GSM Introduction

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History Of GSM In the early 1980s, many countries in Europe

witnessed a rapid expansion of analog cellular telephone systems. However, each country developed its own system, and interoperability across borders became a limiting factor.

In 1982, the Conference of European Post and Tele communications (CEPT), established a working group called the Groupe Spécial Mobile (GSM).

The task of GSM is to define a new standard for mobile communications in the 900 MHz range using digital technology.

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History Of GSM contd..,1986

Field tests were held in Paris to select which digital transmission technology to use.

The choice was Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).1987

A combination of TDMA and FDMA was selected as the transmission technology for GSM.

Operators from 12 countries signed a Memorandum of Understanding committing themselves to introducing GSM by 1991.

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History Of GSM contd..,

1988 CEPT began producing GSM

specifications for a phased implementation.

Another five countries signed the MoU.1989

European Telecommunication Standards Institute (ETSI) took over responsibility for GSM specification.

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History Of GSM contd..,1990

Phase 1 specifications were frozen to allow manufacturers to develop network equipment.1991

The GSM standard was released. An addition was made to the MoU allowing

countries outside CEPT to sign. The year 1991 also saw the definition of the

first derivative of GSM, GSM 1800 or DCS 1800, which more or less translates the GSM system in to the 1800MHz frequency band.

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History Of GSM contd.., The meaning of acronym GSM was

changed to Global System for Mobile communications the same year.1992

Phase 1 specifications were completed. First commercial Phase 1 GSM networks

were launched. The first international roaming

agreement was established between Telecom Finland and Vodafone in UK.

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GSM SPECIFICATIONS GSM was designed to be platform-

independent. The GSM specifications do not specify

the actual hardware requirements, but instead specify the network functions and interfaces in detail.

This allows hardware designers to be creative in how they provide the actual functionality, but at the same time makes it possible for operators to buy equipment from different suppliers.

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GSM PHASES In the late 1980s, the groups involved in

developing the GSM standard realized that within the given time-frame they could not complete the specifications for the entire range of GSM services and features as originally planned.

Because of this, it was decided that GSM would be released in phases with phase 1 consisting of a limited set of services and features.

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GSM frequency Ranges

For GSM 900 system 890-915 MHz is uplink range (MS

transmit) 935-960 MHz is downlink range

(MS receive) Bandwidth is 25 MHz.

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For GSM 1800 system 1710-1785 MHz is uplink frequency

range (MS transmit) 1805-1880 MHz is downlink

frequency range (MS receive) Bandwidth is 75 MHz

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System Architecture of GSM

GSM makes use of Cellular Structure The basic idea of cellular network is to

partition the available frequency range – allocate only part of frequency spectrum to BTS – keep the range of base station small to reuse the frequency.

One more important aspect of network planning is to reduce interference between different base stations.

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Overview of GSM architecture- Subsystems

Mobile Station – MS Subscriber Identity Module –SIM Base transceiver station (BTS) Base station controller (BSC) Transcoding rate and adaptation unit (TRAU) Mobile services switching center (MSC) Home location register (HLR), Visitor location register (VLR) Equipment identity register (EIR). Together, they form a public land

mobile network (PLMN).

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GSM Network Architecture

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GSM Architecture contd.., MS – Mobile Station GSM-PLMN contains as many MSs as

possible, available in various styles and power classes. In particular, the handheld and portable stations need to be distinguished.

SIM-Subscriber Identity Module Identity of the subscriber =/= Identity of

the mobile equipment. SIM provides portability to the subscriber.

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GSM Architecture contd.., SIM determines the directory

number and the calls billed to a subscriber.

SIM is a database on the user side. It consists of a chip, which the user must insert into the GSM telephone

before it can be used. SIM communicates directly with the

VLR and indirectly with the HLR.

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GSM Architecture contd.., BTS - Base Transceiver Station A large number of BTSs take care of the

radio-related tasks and provide the connectivity between the network and the mobile station via the Air-interface.

BSC- Base Station Controller The BTSs of an area are connected to

the BSC via an interface called the Abis-interface.

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GSM Architecture contd..,

BSC takes care of all the central functions and the control of the subsystem, referred to as the base station subsystem (BSS).

The BSS comprises the BSC itself and the connected BTSs.

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GSM Architecture contd..,TRAU (Transcoding Rate and Adaptation Unit):

One of the most important aspects of mobile network is the effectiveness with which it uses the available frequency resources.

Effectiveness addresses how many calls can be made by using a certain bandwidth which in turn

translates in to -Necessity to compress data, at least over the Air-interface.

Information compression is performed in both the MS and the TRAU.

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GSM Architecture contd..,

MSC (Mobile Switching Center) MSC is similar to wireline Digital exchange A large number of BSCs are connected to

the MSC via the A-interface. Major tasks of an MSC is: Routing of incoming and outgoing calls

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GSM Architecture contd..,

HLR (Home Location Register): A repository that stores the data of

a large number of subscribers. An HLR can be regarded as a large

database that administers the data of literally hundreds of thousands of subscribers.

Every PLMN requires at least one HLR.

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VLR (Visitor Location Register) The Visitor Location Register (VLR) is a

database containing information about all MSs that currently are located in the MSC service area.

The VLR contains temporary subscriber information needed by the MSC to provide service for visiting subscribers.

VLR is always integrated with MSC in a GSM network.

The VLR can be seen as a distributed HLR.

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VLR contd.., VLR area = MSC area When a Mobile Station (MS) roams into a

new MSC service area, the VLR connected to that MSC requests data about the MS from the HLR and stores it.

When the MS makes a call, the VLR already has the information needed for call set-up.

So in this way the no of queries made to HLR is reduced and there by reducing the load.

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EIR (Equipment Identity Register) GSM terminal equipment unique

identifier is - International mobile equipment identity (IMEI).

EIR keeps track of IMEI of all the subscribers.

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Signaling and Payload

Intercommunication between nodes and databases.

Different signaling protocols between different subsystems.

Out of Band signaling.

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PCM The A/D conversion is performed by

using a process called Pulse Code Modulation (PCM). PCM involves three main steps:

Sampling Quantization Coding

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Step1: Sampling Sampling involves measuring the analog signal

at specific time intervals.

The accuracy of describing the analog signal in digital terms depends on how often the analog signal is sampled.

This is expressed as the sampling frequency.

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Step2: Quantization The next step is to give each sample a value. For this reason, the amplitude of the signal at

the time of sampling is measured and approximated to one of a finite set of values.

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Step 3: Coding

Coding involves converting the quantized values into binary.

Every value is represented by a binary code.

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Pulse Code Modulation -PCM

Pulse code modulation (PCM) is the worldwide process for transmission

of digital signals. PCM is used to transmit both

signaling data and payload. PCM is categorized into hierarchies,

depending on the transmission rate.

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Consider a 2Mbps PCM link be partitioned in 32 independent channels of 64kbps each

One 64-Kbps time slot out of a 2-Mbps PCM link typically is used for signaling data

A call setup consumes about 1 to 2 Kbps.

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Block diagram of MOBILE Station

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Subscriber Identity Module (SIM)

SIM is a microchip Except for emergency calls, a GSM

mobile phone cannot be used without the SIM.

In GSM terminology, the term MS refers to the combination of a SIM and an ME.

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SIM

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SIM contd.., The SIM stores three types of subscriber

related information: Fixed data stored before the subscription is

sold: e.g. IMSI, authentication key and security algorithms

Temporary network data: e.g. the location area of the subscriber and forbidden PLMNs

Service data: e.g. language preference etc..,

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SIM contd.., A SIM contains information for GSM

network operations. This information can be related to

the mobile subscriber, GSM services or PLMN.

The data storage requirements of a SIM are divided into two categories: mandatory and optional.

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SIM as Database The major task of a SIM is to store data. The SIM has an area of non-volatile

memory which is used to store information specific to a particular subscriber and this includes the subscriber’s unique international mobile subscriber identity (IMSI) number.

This number is used to identify each individual subscriber within the GSM network.

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SIM contd.., The SIM will also contain the subscriber’s

secret authentication key, Ki, the authentication algorithm, A3, and the cipher key generation algorithm, A8.

The language preference indicator is also located in the SIM and this is used to indicate the language to be used on the MS screen.

The items described above are mandatory and must be present in any SIM that conforms to the GSM specifications.

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SIM contd.., The SIM may also contain a number of

optional items which will include the subscriber’s abbreviated dialling numbers.

The SIM may also contain a list of the last number(s) that the subscriber has dialled and an area of storage for the subscriber’s short messages.

Inserting an SIM card into an ME effectively personalises the equipment to the particular subscriber.

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Info stored on SIM – Administrative data

PIN Mandatory/changeable

Personal identification number; requested at every powerup (PIN or PIN2

PUK Mandatory/fixed value

PIN unblocking key; required to unlock a SIM

SIM service table

Mandatory/fixed value

List of the optional functionality of the SIM

Last dialed number

Optional/changeable

Redial

Charging meter Optional/changeable

Charges and time increments can be set

Language Mandatory/changeable

Determines the language for prompts by the mobile station

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Security related dataAlgorithm A3 and A8 Mandatory /fixed value Required for

authentication and to determine Kc

Key Ki Mandatory /fixed value Individual value; known only on SIM and the HLR

Key Kc Mandatory /fixed value Result of A8, Ki, and random number (RAND)

CKSN Optional /changeable value

Ciphering key sequence number

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Subscriber DataIMSI Mandatory /fixed value International mobile

subscriber identity

MSISDN Optional /fixed value Mobile subscriber ISDN; directory number of a

subscriber

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Roaming DataTMSI Mandatory / changeable value Temporary mobile subscriber identity

LAI Mandatory /changeable value Location area information

Network color codes (NCCs) of restricted

PLMNs

Mandatory /changeable Maximum of 4 PLMNs can be entered on a SIM after unsuccessful location update; cause “PLMN not allowed.” Oldest entry deleted when more than 4 restricted PLMNs are found.

NCCs of preferredPLMNs

Optional / changeable What PLMN should the MS select, ifthere is more than one to choosefrom and the home PLMN is not available?

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PLMN dataNCC, mobile countrycode (MCC), and mobile network code (MNC) of the home PLMN

Mandatory /Fixed Network identifier

Absolute radiofrequency channelnumbers (ARFCNs) ofhome PLMN

Mandatory /Fixed Frequencies for which the home PLMN is licensed.

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Mobile Station characteristics Technical Marvel

Falling Prices Availability of different devices Interaction with BTS Channel negotiation ,modulation/demodulation and

coding/decoding functionality Efficient use of battery power Communicates directly with MSC and VLR as well

via MM (mobility management) and CC (call control)

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Mobile Station Classes Different types of MSs have different

output power capabilities and therefore different ranges.

Hand-held phones generally have a lower output power and consequently a shorter range than a vehicle-mounted phone.

According to GSM specifications, MSs are categorized into five classes according to MS output power.

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MS Power Classes

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Mobile Station Classes contd.., The location of the MS also affects the

received power of the transmitted signal.

An MS located at the top of a high building has a greater range than one that is located at or below ground level.