Seminar Report

DEPARTMENT OF COMPUTER ENGINEERINGRegional College For Engineering Research and Technology- Jaipur

Seminar Session- 2010 - 2011

A

SEMINAR REPORT

ON

GLOBAL SYSTEM FOR MOBILE

COMMUNICATION (GSM)

BY

Arpit Sharma

Guided by

Mr. Amit Bairva



PrefaceThis report discusses the mobile telephony system being followed throughout

the globe as Global System for Mobile Telecommunication (GSM), which is

increasingly popular among all the strata of life. The system has connected all

the inhabitants not only on the Earth but even in space. In this report a brief

Introduction about GSM is given in Chapter 1. and Chapter 2 deals with Basic

Concepts about Principles of Cellular Communication, GSM, Its features and

some knowledge about the Channels. Chapter 3 deals with GSM Basic Call

Sequence.

The report is expected to fulfill the expectations of the readers about GSM to

larger extent even though the information provided in precise and subtle form.



CERTIFICATE

This is to certify that the Seminar entitled “GLOBAL SYSTEM FOR

MOBILE COMMUNICATION (GSM)” has been carried out by ARPIT

SHARMA under my guidance in partial fulfillment of the degree of Bachelor

of Engineering in Computer Engineering / Information Technology of

university of Rajasthan during the academic year 2010-2011. To the best of my

knowledge and belief this work has not been submitted elsewhere for the

award of any other degree.

Guide Examiner Head of the Department

Mr. Amit Bairva Mr. Gaurav Jain Mr. Bhuwan Chandra



ACKNOWLEDGEMENT

I express my sincere thanks to Mr.Gaurav Jain, Mr. Amit Bairva, and Mr.

Ranjeet Prasad, all eminent Lecturers, Computer Science Department of

Regional College For Engineering Research and Technology, for extending

their invaluable guidance, support for literature, critical reviews and report

preparation. I am indebted to Mr. Bhuwan Chandra Head Computer Science

Department for his unconditional guidance and timely support in the

preparation of this report. guiding me right form the inception till the

successful completion of the Seminar. I pay my thanks to all others who have

been related to this work directly or indirectly.

(Signature of Student)

Arpit Sharma

(Name of student)



INDEX TOPIC PAGE NO.

1. Introduction 1

2. Basic Concept 2

2.1 Principles of Cellular Communication 2

2.2 Global System For mobile comminication (GSM) 7

2.3 Features Of GSM 20

2.4 Basic knowledge of channels 27

3. Hands On Experience: GSM Basic Call Sequence 31

4. Conclusion 32

5. Bibliography 35



1. INTRODUCTION

This report explains the basic components, technologies used, and operation of

GSM systems. We will discover why mobile telephone service providers have

upgraded from 1st generation analog systems to more efficient and feature rich

2nd generation GSM systems. We will also discover how 2nd generation

systems are gradually evolving into 3rd generation broadband multimedia

systems. We will learn that the key types of GSM devices include single mode

and dual mode mobile telephones, wireless PCMCIA cards, embedded radio

modules, and external radio modems. We will then discover the different types

of available services such as voice services, data services, group call, and

messaging services. Described are the fundamental capabilities and operation

of the GSM radio channel. Because the needs of voice and data communication

are different, we will discover that the GSM system essentially separates

circuit switched (primarily voice) and packet switched (primarily data)

services. Described are key functional sections of a GSM network and how

they communicate with each other. We will learn how and why GSM is

evolving into 3rd generation broadband systems including GPRS, EDGE, and

WCDMA. The term GSM usually means the GSM standard and protocols in

the frequency spectrum around 900MHz. There is also DCS1800 - GSM

protocols but at different air frequencies around 1800 MHz - and in the United

States, where spectrum for Personal Communication Services (PCS) was

auctioned at around 1900MHz, operators using the aptly-named GSM1900 are

competing against a plethora of other standards. As a result of this, the original

and most widely-used GSM frequency implementation is also becoming

known as GSM900, and DCS1800 is also known as GSM1800. However,



although the physical frequencies used differ, the protocols and architecture

remain the same.

2. BASIC CONCEPTS

2.1 Principles of Cellular Telecommunications

OverviewA cellular telephone system links mobile station (MS) subscribers into the

public telephone system or to another cellular system’s MS subscriber.

Information sent between the MS subscriber and the cellular network uses

radio communication. This removes the necessity for the fixed wiring used in a

traditional telephone installation.

Due to this, the MS subscriber is able to move around and become fully

mobile, perhaps travelling in a vehicle or on foot.

Advantages of Cellular CommunicationsCellular networks have many advantages over the existing “land” telephone

networks.

There are advantages for the network provider as well as the mobile subscriber.

Network ComponentsGSM networks are made up of Mobile services Switching Centers (MSC),

Base Station Systems (BSS) and Mobile Stations (MS). These three entities

can be broken down further into smaller entities; such as, within the BSS we

have Base Station Controllers, Base Transceiver Stations and Transcoders.

With the MSC, BSS and MS we can make calls, receive calls, perform billing

etc, as any normal PSTN network would be able to do. The only problem for



the MS is that all the calls made or received are from other MSs. Therefore, it

is also necessary to connect the GSM network to the PSTN.

Mobile Stations within the cellular network are located in “cells”, these cells

are provided by the BSSs. Each BSS can provide one or more cells, dependent

on the manufacturers Equipment.

The cells are normally drawn as hexagonal, but in practice they are irregularly

shaped, this is as a result of the influence of the surrounding terrain, or of

design by the network Planners.

Cell SizeThe number of cells in any geographic area is determined by the number of

MS subscribers who will be operating in that area, and the geographic layout

of the area (hills, lakes, buildings etc).

Large CellsThe maximum cell size for GSM is approximately 70 km in diameter, but this

is dependent on the terrain the cell is covering and the power class of the MS.

Small CellsSmall cells are used where there is a requirement to support a large number of

MSs, in a small geographic region or where a low transmission power may be

required to reduce the effects of interference. Small cells currently cover 200 m

and upwards.

Frequency Re-useStandard GSM has a total of 124 frequencies available for use in a network.

Most network providers are unlikely to be able to use all of these frequencies

and are generally allocated a small subset of the 124.

Example:



A network provider has been allocated 48 frequencies to provide coverage over

a large area, let us take for example Great Britain.

As we have already seen, the maximum cell size is approximately 70 km in

diameter, thus the 48 frequencies would not be able to cover the whole of

Britain.

To overcome this limitation the network provider must re-use the same

frequencies over and over again, in what is termed a “frequency re-use

pattern”.

When planning the frequency re-use pattern the network planner must take into

account how often to use the same frequencies and determine how close

together the cells are, otherwise co-channel and/or adjacent channel

interference may occur. The network provider will also take into account the

nature of the area to be covered. This may range from a densely populated city

(high frequency re-use, small cells, high capacity) to a sparsely populated rural

expanse (large omni cells, low re-use, low capacity).

SectorizationThe cells we have looked at up to now are called Omni-directional cells. That

is each site has a single cell and that cell has a single transmit antenna which

radiates the radio waves to 360 degrees.

The problem with employing Omni-directional cells is that as the number of

MSs increase in the same geographical region, we have to increase the number

of cells to meet the demand. To do this, as we have seen, we have to decrease

the size of the cell and fit more cells into this geographical area. Using Omni-

directional cells we can only go so far before we start introducing co-channel

and adjacent channel interference, both of which degrade the cellular

network’s performance.



To gain a further increase in capacity within the geographic area we can

employ a technique called “sectorization”. A sectorization split a single site

into a number of cells, each cell has transmit and receive antennas and behaves

as an independent cell.

Each cell uses special directional antennas to ensure that the radio propagation

from one cell is concentrated in a particular direction. This has a number of

advantages: firstly, as we are now concentrating all the energy from the cell in

a smaller area 60, 120, 180 degrees instead of 360 degrees, we get a much

stronger signal, which is beneficial in locations such as “in-building coverage”.

Secondly, we can now use the same frequencies in a much closer re-use

pattern, thus allowing more cells in our geographic region which allows us to

support more MSs.

Switching and ControlHaving established radio coverage through the use of cells, both omni-

directional and directional (sectored sites), now consider what happens when

the MS is in motion (as MSs tend to be).

At some point the MS will have to move from one cell’s coverage area to

another cell’s coverage area. Handovers from one cell to another could be for a

number of reasons (e.g. the signal strength of the “serving cell” is less than the

signal strength of a “neighbor cell”, or the MS is suffering a quality problem in

the serving cell) and by handing over to one of its neighbors this may stop the

quality problem.

Regardless of the reason for a “handover” it has to be controlled by some

entity, and in GSM that entity is the Mobile services Switching Centre (MSC).

To perform a handover, the network must know which neighbor cell to hand

the MS over to. To ensure that we handover to the best possible candidate the



MS performs measurements of its surrounding neighbor cells and reports its

findings to the network.

These are then analyzed together with the measurements that the network

performs and a decision is made on a regular basis as to the need for a

handover. If a handover is required then the relevant signal protocols are

established and the handover is controlled by the MSC.

Handovers must be transparent to the MS subscriber. That is the subscriber

should be unaware that a handover has occurred.

As we will see later in this course, handovers are just one of the functions of

the MSC, many more are performed by the MSC and its associated entities

(e.g. such as authentication of MS, ciphering control, location updating,

gateway to PSTN).

Note:Some networks may allow certain handovers to be performed at the BSS level.

This would be dependent on the manufacturer’s equipment.



2.2 GLOBAL SYSTEM FOR MOBILE COMMUNICATION

(GSM)HISTORY OF GSM

This history of GSM is outlined in the following

1982-1985

Conférence Européenne des Postes et Télécommunications (CEPT) begin

specifyinga European digital telecommunications standard in the 900 MHz

frequency band. This standard later became known as Global System for

Mobile communication (GSM).

1986

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

either Time Division Multiple Access (TDMA) or Frequency Division

Multiple Access (FDMA).

1987

A combination of TDMA and FDMA selected as the transmission technology

for GSM.

Operators from 12 countries sign a Memorandum of Understanding (MoU)

committing to introduce GSM by 1991.

1988

CEPT begins producing GSM specifications for a phased implementation.

Another five countries sign the MoU.



1989

European Telecommunication Standards

Institute (ETSI) takes over responsibility for GSM specification.

1990

Phase 1 specifications frozen to allow manufacturers to develop network

equipment.

1991

The GSM 1800 standard was released.

An addendum was added to the MoU allowing countries outside CEPT to sign.

1992

Phase 1 specifications are completed.

First commercial Phase 1 GSM networks launched.

First international roaming agreement between Telecom Finland and Vodafone

in UK.

1993

Australia becomes the first non-European country to sign the MoU.

The MoU now had a total of 70 signatories. GSM networks launched in

Norway, Austria, Ireland, Hong Kong and Australia.

The number of GSM subscribers reaches one million.

The first commercial DCS 1800 system is launched in the U.K.



1994

The MoU now has over 100 signatories covering 60 countries.

More GSM networks are launched.

The total number of GSM subscribers exceeded 3 million.

1995

The specification for the Personal Communications Services (PCS) developed

in the U.S.A. This version of GSM operates at 1900 MHz.

GSM growth trends continue steadily through 1995, with the number of GSM

subscribers increasing at the rate of 10,000 per day and rising.

In April 1995, there are 188 members of the MoU from 69 countries.

1996

The first GSM 1900 systems become available. These comply with the PCS

1900 standard.

1998

The MoU has a total of 253 members in over 100 countries and there are over

70 million GSM subscribers world-wide. GSM subscribers account for 31% of

the world's mobile market.

1999

GSM networks now exist in over 179 countries.

2002

Functionality of GSM extended to incorporate EDGE, AMR, and support for

flexible positioning services.

2003

Total number of subscribers expected to soar to over 1 billion.



GSM SPECIFICATIONSGSM 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.

The GSM recommendations consist of twelve series listed in the table below.

Each series was written by different working parties and a number of expert

groups. A permanent nucleus was established in order to coordinate the

working parties and to manage the editing of the recommendations. All these

groups were organized by ETSI. The GSM 1800 section is written as a delta

part within the GSM recommendations, describing only those differences

between GSM 900 and GSM 1800. GSM 1900 is based on GSM 1800 and has

been adapted to meet the American National Standards Institute (ANSI)

standard. As we shall see GSM frequencies have expanded to include GSM at

800 MHz.



GSM PHASESIn 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. Each new phase builds on

the services offered by existing phases

Phase 1Phase 1 contains the most common services including:

Voice telephony

International roaming

Basic fax/data services (up to 9.6 kbits/s)

Call forwarding

Call barring

Short Message Service (SMS)

Phase 1 also incorporated features such as ciphering and Subscriber Identity

Module (SIM) cards. Phase 1 specifications were then closed and cannot be

modified.

Phase 2Additional features were introduced in GSM phase 2 including:

Advice of charge

Calling line identification

Call waiting

Call hold

Conference calling

Closed user groups



Additional data communications capabilities

Phase 2+The standardization groups have already defined the next phase, 2+. This

program covers multiple subscriber numbers and a variety of business oriented

features. Some of the enhancements offered by Phase 2+ include:

Multiple service profiles

Private numbering plans

Access to Centrex services

Interworking with GSM 1800, GSM 1900 and the Digital

Enhanced Cordless Telecommunications (DECT) standard Priorities and time

schedules for new features and functions depend primarily on the interest

shown by operating companies and manufacturers and technical developments

in related areas.

Phase 2++ This phase includes sophisticated enhancements to the radio

interface including:

Enhanced Datarates for Global Evolution (EDGE), a new modulation

method which increases capacity on the air interface.

Customized Application for Mobile Enhanced Logic (CAMEL), a stan-

dard, governing IN service access while roaming internationally.



GSM NETWORK COMPONENTSThe GSM network is divided into two systems. Each system comprises a

number of functional units or individual components of the mobile network.

The two systems are:

Switching System (SS)

Base Station System (BSS)

In addition, as with all telecommunications networks, GSM networks are

operated, maintained and managed from computerized centers.

Figure 1



Abbreviations:AUC Authentication Center

BSC Base Station Controller

BTS Base Transceiver Station

EIR Equipment Identity Register

HLR Home Location Register

MS Mobile Station

MSC Mobile services Switching Center

NMC Network Management Center

OMC Operation and Maintenance Center

VLR Visitor Location Register

SWITCHING SYSTEM (SS) COMPONENTS

Mobile services Switching Center (MSC)The MSC performs the telephony switching functions for the mobile network.

It controls calls to and from other telephony and data systems, such as the

Public Switched Telephone

Network (PSTN), Integrated Services Digital Network (ISDN), public data

networks, private networks and other mobile networks.

Gateway FunctionalityGateway functionality enables an MSC to interrogate a network's HLR in order

to route a call to a Mobile Station (MS).

Such an MSC is called a Gateway MSC (GMSC). For example, if a person

connected to the PSTN wants to make a call to a



GSM mobile subscriber, then the PSTN exchange will access the GSM

network by first connecting the call to a GMSC. The same is true of a call from

an MS to another MS.

Any MSC in the mobile network can function as a gateway by integration of

the appropriate software.

Home Location Register (HLR)The HLR is a centralized network database that stores and manages all mobile

subscriptions belonging to a specific operator. It acts as a permanent store for a

person's subscription information until that subscription is canceled. The

information stored includes:

Subscriber identity

Subscriber supplementary services

Subscriber location information

Subscriber authentication information

The HLR can be implemented in the same network node as the MSC or as a

stand-alone database. If the capacity of the HLR is exceeded, additional HLRs

may be added.

Visitor Location Register (VLR)The VLR database contains information about all the mobile subscribers

currently located in an MSC service area. Thus, there is one VLR for each

MSC in a network. The VLR temporarily stores subscription information so

that the MSC can service all the subscribers currently visiting that MSC

service area. The VLR can be regarded as a distributed HLR as it holds a copy

of the HLR information stored about the subscriber.

When a subscriber roams into a new MSC service area, the VLR connected to

that MSC requests information about the subscriber from the subscriber's HLR.



The HLR sends a copy of the information to the VLR and updates its own

location information. When the subscriber makes a call, the VLR will already

have the information required for call set-up.

Authentication Center (AUC)The main function of the AUC is to authenticate the subscribers attempting to

use a network. In this way, it is used to protect network operators against fraud.

The AUC is a database connected to the HLR which provides it with the

authentication parameters and ciphering keys used to ensure network security.

Equipment Identity Register (EIR)The EIR is a database containing mobile equipment identity information which

helps to block calls from stolen, unauthorized, or defective MSs. It should be

noted that due to subscriber-equipment separation in GSM, the barring of MS

equipment does not result in automatic barring of a subscriber

BASE STATION SYSTEM (BSS) COMPONENTS

Base Station Controller (BSC)The BSC manages all the radio-related functions of a GSM network. It is a

high capacity switch that provides functions such as MS handover, radio

channel assignment and the collection of cell configuration data. A number of

BSCs may be controlled by each MSC.

Base Transceiver Station (BTS)The BTS controls the radio interface to the MS. The BTS comprises the radio

equipment such as transceivers and antennas which are needed to serve each

cell in the network. A group of BTSs are controlled by a BSC.



MOBILE STATION (MS)An MS is used by a mobile subscriber to communicate with the mobile

network. Several types of MSs exist, each allowing the subscriber to make and

receive calls. Manufacturers of MSs offer a variety of designs and features to

meet the needs of different markets.

The range or coverage area of an MS depends on the output power of the MS.

Different types of MSs have different output power capabilities and

consequently different ranges. For example, hand-held MSs have a lower

output power and shorter range than car-installed MSs with a roof mounted

antenna.

GSM MSs consist of:

A mobile terminal

A Subscriber Identity Module (SIM)

Unlike other standards, in GSM the subscriber is separated from the mobile

terminal. Each subscriber's information is stored as a "smart card" SIM. The

SIM can be plugged into any GSM mobile terminal. This brings the advantages

of security and portability for subscribers. For example, subscriber A's mobile

terminal may have been stolen. However, subscriber A's own

SIM can be used in another person's mobile terminal and the calls will be

charged to subscriber A.

GSM FREQUENCY BANDS

As GSM has grown worldwide, it has expanded to operate at four main

frequency bands: 900, 1800, 1900 and 800.



GSM 900

The original frequency band specified for GSM was 900 MHz. Most GSM

networks worldwide use this band. In some countries and extended version of

GSM 900 can be used, which provides extra network capacity. This extended

version of GSM is called E-GSM, while the primary version is called P-GSM.

GSM 1800

In 1990, in order to increase competition between operators, the

United Kingdom requested the start of a new version of GSM adapted to the

1800 MHz frequency band. Licenses have been issued in several countries and

networks are in full operation. By granting licenses for GSM 1800 in addition

to GSM 900, a country can increase the number of operators. In this way, due

to increased competition, the service to subscribers is improved.

GSM 1900

In 1995, the Personal Communications Services (PCS) concept was specified

in the United States. The basic idea is to enable "person-to-person"

communication rather than "station to station ". PCS does not require that such

services be implemented using cellular technology, but this has proven to be

the most effective method. The frequencies available for PCS are around 1900

MHz As GSM 900 could not be used in North America due to prior allocation

of the 900 MHz frequencies, GSM 1900 MHz is seen as an opportunity to

bridge this gap.

The main difference between the American GSM 1900 standard and GSM 900

is that it supports ANSI signaling.

GSM 800

The multi band support in Ericsson’s GSM system is now enhanced to include

support for the GSM 800 MHz band, thus increasing capacity for operators



with a licence for this frequency. This frequency band was traditionally used

by TDMA in USA.

MS REGISTRATION AND ROAMING

When an MS is powered off it is detached from the network. When the

subscriber switches power on, the MS scans the GSM frequencies for special

channels called control channels. When it finds a control channel, the MS

measures the signal strength it receives on that channel and records it. When

all control channels have been measured, the MS tunes to the strongest one.

When the MS has just been powered on, the MS must register with the network

which will then update the MS's status to idle. If the location of the MS is

noticed to be different from the currently stored location then a location update

will also take place.

As the MS moves through the network, it continues to scan the control

channels to ensure that it is tuned to the strongest possible channel. If the MS

finds one which is stronger, then the

MS re-tunes to this new control channel2. If the new control channel belongs

to a new LA, the MS will also inform the network of its new location.

Note: In idle mode, it is the MS which decides which cell to move into. In

active mode, the network makes this decision.



2.3 FEATURES OF GSM

CompatibilityThe rapid development of analogue cellular networks during the 1980s resulted in many different cellular systems which were incompatible with one another.The need for a common standard for mobile telecommunications was therefore obvious, and so an executive body was set up to co-ordinate the complicated task of specifying the new standardized network.

GSM has been specified and developed by many European countries working in co-operation with each other. The result is a cellular system which has been implemented throughout Europe and many parts of the world.An additional advantage resulting from this is that there is a large market for

GSM equipment. This means that manufacturers can produce equipment in higher quantities and of better quality, and also, due to the number of manufacturers, a competitive and aggressive pricing structure exists. This results in lower costs for the MS subscriber and the network operators.

Noise RobustIn cellular telephone systems, such as AMPs, TACs or NMT the MS communicates with the cell site by means of analogue radio signals. Although this technique can provide an excellent audio quality (it is widely used for stereo radio broadcasting, for example), it is vulnerable to noise, as anyone who has tried to receive broadcast stereo with a poor aerial will testify!

The noise which interferes with the current system may be produced by any of the following sources:

A powerful or nearby external source (a vehicle ignition system or a lightning bolt, perhaps);

Another transmission on the same frequency (co-channel interference); Another transmission “breaking through” from a nearby frequency

(adjacent channel interference); Background radio noise intruding because the required signal is too

weak to exclude it.



In order to combat the problems caused by noise, GSM uses digital technology instead of analogue.

By using digital signals, we can manipulate the data and include sophisticated error protection, detection and correction software. The overall result is that the signals passed across the GSM air interface withstand more errors (that is, we can locate and correct more errors than current analogue systems). Due to this feature, the GSM air interface in harsh RF environments can produce a usable signal, where analogue systems would be unable to. This leads to better frequency re-use patterns and more capacity.

Flexibility and Increased CapacityWith an analogue air interface, every connection between an MS and a cell site requires a separate RF carrier, which in turn requires a separate set of RF hardware. In order to expand the capacity of a cell site by a given number of channels, an equivalent quantity of hardware must be added. This makes system expansion time consuming, expensive and labour intensive.

Re-configuration of an analogue site suffers similar problems since much of the equipment requires manual re-tuning and this makes the system inflexible.GSM equipment is fully controlled by its software. Network re-configurations can be made quickly and easily with a minimum of manual intervention required. Also, since one carrier can support eight users, expansion can be made with less equipment.

An enhancement soon to be realized is the half rate speech channel, where mobiles will use new speech algorithms requiring half as much data to be sent over the air interface.

By implementing half rate, one carrier will be able to support 16 users, effectively doubling the capacity of the network. However, this is the optimum since the mobile, as well as the BTS, will need to be modified to support half rate.



GSM networks also offer the flexibility of international roaming. This allows the mobile user to travel to foreign countries and still use their mobiles on the foreign network. If necessary, the user may leave their mobile equipment at home and carry only the SIM card, making use of a hired mobile or any available equipment.

GSMs use of a digital air interface makes it more resilient to interference from users on the same or nearby frequencies and so cells can be packed closer together, which means more carriers in a given area to give better frequency re-use.

Multi-band networks and mobiles are available where a user can make use of both the 900 MHz network and the 1800/1900 networks. The mobile must be capable of operation in dual frequency bands, however, to the user it will be transparent. This enables network operators to add in capacity and reduce network interference by using cells operating in different frequency bands. The operator will be required to show that they have made efficient use of their existing frequencies before they will be granted access to frequencies in another band. This means using techniques like sectorisation, microcells and frequency hopping.

GSM is highly software dependent and, although this makes it very complex, it also provides for a high degree of flexibility.

Use of Standardised Open InterfacesThe equipment in each of the analogue cellular networks tends to be produced by one manufacturer. This is because the equipment is only designed to communicate with other equipment made by that manufacturer. This situation is very profitable for the manufacturers as they have a great deal of influence over the pricing of their product.

Unfortunately for the MS user and the network provider, this means high prices.The situation is very different with GSM, where standard interfaces such as C7 and X.25 are used throughout the network. This means that network planners



can select different manufacturers for different pieces of hardware. Competition between manufacturers is therefore intense in the GSM market and manufacturers must ensure they support the latest developments at a competitive price.

In addition, network planners have a great deal of flexibility in where the network components are situated. This means that they can make the most efficient use of the terrestrial links which they operate.

Improved Security and ConfidentialitySecurity figures high on the list of problems encountered by some operators of analogue systems. In some systems, it is virtually non-existent and the unscrupulous were quick to recognize this. With some of the “first generation” systems, it has been estimated that up to 20% of cellular phone calls are stolen.Extensive measures have been taken, when specifying the GSM system, to substantially increase security with regard to both call theft and equipment theft.

With GSM, both the Mobile Equipment (ME) and Mobile Subscriber are identified. The ME has a unique number coded into it when it is manufactured. This can be checked against a database every time the mobile makes a call to validate the actual equipment.

The subscriber is authenticated by use of a smart card known as a Subscriber Identity Module (SIM), again this allows the network to check a MS subscriber against a database for authentication.

Flexible Handover ProcessesHandovers take place as the MS moves between cells, gradually losing the RF signal of one and gaining that of the other.

The MS switches from channel to channel and cell to cell as it moves to maintain call continuity. With analogue systems, handovers are frequently a problem area and the subscriber is often aware that a handover has occurred!



When GSM was specified a great deal of thought went into the design and implementation of handovers. Although the GSM system is more complicated than analogue in this area, the flexibility of the GSM handover processes offer significant improvements which provide a much better quality of service to the subscriber.GSM provides handover processes for the following:

Quality (uplink/downlink). Interference (uplink/downlink). RF level (uplink/downlink). MS distance. Power budget.

More handover algorithms have been developed for specific applications, such as microcellular, and are currently being implemented.

ISDN CompatibilityIntegrated Services Digital Network (ISDN) is a standard that most developed countries are committed to implement. This is a new and advanced telecommunications network designed to carry voice and user data over standard telephone lines.

Major telephone companies in Europe, North America, Hong Kong, Australia and Japan are committed to commercial enterprises using ISDN.

The GSM network has been designed to operate with the ISDN system and provides features which are compatible with it. GSM can provide a maximum data rate of 9.6 kbit/s while ISDN provides much higher data rates than this (standard rate 64 kbit/s, primary rate 2.048 Mbit/s).

2B+DThis refers to the signals and information which may be carried on an ISDN line. There are effectively three connections, one for signalling (‘D’) and the other two for data or speech (‘2B’



Enhanced Range Of ServicesGSM has the potential to offer a greatly enhanced range of services compared to existing analogue cellular systems. As well as a full range of data transmission options and fax, there will be a wide range of supplementary services.

The basic call services which are already provided within analogue systems such as Call Forwarding, Voice Message Services etc, are already available in some operational systems. Whether these services and others are provided as part of the basic service or at additional cost to the subscriber will depend on the network provider.

When services were specified on GSM, the current land PSTN and ISDN system had to be taken into consideration; after all it is these systems we are most likely to be communicating with.The services available to a subscriber will be determined by three factors:

The level of service provided by the network provider. The level of service purchased by the subscriber. The capabilities of the subscriber’s mobile equipment.

Data ServicesData can be sent over the air using some of the present systems, but this requires specially designed “add ons” to protect the data content in the harsh environment of the air interface.

Special provision is made in the GSM technical specifications for data transmission.

Therefore, like ISDN, GSM is “specially designed” for data transmission. GSM can be considered as an extension of ISDN into the wireless environment.

Text files, images, messages and fax may all be sent over the GSM network. The data rates available are 2.4 kbit/s, 4.8 kbit/s and 9.6 kbit/s.



In addition to supporting data transmission, GSM also provides for Group 3 Fax transmission.

ServicesA supplementary service is a modification of, or a supplement to, a basic telecommunication service. The network provider will probably charge extra for these services or use them as an incentive to join their network.

Here is a list of some of the optional supplementary subscriber services that could be offered to GSM subscribers:

Number Identification Receiving party requests calling number to be shown. Calling party requests calling number not to be shown.

Call Barring Bar all incoming or all outgoing calls. Bar specific incoming or outgoing calls.

Call Forwarding Forward all calls. Forward calls when subscriber is busy. Forward calls if subscriber does not answer. Forward calls if subscriber cannot be located.

Call Completion Enable incoming call to wait until subscriber completes current call. Enable subscriber to place incoming calls on hold.

Charging Display current cost of call.

Multi-party Three party service. Conference calling.



2.4 BASIC KNOWLEDGE ON CHANNELS

BCH- BROADCAST CHANNEL

One ARFCN, On all the time, in every cell

Uses Timeslot 0 on a channel, in Downlink.

Allows Mobiles to tune to BTS freq. - FCCH This channel carries a 142 bit zero sequence and repeats once in every 10 frames on BCH

Allows Mobile to Synchronise - SCH This channel carries the Frame number and BSIC in encrypted data format. Amidamble of 64 bits helps mobiles to synchronize. SCH also repeats once every 10 Frames.

Allows Mobiles to identify Network - BCCH Used to broadcast Cell and Network identity. BCCH occupies 4 frames on BCH. and repeats once every Multiframe.

Figure 2



COMMON CONTROL CHANNEL- CCH

CCCH shares Timeslot 0 with BCH on a Multiframe

CCCH consists of PCH , RACH & AGCH.

PCH – Paging Channel is used to alert mobiles on Signaling calls. PCH carries IMSI to page for Mobiles in the cell. PCH is Downlink channel.

RACH – Random Access Channel - is a short burst sent by mobile to BTS , to initiate a call request . RACH uses Timeslot 0 on reverse BCH channel on Uplink.

AGCH – Access Grant Channel - When mobile sends a RACH to BTS, BTS responds by allocating a SDCCH channel to mobile over AGCH

RANDOM ACCESS CHANNELUsed by the MOBILE to get attention from BASE STATION in the Uplink.

Several mobiles might originate RACH simultaneously.

RACH uses a Slotted ALOHA access scheme.

Mobile doesn’t know path delay– So RACH has to be a special SHORT BURST– Mobile sends normal burst only after getting Timing Advance on Downlink SACCH

DEDICATED CONTROL CHANNELDedicated Control Channels have a TCH like allocation. DCCH have three Sub Channels.

SDCCH - Standalone Dedicated Control Channel This is used as an interim channel before final assignment of TCH. SDCCH is used for Signaling and Authentication message transfers.



FACCH - Fast Associated Control Channel . FACCH is used by BTS to command a handoff to the mobile. A TCH frame is used up by FACCH , since handoff has to take place on priority. SACCH - Slow Associated Control Channel – SACCH flows at a slower rate on Uplink & Downlink along with TCH or SDCCH. During a call, SACCH flows once for every 24 Frames of TCH .

Figure3

SACCH - Slow Associated Control Channel

DOWNLINK ( BTS - MS )– Mobile Tx Power Commands – Mobile Timing Advance– Cell's Channel ConfigurationUPLINK ( MS - BTS )– Received signal quality report (RXQual)– Received signal level report (RXLev)– Adjacent BCH power measurements



– Mobile's status

Figure 4

TCH - Traffic Channel

Traffic Channel carries the Voice data.

Two blocks of 57 bits contain voice data.

One TCH is allocated for every active call. While call is in progress if there is degradation in quality of current channel, BTS may shift the communication to another TCH on a different Carrier and/or Time slot.

A Full rate TCH carries 13 KB/s voice data , and Half rate TCH carries a 6.5 KB/s voice data



3. HANDS ON EXPERIENCE: GSM BASIC CALL SEQUENCE

In the MS to Land directionThe BTS receives a data message from the MS which it passes it to the BSC. The BSC relays the message to the MSC via C7 signaling links, and the MSC then sets up the call to the land subscriber via the PSTN. The MSC connects the PSTN to the GSM network, and allocates a terrestrial circuit to the BSS serving the MS’s location. The BSC of that BSS sets up the air interface channel to the MS and then connects that channel to the allocated terrestrial circuit, completing the connection between the two subscribers.

In the Land to MS directionThe MSC receives its initial data message from the PSTN (via C7) and then establishes the location of the MS by referencing the HLR. It then knows which other MSC to contact to establish the call and that MSC then sets up the call via the BSS serving the MS’s location.

The actual processes are, of course, considerably more complex than described above. Also, there are many different GSM call sequence and handover scenarios – enough to form the subject of their own training programme! In this course we consider in detail just the MS to Land and Land to MS call sequences and the intra-MSC (inter-BSS) handover sequence. This will give you a good appreciation of the messaging that occurs in theGSM system, and how the PLMN interacts with the PSTN.



4. CONCLUSION

From the above it is clear what GSM is meant for and how it functions for the

advantage of the society. The radio frequencies are used to keep the people in

touch with each other using value added services. Voice quality and Noise

control can be enhanced through digital technology. As HLR and VLR

components are there so the security and confidentiality also become

important. Here lies the importance of SIM. Apart from voice contacts, GSM

cab be utilized in many other ways for Data services like text files, messages,

faxes, images etc. through the report it is highlighted how different types of

CCH, RAC, DCC, FACCH, SACCH are being used to improve the services of

GSM.



SEMINAR GUIDE INTERACTION REPORT

Phase Interaction Remarks

Topic Decision

Resources search

Compilation of resources

Abridged report

Rough draft

Intermediate draft

Signature

Seminar Guide



Seminar Observer’s Report

Name Observations Remarks/Marking (10)

Note: Seminar will be treated as Void in case of non presence of Observer, Guide, HoD

Observers can be Principal sir, Director sir, Dean sir.



5. BIBLIOGRAPHY1. Communication System for the Mobile Information Society By: Martin Sauter

2. An Introduction to GSM By: REDL & WEBER

3. Wikipedia

Seminar Report

Documents

jaipur seminar session

personal communications services

39encrypted data bits

base station system

subscriber identity module

adjacent channel interference

analogue cellular networks

cells coverage area