Industrial Report

On

“GSM Technology”

At

Submitted towards the partial fulfillment of requirement

Of

Bachelor of Technology in Electronics & Communication Engineering

Submitted by

ANSHUL JOSHI

Roll No.: 110070102032

B. Tech ECE IV Year

Batch -2011-2015

Submitted to:

DR. SANDEEP SHARMA

Department of Electronics & Communication Engineering

Dehradun Institute of Technology

Autonomous College Affiliated to

Uttarakhand Technical University

GSM Technology

Page 2

ACKNOWLEDGEMENT

It is my pleasure to be indebted to various people, who directly or indirectly contributed in the

development of this work and who influenced my thinking, behavior, and acts during the course

of study.

I express my sincere gratitude to Mr. Shishir Kumar, worthy Director of DIT Dehradun for

providing me an opportunity to undergo summer training at BSNL Haldwani.

I am thankful to Mr. Hem Tripathi, Training Supervisor at BSNL Haldwani for his support,

cooperation and motivation provided to me during the training for constant inspiration, presence

and blessings.

I also extend my sincere appreciation to Dr. Sandeep Sharma, HOD Electronics Department at

DIT Dehradun who provided his valuable suggestions and precious time in accomplishing my

project report.

Lastly, I would like to thank the almighty and my parents for their moral support and my friends

with whom I shared my day-to-day experience and received lots of suggestions that improved

my quality of work.

Anshul Joshi

DIT, Dehradun

GSM Technology

Page 3

DECLARATION

I, Anshul Joshi, student of B.Tech 7th Semester, studying at Dehradun Institute Of

Technology (DIT), Dehradun, hereby declare that the summer training report on “GSM

Technology” submitted to BSNL, Haldwani in partial fulfillment of Degree of Bachelors Of

Technology is the original work conducted by me.

The information and data given in the report is authentic to the best of my knowledge.

This summer training report is not being submitted to any other University for award of any

other Degree, Diploma and Fellowship.

Anshul Joshi

---------------------

(Signature)

GSM Technology

Page 4

Contents

S.No CHAPTER PAGE

1. Company Profile

About Company

Services Provided By BSNL

6

6-8

2. Introduction & Basic Concepts

Definition

Evolution Of Mobile Telephone System

GSM

GSM Network

The Switching System

The Operator And Support System

Additional Functional Elements

GSM Network Area

GSM Specifications

9

9-10

10

10-11

11-13

13

13-14

14-15

16-17

3. GSM Architecture 18-22

4. Other GSM Entities

GSM Interface

Signal Processing In GSM

Frame Structure Of GSM

Channels Used In GSM

Handoff

23-24

24-29

29-32

32-35

35-36

GSM Technology

Page 5

GSM Subscriber Services

Supplementary Services

36-37

37-39

5. Conclusion 40

6. Bibliography 41

List of Figures

S.No Figure Name Page No.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Cellular Subscriber Growth Worldwide

GSM Network Elements

Network Areas

Location Areas

MSC/VLR Service Areas

PLMN Network Areas

GSM Architecture

GSM Speech Operation

Interleaving

GSM Frame Structure

Channels Used in GSM

9

11

14

15

15

15

18

24

26

30

32

GSM Technology

Page 6

CHAPTER 1- COMPANY PROFILE

About Company

Bharat Sanchar Nigam Limited (abbreviated BSNL) is an Indian state-

owned telecommunications company headquartered in New Delhi, India. It was incorporated on

15 September 2000. It took over the business of providing of telecom services and network

management from the erstwhile Central Government Departments of Telecom Services (DTS)

and Telecom Operations (DTO), with effect from 1 October 2000 on going concern basis. It is

the largest provider of fixed telephony and fourth largest mobile telephonyprovider in India, and

is also a provider of broadband services. However, in recent years the company's revenue and

market share plunged into heavy losses due to intense competition in the Indian

telecommunications sector.[2][3]

BSNL is India's oldest and largest communication service provider (CSP). It had a customer base

of 117 million as of Jan 2014.[4]

It has footprints throughout India except for the metropolitan

cities of Mumbai and New Delhi, which are managed by Mahanagar Telephone Nigam (MTNL).

Services Provided By BSNL

BSNL provides almost every telecom service in India. Following are the main telecom services

provided by BSNL:

Optical Infrastructure and DWDM : BSNL owns the biggest OFC network in India.

Also the DWDM network is one of the biggest in the world. The DWDM equipments

purchased in open tender at BSNL are mainly of United Telecoms Limited ( UTL) )

make, which was declared lowest cost in competitive bidding. Rest DWDM equipments

are from Huawei. The SDH equipments are mainly from Tejas Networks, Huawei, ZTE,

ECI, UT STAR etc.

Market Share : As of 30 November 2013, BSNL had 12.9% marketshare in India and

stands as 5th Telecom Operator in India and 67% market share in ADSL Services.

GSM Technology

Page 7

Managed Network Services : BSNL is providing complete Telecom Services Solution

to the Enterprise Customers i.e. MPLS Connectivity, Point to Point Leased

Lines and Internet Leased Lines .

Universal Telecom Services : Fixed wireline services and landline in local loop (WLL)

using CDMA Technology called bfone andTarang respectively. As of 30 June 2010,

BSNL had 75% marketshare of fixed lines.

Cellular Mobile Telephone Services: BSNL is major provider of Cellular Mobile

Telephone services using GSM platform under the brand name Cellone & Excel (BSNL

Mobile). As of 30 June 2010 BSNL has 13.50% share of mobile telephony in the

country.[5]

It has 95.54 million customers using BSNL mobile.[4]

WLL-CDMA Telephone Services: BSNL's WLL (Wireless in Local Loop) service is a

service giving both fixed line telephony & Mobile telephony.

Internet: BSNL provides Internet access services through dial-up connection (as

Sancharnet through 2009) as Prepaid, NetOne as Postpaid and ADSL broadband as

BSNL Broadband BSNL held 55.76% of the market share with reported subscriber base

of 9.19 million Internet subscribers with 7.79% of growth at the end of March 2010 Top

12 Dial-up Service providers, based on the subscriber base, It Also Provides Online

Games via its Games on Demand (GOD)

Intelligent Network (IN): BSNL offers value-added services, such as Free

Phone[9]

Service (FPH), India Telephone Card (Prepaid card), Account Card Calling

(ACC), Virtual Private Network (VPN), Tele-voting, Premium Rae Service (PRM),

Universal Access Number (UAN).

3G:BSNL offers the '3G' or the'3rd Generation' services which includes facilities like

video calling, mobile broadband, live TV, 3G Video portal, streaming services like online

full length movies and video on demand etc.

IPTV:BSNL also offers the 'Internet Protocol Television' facility which enables

customers to watch television through internet.

FTTH:Fibre To The Home facility that offers a higher bandwidth for data transfer. This

idea was proposed on post-December 2009

GSM Technology

Page 8

Helpdesk: BSNL's Helpdesk (Helpdesk) provide help desk support to their customers for

their services.

VVoIP: BSNL, along with Sai Infosystem - an Information and Communication

Technologies (ICTs) provider - has launched Voice and Video Over Internet Protocol

(VVoIP). This will allow to make audio as well as video calls to any landline, mobile, or

IP phone anywhere in the world, provided that the requisite video phone equipment is

available at both ends.[10]

WiMax: BSNL has introduced India's first 4th Generation High-Speed Wireless

Broadband Access Technology with the minimum speed of 256kbit/s. The focus of this

service is mainly rural customer where the wired broadband facility is not available.

GSM Technology

Page 9

CHAPTER 2-INTRODUCTION AND BASIC CONCEPTS

Definition

Global system for mobile communication (GSM) is a globally accepted standard for digital

cellular communication. GSM is the name of a standardization group established in 1982 to

create a common European mobile telephone standard that would formulate specifications for a

pan-European mobile cellular radio system operating at 900 MHz. It is estimated that many

countries outside of Europe will join the GSM partnership.

The Evolution of Mobile Telephone Systems

Cellular is one of the fastest growing and most demanding telecommunications applications.

Today, it represents a continuously increasing percentage of all new telephone subscriptions

around the world. Currently there are more than 45 million cellular subscribers worldwide, and

nearly 50 percent of those subscribers are located in the United States. It is forecasted that

cellular systems using a digital technology will become the universal method of

telecommunications. By the year 2015, forecasters predict that there will be more than 200

million cellular subscribers worldwide. It has even been estimated that some countries may have

more mobile phones than fixed phones by the year 2000 (see Figure 1).

Figure 1. Cellular Subscriber Growth Worldwide

The concept of cellular service is the use of low-power transmitters where frequencies can be

reused within a geographic area. The idea of cell-based mobile radio service was formulated in

the United States at Bell Labs in the early 1970s. However, the Nordic countries were the first to

GSM Technology

Page 10

introduce cellular services for commercial use with the introduction of the Nordic Mobile

Telephone (NMT) in 1981. Cellular systems began in the United States with the release of the

advanced mobile phone service (AMPS) system in 1983. The AMPS standard was adopted by

Asia, Latin America, and Oceanic countries, creating the largest potential market in the world for

cellular. In the early 1980s, most mobile telephone systems were analog rather than digital, like

today's newer systems. One challenge facing analog systems was the inability to handle the

growing capacity needs in a cost-efficient manner. As a result, digital technology was welcomed.

The advantages of digital systems over analog systems include ease of signaling, lower levels of

interference, integration of transmission and switching, and increased ability to meet capacity

demands.

GSM

Throughout the evolution of cellular telecommunications, various systems have been developed

without the benefit of standardized specifications. This presented many problems directly related

to compatibility, especially with the development of digital radio technology. The GSM standard

is intended to address these problems. From 1982 to 1985 discussions were held to decide

between building an analog or digital system. After multiple field tests, a digital system was

adopted for GSM. The next task was to decide between a narrow or broadband solution. In May

1987, the narrowband time division multiple access (TDMA) solution was chosen.

The GSM Network

GSM provides recommendations, not requirements. The GSM specifications define the functions

and interface requirements in detail but do not address the hardware. The reason for this is to

limit the designers as little as possible but still to make it possible for the operators to buy

equipment from different suppliers. The GSM network is divided into three major systems: the

switching system (SS), the base station system (BSS), and the operation and support system

(OSS). The basic GSM network elements are shown in Figure 2.

GSM Technology

Page 11

Figure 2. GSM Network Elements

The Switching System

The switching system (SS) is responsible for performing call processing and subscriber-related

functions. The switching system includes the following functional units:

Home location register (HLR)—The HLR is a database used for storage and

management of subscriptions. The HLR is considered the most important database, as it

stores permanent data about subscribers, including a subscriber's service profile, location

information, and activity status. When an individual buys a subscription from one of the

PCS operators, he or she is registered in the HLR of that operator.

GSM Technology

Page 12

Mobile services switching center (MSC)—The MSC performs the telephony switching

functions of the system. It controls calls to and from other telephone and data systems. It

also performs such functions as toll ticketing, network interfacing, common channel

signaling, and others.

Visitor location register (VLR)—The VLR is a database that contains temporary

information about subscribers that is needed by the MSC in order to service visiting

subscribers. The VLR is always integrated with the MSC. When a mobile station roams

into a new MSC area, the VLR connected to that MSC will request data about the mobile

station from the HLR. Later, if the mobile station makes a call, the VLR will have the

information needed for call setup without having to interrogate the HLR each time.

Authentication center (AUC)—A unit called the AUC provides authentication and

encryption parameters that verify the user's identity and ensure the confidentiality of each

call. The AUC protects network operators from different types of fraud found in today's

cellular world.

Equipment identity register (EIR)—The EIR is a database that contains information

about the identity of mobile equipment that prevents calls from stolen, unauthorized, or

defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as

a combined AUC/EIR node.

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers

(BSCs) and the base transceiver stations (BTSs).

BSC—The BSC provides all the control functions and physical links between the MSC

and BTS. It is a high-capacity switch that provides functions such as handover, cell

configuration data, and control of radio frequency (RF) power levels in base transceiver

stations. A number of BSCs are served by an MSC.

GSM Technology

Page 13

BTS—The BTS handles the radio interface to the mobile station. The BTS is the radio

equipment (transceivers and antennas) needed to service each cell in the network. A

group of BTSs are controlled by a BSC.

The Operation and Support System

The operations and maintenance center (OMC) is connected to all equipment in the switching

system and to the BSC. The implementation of OMC is called the operation and support system

(OSS). The OSS is the functional entity from which the network operator monitors and controls

the system. The purpose of OSS is to offer the customer cost-effective support for centralized,

regional, and local operational and maintenance activities that are required for a GSM network.

An important function of OSS is to provide a network overview and support the maintenance

activities of different operation and maintenance organizations.

Additional Functional Elements

Other functional elements shown in Figure 2 are as follows:

Message center (MXE)—The MXE is a node that provides integrated voice, fax, and

data messaging. Specifically, the MXE handles short message service, cell broadcast,

voice mail, fax mail, email, and notification.

Mobile service node (MSN)—The MSN is the node that handles the mobile intelligent

network (IN) services.

Gateway mobile services switching center (GMSC)—A gateway is a node used to

interconnect two networks. The gateway is often implemented in an MSC. The MSC is

then referred to as the GMSC.

GSM Technology

Page 14

GSM interworking unit (GIWU)—The GIWU consists of both hardware and software

that provides an interface to various networks for data communications. Through the

GIWU, users can alternate between speech and data during the same call. The GIWU

hardware equipment is physically located at the MSC/VLR.

GSM Network Areas

The GSM network is made up of geographic areas. As shown in Figure 3, these areas include

cells, location areas (LAs), MSC/VLR service areas, and public land mobile network (PLMN)

areas.

Figure 3. Network Areas

The cell is the area given radio coverage by one base transceiver station. The GSM network

identifies each cell via the cell global identity (CGI) number assigned to each cell. The location

area is a group of cells. It is the area in which the subscriber is paged. Each LA is served by one

or more base station controllers, yet only by a single MSC (see Figure 4). Each LA is assigned a

location area identity (LAI) number.

GSM Technology

Page 15

An MSC/VLR service area represents the part of the GSM network that is covered by one MSC

and which is reachable, as it is registered in the VLR of the MSC (see Figure 5).

Figure 4. Location Areas

Figure 5. MSC/VLR Service Areas

The PLMN service area is an area served by one network operator (see Figure 6).

Figure 6. PLMN Network Areas

GSM Technology

Page 16

GSM Specifications

Before looking at the GSM specifications, it is important to understand the following basic

terms:

Bandwidth—the range of a channel's limits; the broader the bandwidth, the faster data

can be sent

Bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte

Frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)

Kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits

per second

Megahertz (MHz)—1,000,000 hertz (cycles per second)

Milliseconds (ms)—one-thousandth of a second

Watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary among the

different PCS networks. Listed below is a description of the specifications and characteristics for

GSM.

Frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz

(mobile station to base station).

Duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance

between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz

apart.

GSM Technology

Page 17

Channel separation—The separation between adjacent carrier frequencies. In GSM, this

is 200 kHz.

Modulation—Modulation is the process of sending a signal by changing the

characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift

keying (GMSK).

Transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.

Access method—GSM utilizes the time division multiple access (TDMA) concept.

TDMA is a technique in which several different calls may share the same carrier. Each

call is assigned a particular time slot.

Speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to

reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract.

The signal passes through this filter, leaving behind a residual signal. Speech is encoded

at 13 kbps.

GSM Technology

Page 18

CHAPTER 3- GSM ARCHITECTURE

GSM Architecture

Global System for Mobile (GSM) is a second generation cellular system standard that was

developed to solve the fragmentation problems of the first cellular systems in Europe. GSM was

the world’s first cellular system to specify digital modulation and network level architectures and

services, and is the world’s most popular 2G technology. Before GSM, European countries used

different cellular standards throughout the continent, and it was originally developed to serve as

the pan-European cellular service and promised a wide range of network services through the use

of ISDN. GSM’s success has exceeded the expectations of virtually everyone, and it is now the

world’s most popular standard for new cellular radio and personal communications equipment

throughout the world. The task of specifying a

common mobile communication system for Europe in the 900 MHz band was taken up in the

mid – 1980s by the GSM (Groupe special mobile) committee.

Figure 7- GSM Architecture

GSM Architecture is divided into three main parts:

GSM Technology

Page 19

1. Mobile Station (MS)

2. Base Station System (BSS)

3. Network and Switching Subsystem (NSS)

In this architecture, a MS communicates with a BSS through the radio interface. The BSS is

connected to the NSS by communicating with a mobile switching center (MSC).

Moblile Station

The MS consists of two parts: the subscriber identity module (SIM) and the mobile equipment

(ME). Sometime it also contains a third part called terminal equipment (TE), which can be a

PDA or PC connected to the ME. In this case, the first two parts (i.e., ME and SIM) are called

the mobile terminal (MT).

Characteristic of Subscriber Identity Module (SIM)

A SIM can be a smart card that size is usually the size of a credit card. A SIM can be a

smaller-sized “plug-in SIM”.

The SIM is protected by a personal identity number (PIN) between four to eight digits in

length. The PIN is initially loaded by the network operator at the subscription time. This

PIN can be deactivated or changed by the user.

At the time of switch ON the MS, the user is asked to enter the PIN. If user enters the

wrong PIN upto three consecutive attempts, the SIM will be blocked and the MS cannot

be used. To unblock the SIM, the user is asked to enter the eight PIN unblocking key

(PUK).

GSM Technology

Page 20

A SIM contains the subscriber-related information, including the PIN and PUK codes.

The subscriber-related data also include a list of abbreviated and customized short dialing

numbers, short messages received.

Parts of the SIM information can be modified by the subscriber either by using the

keypad of an MS or a personal computer using an RS232 connections data retrieved by

using software on a PC.

The SIM card can be updated over the air through SIM toolkit, with which network

operators can remotely upgrade an MS by sending codes through short messages.

These messages are issued from a SimCard server and are received by MSs equipped

with SIM-toolkit capability.

SIM Toolkit provides security-related functions so that SIM cards are not falsely

modified.

Characteristic of Mobile Equipment (ME)

The ME contains the noncustomer-related hardware and software specific to the radio

interface.

When the SIM is removed from an MS, the ME cannot be used for reaching the service,

except for emergency calls.

At every new connection between MS (SIM) and the network, the characteristic

indication of the ME, called classmark, is given to the network.

This SIM-ME design supports portability, as well as enhancing security. Usually, the ME

is the property of the subscriber. The SIM, although loaned to the subscriber, is the

property of the service provider.

GSM Technology

Page 21

Base Station System

The BSS consists of two parts:

A) Base Transceiver Station (BTS)

B) Base Station Controller(BSC)

The BTS contains transmitter, receiver, signaling equipment specific to the radio interface in

order to contact the MSs. An important part of the BTS is the transcoder/rate adapter unit

(TRAU) that carries out GSM-specific speech encoding/decoding and rate adaption in data

transmission.

Functions of BSC

The BSC is responsible for the switching functions in the BSS, and is in turn connected

to an MSC in the NSS.

The BSC supports radio channel allocation/release and handoff management.

A BSC may connect to several BTSs and maintain cell configuration data of these BTSs.

The BSC communicates with the BTSs using ISDN protocols via the A-bis interface.

Processor load of a BSC, In busy hours

Call activities = around 20-25 percent

Paging and Short Message Service (SMS) = around 10-15 percent

Mobility management (handoff and location update) = around 20-25 percent

Hardware checking/network-triggered events = around 15-20 percent.

GSM Technology

Page 22

A BSC is typically engineered at 80 percent utilization. When a BSC is overloaded, it first

rejects location update, next MS originating calls, and then handoffs.

Network and Switching Subsystem (NSS)

The NSS supports the switching functions, subscriber profiles, and mobility management. The

basic switching function in the NSS is performed by the MSC.

This interface follows a signaling protocol used in the telephone network The MSC also

communicates with other network elements external to GSM utilizing the same signaling

protocol.

The current location of an MS is usually maintained by HLR and VLR. When an MS

move from the home system to a visited system, its location is registered at the VLR of

the visited system. The VLR then informs the MS’s HLR of its current location.

The authentication center (AuC) is used in the security data management for the

authentication of subscribers. The AuC may be connected with the HLR.

An incoming call is routed to an MSC, unless the fixed network is able to

inteterrogate the HLR directly. That MSC is called the gateway MSC (GMSC).

The GMSC obtains the location information routes the calls to the visited MSC of the

subscribers to receive the calls.

GSM Technology

Page 23

CHAPTER 4- OTHER GSM ENTITIES

GSM Interfaces

GSM interfaces are used for connection of various nodes in GSM network. There are different

GSM interfaces.

Um Interface:

It is also known as Air Interface or Radio interface. It is the most important part in any

mobile radio system and interfaces MS and BTS.

It supports maximum spectral efficiency and universal use of any compatible mobile

station in a GSM network.

The radio interface uses the Link Access Protocol on D channel (LAPD).

Abis Interface:

Abis interfaces are vendor specific. It interfaces BSC and BTS. The interface comprises

traffic and control channels. Functions implemented at Abis interface are:

Traffic channel transmission, terrestrial and radio channel management.

Voice-data traffic exchange.

Signaling exchange between BSC and BTS.

Transporting synchronization information from BSC to BTS.

This interface supports two types of communication links:

Traffic channels at 64 kbps.

Signaling channels at 16 kbps

GSM Technology

Page 24

The two messages handheld by traffic management procedure part of the signaling interface are

transparent and non-transparent. Messages between MS and BSC-MSC are transparent messages

and they do not require analysis by BTS. But, BTS analysis is required by non-transparent

messages.

A-interface

It is the interface between BSC and MSC. The physical layer of A-interface is a 2Mbps

standard CCITT digital connection.

Proprietary M-Interface

It is the interface between physical BSC and the TRAU. TRAU is included in BSC in the

GSM network implementation of lucent technologies. The TRAU adapts transmission bit

rate of A-interface (64 kbps) to A-bis interface (16 kbps).

Interface between other GSM Entities

MAP (Media Application Protocol) is used to transfer information between GSM PLMN

entities. Mobile application and several Application Service Elements (ASEs) are

contained in MAP.

Signal Processing in GSM

Figure 8- GSM operations from speech input to speech output

GSM Technology

Page 25

There are following operations performed from transmitter to receiver in signal processing in

GSM.

Speech Coding-

The GSM speech coder is based on the Residually Excited Linear Predictive Coder

(RELP), which is enhanced by including a Long-Term Predictor (LTP).

The coder provides 260 bits for each 20 ms blocks of speech, which yields a bit rate of 13

kbps.

The GSM speech coder takes advantage of the fact that in a normal conversation, each

person speaks on average for less than 40% of the time.

By incorporating a voice activity detector (VAD) in the speech coder, GSM systems

operate in a discontinuous transmission mode (DTX),

Which provides a longer subscriber battery life and reduces instantaneous radio

interference since the GSM transmitter is not active during silent periods.

A comfort noise subsystem (CNS) at the receiving end introduces a background acoustic

noise to compensate for the annoying switched muting which occurs due to DTX.

Channel Coding for Data Channels - The coding provided for GSM full rate data channels

(TCH/F9.6) is based on handling 60 bits of user data at 5 ms intervals, in accordance with the

modified CCITT V.110 modem standard.

Channel Coding for Control Channels - GSM control channel messages are defined to be 184

bits long, and are encoded using a shortened binary cyclic fire code, filled by a half-rate

convolution coder.

GSM Technology

Page 26

Interleaving-

In order to minimize the effect of sudden fades on the received data, the total of 456 encoded bits

within each 20 ms speech frame or control message frame are broken into eight 57 bit sub-

blocks.

These eight sub-blocks which make up a single speech frame are spread over eight

consecutive TCH time slots.

If a burst is lost due to interference or fading, channel coding ensures that enough bits

will still be received correctly to allow the error correction to work.

Each TCH time slot carries two 57 bit blocks of data from two different 20 ms (456 bit)

speech (control) segments.

Figure illustrates exactly how the speech frames are diagonally interleaved within the

time slots. And here eight speech sub-blocks are spread over eight successive TCH time

slots for a specific time slot number.

Figure 9- Diagonal interleaving used for TCH/SACCH/FACCH data

GSM Technology

Page 27

Here TS 0 contains 57 bits of data from 0th sub-block of nth speech coder frame (denoted

as “a” in figure) and 57 bits of data from the 4th sub-block of the (n-1) speech coder

frame (denoted as “b” in figure).

Burst Formatting-

Burst formatting adds binary data to the ciphered blocks, in order to help synchronization

and equalization of the received signal.

Ciphering-

Ciphering modifies the contents of the eight interleaved blocks through the use of

encryption techniques known only to the particular mobile station and base transceiver

station.

Security is further enhanced by the fact that the encryption algorithm is changed from call

to call.

Two types ciphering algorithms, called A3 and A5, are used in GSM to prevent

unauthorized network access and privacy for the radio transmission respectively.

The A3 algorithm is used to authenticate each mobile by verifying the user’s passcode

within the SIM with the cryptographic key at the MSC.

The A5 algorithm provides the scrambling for the 114 coded data bits sent in each TS

Modulation-

The modulation scheme used by GSM is 0.3 GMSK, where 0.3 describes the 3 db

bandwidth of the Gaussian pulse shaping filter with relation to the bit rate (BT = 0.3) .

GSM Technology

Page 28

GMSK is a special type of digital FM modulation. Binary ones and zeroes are

represented in GSM by shifting RF carrier by 67.708 kHz.

The channel data rate of GSM is 270.834 kbps, which is exactly four times the RF

frequency shift.

This minimizes the bandwidth occupied by the modulation spectrum and hence improves

channel capacity.

The MSK modulated signal is passed through a Gaussian filter to smooth the rapid

frequency transitions which would otherwise spread energy into adjacent channels.

Frequency Hoping-

Under normal conditions, each data burst belonging to a particular physical channel is

transmitted using the same carrier frequency.

However, if uses in a particular cell have severe multipath problems, the cell may be

defined as a hoping cell by the network operator,

In which case slow frequency hoping may be implemented to combat the multipath or

hopping occurs at a maximum rate of 217.6 hops per second.

As many as 64 different channels may be used before a hopping sequence is repeated.

Frequency hoping is completely specified by the service provider.

Equalization-

Equalization is performed at the receiver with the help of the training sequences

transmitted in the mixable of every time slot.

GSM Technology

Page 29

The type of equalizer for GSM is not specified and is left up to the manufacturer.

Demodulation-

The portion of the transmitted forward channel signal which is of interest to a particular

user is determined by the assigned TS and ARFCN.

The appropriate TS are demodulated with the aid of synchronization data provided by the

burst formatting.

After demodulation, the binary information is deciphered, de-interleaved, channel

decoded, and speech decoded. Here at the receiver, the process of De-ciphering, Burst

Formatting, De-interleaving, Channel decoding, source decoding are exactly opposite to

the transmitter and finally speech as an output.

Frame structure of GSM

The most interesting interface in a GSM system is Um, GSM implements SDMA using

cells with BTS and assigns an MS to a BTS, Furthermore, FDD is used to separate

downlink and uplink as shown in figure.

Media access combines TDMA and FDMA. In GSM 900 (Uplink are 935-960 MHz and

Downlink are 890-915 MHz), 124 channels, each 200 kHz wide, are used for FDMA,

where as GSM 1800 uses, 374 channels.

Due to technical reasons, channels 1 and 124 are not used for transmission in GSM 900.

Typically, 32 channels are reserved for organizational data; the remaining 90 are used for

customers.

Each BTS then manages a single channel for organizational data and, e.g., up to 10

channels for user data.

GSM Technology

Page 30

Note that, for a given distance, less power is required to transmit signal over a lower

frequency. To save MS (Mobile Station) power, uplink frequencies in mobile systems are

always the lower band of frequencies.

Discontinuous transmission is used in GSM to save power consumption of the MS. With

this function, an MS turns the transmitter on only while voice is present. When there is no

voice input, the transmitter is turned off.

GSM also supports discontinuous reception where the MS needs to listen only to its

subchannel for paging.

The following example is based on the GSM 900 system, but GSM works in a similar

way at 1800 and 1900 MHz.

The duration of a frame is 4.615 ms. A frames is again subdivided into 8 GSM time slots,

where each slot represents a physical TDM channel and lasts for 577 microseconds. Each

TDM channel occupies the 200 kHz carrier for 577 microseconds every 4.615

millisecond.

The time slots in the uplink are derived from the downlink by a delay of three time slots.

This arrangement prevents an MS from transmitting and receiving at the same time.

Figure 10-GSM Frame Structure

GSM Technology

Page 31

The duration of a frame is 4.615 ms. A frames is again subdivided into 8 GSM time slots,

where each slot represents a physical TDM channel and lasts for 577 microseconds. Each

TDM channel occupies the 200 kHz carrier for 577 microseconds every 4.615

millisecond.

The time slots in the uplink are derived from the downlink by a delay of three time slots.

This arrangement prevents an MS from transmitting and receiving at the same time.

An MS does not need a full-duplex transmitter, a simpler half-duplex transmitter

switching between receiving and sending is enough.

To avoid frequency selective fading, GSM specifies an optional slow frequency hopping

mechanisms and BTS may change the carrier frequency after each frame based on a

common hopping sequence. An MS changes it frequency between up and downlink slots

respectively.

Data is transmitted in small portions, called bursts. Normal burst are used for data

transmission inside a time slot (user and signaling data).The burst is only 546.5

microsecond long and contains 148 bits.

The remaining 30.5 microsecond are used as guard space to avoid overlapping with other

burst due to different path delays and to give the transmitter time to turn on and off.

The first and last three bits of a normal burst (tail) are all set to 0 and can be used to

enhance the receiver performance.

The training sequence in the middle of a slot is used to adapt the parameters of the

receiver to the current path propagation characteristic and to select the strongest signal in

case of multi-path propagation.

GSM Technology

Page 32

A flag indicates whether the data field contains user or network control data.

Apart from the normal burst, ETSI defines four more bursts for data transmission,

frequency correction bursts allows the MS to correct the local oscillator to avoid

interference with neighboring channels, a synchronization burst with an extended training

sequence synchronizes the MS with the BTS in time, an access burst is used for the initial

connection setup between MS and BTS, and finally a dummy burst is used if no data is

available for a slot.

Channels used in GSM

Figure 11-Channels in GSM

Traffic channels (TCHs)

Traffic channels are used to carry user information (speech or data).Two kinds of TCHs are

defined:

a) Full-rate TCH (TCH/F)

Provides transmission speed of 13 kbps for speech or 9.6, 4.8, or 2.4 kbps for data. Enhanced

full-rate (EFR) speech coders have been implemented to improve the speech quality of a TCH/F.

GSM Technology

Page 33

b) Half-rate TCH (TCH/H)

Allows transmission of 6.5 kbps speech, or 4.8 or 2.4 kbps of data.

1) Control Channels (CCHs)

The CCHs are intended to carry signaling information. Three types of CCHs are defined in

GSM:

a) Common control channels (CCCHs): Include the following channel types:

Paging channel (PCH): The PCH provides paging signals from the base station to all

mobiles in the cell, and notifies a specific mobile of an incoming call which

originates from the PSTN. The PCH transmits the IMSI of the target subscriber, along

with a request for acknowledgment from the mobile unit on the RACH. Alternatively;

the PCH may be used to provide cell broadcast ASCII text messages to all

subscribers, as part of the SMS feature of GSM.

Access grant channel (AGCH): The AGCH is used by the base station to provide

forward link communication to the mobile, and carries data which instructs the

mobile to operate in a particular physical channel (time slot and ARFCN) with a

particular dedicated control channel. The AGCH is the final CCCH message sent by

the base station before a subscriber is moved off the control channel. The AGCH is

used by the base station to respond to a RACH sent by a mobile station in a previous

CCCH frame.

Random access channel (RACH): The RACH is a reverse link channel used by a

subscriber unit to acknowledge a page from the PCH, and is also used by mobiles to

originate a call. The RACH uses a slotted ALOHA access Scheme. It is used by the

MSs for initial access to the network. It utilizes the uplink. Same MSs may access the

GSM Technology

Page 34

same RACH, potentially resulting in collisions. The slotted Aloha protocol is adopted

in GSM to resolve access collision.

b) Dedicated control channels: It is supported in GSM for dedicated use by a specific MS.

Standalone dedicated control channel (SDCCH): It is used only for signaling and for

short messages. The SDCCH is used to send authentication and alert messages (but not

speech) as the mobile synchronizes itself with the frame structure and waits for a TCH.

The SDCCH is used in both downlink and uplink.

Slow associated control channel (SACCH): It is associated with either a TCH or an

SDCCH. The SACCH is used for nonurgent procedures, mainly the transmission of

power and time alignment control information over the uplink. A TCH is always

allocated with a control channel SACCH to transport both user information and signaling

data in parallel. The SACCH is used in both downlink and uplink.

Fast associated control channel (FACCH): It is used for time-critical signaling, such as

call-establishing progress, authentication of subscriber, or handoff. The FACCH makes

use of the TCH during a call; thus, there is a loss of user data because the FACCH

“steals” the bandwidth of the TCH. The FACCH is used in both downlink and uplink.

Call broadcast channel (CBCH): It carries only the short message service cell broadcast

messages, which use the same time slot as the SDCCH. The CBCH is used on the

downlink only.

c) Broadcast channels (BCHs): It is used by the BTS to broadcast information to the MSs in its

coverage area.

Frequency correction channel (FCCH): The FCCH allows each subscriber unit to

synchronize the internal frequency standard (local oscillator) to the exact frequency of the

base station. It carries information from the BSS to the MS.

GSM Technology

Page 35

Synchronization channel (SCH): It carry information from the BSS to the MS.It is used

to identify the serving base station while allowing each mobile to frame synchronizes

with the base station.

Broadcast control channel (BCCH): The BCCH is a forward control channel that is

used to broadcast information such as cell and network identity, and operating

characteristics of the cell (current control channel structure, channel availability, and

congestion).The BCCH also broadcasts a list of channels that are currently in use within

the cell.

Handoff:

When a mobile user is engaged in conversation, the MS is connected to a BS via a radio link. If

the mobile user moves to the coverage area of another BS, the radio link to the old BS is

eventually disconnected, and a radio link to the new BS should be established to continue the

conversation. This process is variously referred to as automatic link transfer, handover, or

handoff.

There are five types of handoff:

Intracell handoff The link transfer is performed between two time slots or channels in

the same BS. For a TDMA system, Intracell handoff is also referred to as time slot

transfer (TST).

Intercell handoff or inter-BS handoff The link transfer is performed between two BSs

attached to the same base station controller (BSC).

Inter-BSC handoff The link is transferred between two BSs connected to different BSCs

on the same mobile switching center (MSC).

GSM Technology

Page 36

Intersystem handoff or inter-MSC handoff The link transfer takes place at two BSs

connected to different BSCs on different MSCs.

Intersystem handoff between two PCS networks The link transfer is between two BSs

connected to different MSCs homing to different PCS networks.

Three strategies have been proposed to detect the need for handoff:

In mobile-controlled handoff (MCHO) The MS continuously monitors the signals of

the surrounding BSs and initiates the handoff process when some handoff criteria are

met. MCHO is used in DECT and PACS.

In network-controlled handoff (NCHO) The surrounding BSs measure the signal from

the MS, and the network initiates the handoff process when some handoff criteria are

met. NCHO is used in CT-2 plus and AMPS.

In mobile-assisted handoff (MACHO) The networks ask the MS to measure the signal

from the surrounding BSs. The network makes the handoff decision based on reports

from the MS. MACHO is used in GSM and IS-95 CDMA.

The BSs involved in the handoff may be connected to the same MSC (inter-cell handoff or inter-

BS handoff) or two different MSCs (inter-system handoff or inter-MSC handoff).

GSM Subscriber Services

There are two basic types of services offered through GSM: telephony (also referred to as

teleservices) and data (also referred to as bearer services). Telephony services are mainly voice

services that provide subscribers with the complete capability (including necessary terminal

equipment) to communicate with other subscribers. Data services provide the capacity necessary

to transmit appropriate data signals between two access points creating an interface to the

network. In addition to normal telephony and emergency calling, the following subscriber

services are supported by GSM:

GSM Technology

Page 37

Dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for

various control purposes via the telephone network, such as remote control of an

answering machine. GSM supports full-originating DTMF.

Facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax

machines are designed to be connected to a telephone using analog signals, a special fax

converter connected to the exchange is used in the GSM system. This enables a GSM–

connected fax to communicate with any analog fax in the network.

Short message services—A convenient facility of the GSM network is the short message

service. A message consisting of a maximum of 160 alphanumeric characters can be sent

to or from a mobile station. This service can be viewed as an advanced form of

alphanumeric paging with a number of advantages. If the subscriber's mobile unit is

powered off or has left the coverage area, the message is stored and offered back to the

subscriber when the mobile is powered on or has reentered the coverage area of the

network. This function ensures that the message will be received.

Cell broadcast—A variation of the short message service is the cell broadcast facility. A

message of a maximum of 93 characters can be broadcast to all mobile subscribers in a

certain geographic area. Typical applications include traffic congestion warnings and

reports on accidents.

Voice mail—This service is actually an answering machine within the network, which is

controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box

and the subscriber checks for messages via a personal security code.

Fax mail—With this service, the subscriber can receive fax messages at any fax

machine. The messages are stored in a service center from which they can be retrieved by

the subscriber via a personal security code to the desired fax number.

GSM Technology

Page 38

Supplementary Services

GSM supports a comprehensive set of supplementary services that can complement and support

both telephony and data services. Supplementary services are defined by GSM and are

characterized as revenue-generating features. A partial listing of supplementary services follows.

Call forwarding—This service gives the subscriber the ability to forward incoming calls

to another number if the called mobile unit is not reachable, if it is busy, if there is no

reply, or if call forwarding is allowed unconditionally.

Barring of outgoing calls—This service makes it possible for a mobile subscriber to

prevent all outgoing calls.

Barring of incoming calls—This function allows the subscriber to prevent incoming

calls. The following two conditions for incoming call barring exist: baring of all

incoming calls and barring of incoming calls when roaming outside the home PLMN.

Advice of charge (AoC)—The AoC service provides the mobile subscriber with an

estimate of the call charges. There are two types of AoC information: one that provides

the subscriber with an estimate of the bill and one that can be used for immediate

charging purposes. AoC for data calls is provided on the basis of time measurements.

Call hold—This service enables the subscriber to interrupt an ongoing call and then

subsequently reestablish the call. The call hold service is only applicable to normal

telephony.

Call waiting—This service enables the mobile subscriber to be notified of an incoming

call during a conversation. The subscriber can answer, reject, or ignore the incoming call.

Call waiting is applicable to all GSM telecommunications services using a circuit-

switched connection.

GSM Technology

Page 39

Multiparty service—The multiparty service enables a mobile subscriber to establish a

multiparty conversation—that is, a simultaneous conversation between three and six

subscribers. This service is only applicable to normal telephony.

Calling line identification presentation/restriction—These services supply the called

party with the integrated services digital network (ISDN) number of the calling party.

The restriction service enables the calling party to restrict the presentation. The restriction

overrides the presentation.

Closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a

group of subscribers who are capable of only calling themselves and certain numbers.

GSM Technology

Page 40

CHAPTER 5-CONCLUSION

In analog communication systems the quality of voice and the speed of communication were not

good. Thus an evolution of analog communication systems was needed; Hence GSM

Technology was made which improved the communication process by addressing the problems

of the previous systems.

It increases the capacity, reduces RF transmission power, and provides international roaming

capability, better security against fraud through terminal validation and user authentication. It

also provides encryption capability for information security and privacy, At the same time it is

compatible with the ISDN network.

The training at BSNL covered GSM Technology as a whole with key focus on its architectural

elements, cell types, interfaces, handover etc.

GSM Technology

Page 41

Bibliography

The following are the sources that I used while making the project report.

Books-

1. Marie-Bernadette Paulket and Michel Mouly, ”The GSM System for Mobile

Communication”, Ed. 2003, McGraw Hill Publication.

2. Siegmud M. Redl, Matthias K. Weber and Malcolm W. Oliphant, ”An Introduction to

GSM”, Ed. 2005, Prentice Hall of India.

Journals-

1. Kwok-Keung M. Cheng, IEEE TRANSACTIONS ON GSM Technology, VOL. 56, NO.

2, FEBRUARY 2001.

WEB-

1. The International Engineering Consortium

2. Tutorialspoint.com