & Maintenance Center (OMC) controls and

monitors other network entities.• OMC functions are traffic monitoring, status report of network

entities, subscriber’s security management and billing.• Uses Equipment Identity Register (EIR) to identify valid, invalid

and malfunctioning subscribers.• Uses Authentication Center (AuC) for authentications of a

subscriber. It helps to verify user’s identity.

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Call Routing: Incoming Call

1. Calling a GSM subscribers2. Forwarding call to GSMC3. Signal Setup to HLR4. 5. Request MSRN from VLR6. Forward responsible MSC to

GMSC7. Forward Call to current MSC8. 9. Get current status of MS10.11. Paging of MS12.13. MS answers14.15. Security checks16.17. Set up connection

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Call Routing: Outgoing Call1. MS sends dialed number to BSS 2. BSS sends dialed number to MSC3,4 MSC checks VLR if MS is allowed

the requested service. 5 MSC routes the call to GMSC6 GMSC routes the call to local

exchange of called user7, 8,9,10 Answer back (Ring back) tone is

routed from called user to MS via GMSC,MSC,BSS

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HandoversFour different types of handover in the GSM

• Channels (time Slots) in the same cell.

• Between 1 and 2 – Inter BTS / Intra BSC

• Between 1 and 3 – Inter BSC/ Intra MSC

• Between 1 and 4 – Inter MSC

Internal Handovers - Involved only one base station controller.

External Handovers - These are handled by MSC.

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Mobility Management : Location Management• A powered on mobile is informed of an incoming call by a paging message

sent over the paging channel of the cells within the current location area.• The location updating ,and subsequent call routing ,use the MSC and both

HLR and VLR.• Information send to HLR is normally the SS7 address of the new VLR.• If the subscriber is entitled to service ,the HLR sends a subset of the

subscriber information needed for call control to the new MSC / VLR ,and sends the message to the old MSC/VLR to cancel the old registration.

• An incoming mobile terminating call is directed to the gateway MSC(GMSC) function.

• The GSMC is basically a switch. which is able to interrogate the subscriber’s HLR , though the GMSC function is distinct from the MSC function ,it is usually implemented within the MSC.

• The routing information that is returned to the GMSC is the MSRN( Mobile station roaming Number) it is defined by the E.164 numbering plan .MSRNs are related to the geographical numbering plan ,and not assigned to subscribes.

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• Routing procedure begin with the GMSC querying the called subscriber’s HLR for an MSRN. The HLR typically stores only the SS7 address of the subscriber current VLR.

• The VLR temporarily allocates an MSRN from its pool for the call.

• This MSRN is returned to the HLR and back to the GMSC, which can then route the call to the new MSC.

• At the new MSC, the IMSI corresponding to the MSRN is looked up , and the mobile is paged in its current location area.

• HLR is referred for incoming call , VLR is referred for outgoing call.

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Security in GSM

• On air interface, GSM uses encryption and TMSI

Temporary Mobile Subscriber Identity instead of IMSI.• SIM is provided 4-8 digit PIN to validate the ownership of

SIM• 3 algorithms are specified :

- A3 algorithm for authentication

- A5 algorithm for encryption

- A8 algorithm for key generation

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Authentication in GSMGSM DEC/2014 26

Key generation and EncryptionGSM DEC/2014 27

Characteristics of GSM Standard

• Fully digital system using 900,1800 MHz frequency band.

• TDMA over radio carriers(200 KHz carrier spacing.• 8 full rate or 16 half rate TDMA channels per carrier.• User/terminal authentication for fraud control.• Encryption of speech and data transmission over the

radio path.• Full international roaming capability.• Low speed data services (upto 9.6 Kb/s).• Compatibility with ISDN.• Support of Short Message Service (SMS).

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Advantages of GSM over Analog system

• Capacity increases• Reduced RF transmission power and longer battery life.• International roaming capability.• Better security against fraud (through terminal validation

and user authentication).• Encryption capability for information security and privacy.• Compatibility with ISDN, leading to wider range of

services

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GSM Applications

• Mobile telephony• GSM-R• Telemetry System

- Fleet management

- Automatic meter reading

- Toll Collection

- Remote control and fault reporting of DG sets• Value Added Services

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Future Of GSM

2nd Generation GSM -9.6 Kbps (data rate)

2.5 Generation ( Future of GSM)HSCSD (High Speed ckt Switched data)

Data rate : 76.8 Kbps (9.6 x 8 kbps)

GPRS (General Packet Radio service)Data rate: 14.4 - 115.2 Kbps

EDGE (Enhanced data rate for GSM Evolution)Data rate: 547.2 Kbps (max)

3 GenerationWCDMA (Wide band CDMA)

Data rate : 0.348 – 2.0 Mbps

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