Aircel

Institute of Science & Technology, klawad, 2012-13

CHAPTER-1

INTRODUCTION OF GSM

1.1 INTRODUCTION

GSM (Global System for Mobile Communication) 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 900MHz.It is

used to describe technologies for second generation (2G) digital cellular networks.

Developed as a replacement for first generation (1G) analog cellular networks, the GSM

standard originally described a digital, circuit switched network optimized for full

duplex voice telephony. The standard was expanded over time to include first circuit

switched data transport, then packet data transport via GPRS (General Packet Radio

Services). Packet data transmission speeds were later increased via EDGE (Enhanced Data

rates for GSM Evolution) referred as EGPRS.

Development in wireless technology

Fig1.1: Graph representation

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1.2 COMPANY DESCRIPTION

The Aircel Group is a joint venture between Maxis Communications Berhad of Malaysia

and Apollo Hospital Enterprise Ltd of India, with Maxis Communications holding a

majority stake of 74%. Aircel commenced operations in 1999 and became the leading

mobile operator in Tamil Nadu within 18 months. In December 2003, it launched

commercially in Chennai and quickly established itself as a market leader – a position it

has held since.Aircel is currently operational in 22 telecom circles of India.

Aircel began its outward expansion in 2005 and met with unprecedented success in the

Eastern frontier circles. It emerged a market leader in Assam and in the North Eastern

provinces within 18 months of operations. During this period, the company gained a

foothold in 9 circles including Chennai, Tamil Nadu, Assam, North East, Orissa, Bihar,

Jammu & Kashmir, Himachal Pradesh and West Bengal.

The Company has currently gained a momentum in the space of telecom in India post the

allocation of additional spectrum by the Department of Telecom, Govt. of India for 13 new

circles across India. These include Delhi (Metro), Mumbai (Metro), Andhra Pradesh,

Gujarat, Haryana, Karnataka, Kerala, Madhya Pradesh, Maharashtra & Goa, Rajasthan,

Punjab, UP (West) and UP (East).

Aircel has won many awards and recognitions. Voice and Data gave Aircel the highest

rating for overall customer satisfaction and network quality in 2006. Aircel emerged as the

top mid-size utility company in Businessworld’s ‘List of Best Mid-Size Companies’ in

2007. Additionally,Tele.net recognised Aircel as the best regional operator in 2008.

With over 41million customers in the country, Aircel, the fastest growing telecom company

in India, a full-fledged national operator. Aircel has won 3G spectrum across 13 circles and

BWA spectrum in 8 circles in recently spectrum auction.Aircel holds the highest amount of

next generation spectrum amongst all telecom operators in India

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1.3 GSM Specifications

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; 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

There are three different types in GSM.

1. GSM 900

2. DCS 1800

3. PCS 1900

1.3.1 GSM 900

GSM 900 operates at 900 MHz frequency.

Up link operates on 890 MHz to 915 MHz Band.

Downlink operates on 935 MHz to 960MHz Band.

Up link / Downlink separation: 45 MHz.

GSM takes advantages of both FDMA & TDMA.

In 25MHz BW , 124 carriers are generated with channel spacing of 200KHz

(FDMA)

Each carrier is divided into 8 time slots (TDMA)

At any specific time 992 speech channels are made available in GSM 900

.

1.3.2 DCS 1800

DCS 1800 operates at 1800 MHz frequency.


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Downlink operates on 1805 MHz to1880 MHz Band.


channel spacing : 200KHz (FDMA)

Each carrier is divided into 8 time slots (TDMA)

No. of carrier : 374

1.3.3 PCS 1800

PCS 1900 operates at 1900 MHz frequency.


Downlink operates on 1930 MHzto1990 MHz Band.


1.4 TECHNOLOGIES IN WIRELESS COMMUNICATION

FDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

CDMA (Code Division Multiple Access)

1.4.1 FDMA (Frequency Division Multiple Access)

In FDMA, signals from various users are assigned different frequencies. Frequency

guard bands are maintained between adjacent signal spectra to minimize crosstalk

between channels.

Fig:1.2 frequency division multiplex acess

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1.4.1.1 Advantages of FDMA

1. Capacity increase can be obtained by reducing the information bit rate and using

efficient digital codes.

2. Technological advances required for implementation are simple. A system can be

configured so that improvement in terms of speech code bit rate reduction could be

readily incorporated.

1.4.1.2 Disadvantages of FDMA

The maximum bit rate per channel is fixed and low, inhibiting the flexibility in bit-rate

capability that is needed for computer file transfers. There is a wastage of bandwidth if the

traffic is distributed unevenly

1.4.2 TDMA (Time Division Multiple Access)

In a TDMA system, data from each user is conveyed in time intervals called “Time slots”.

Several slots make up a frame . Each slot is made up of a preamble plus information bits

addressed to various stations .the functions of the preamble are to provide identification and

incidental information and to allow synchronization of the slot at the intended

receiver .Guard times are used between each user’s transmission to minimize crosstalk

between channels.

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Fig:1.3 Time division multiplex acess

1.4.2.1 Advantages of TDMA

1. It permits a flexible bit rate.

2. It offers the opportunity for frame by frame monitoring of the signal strength and

bit error rates.

3. It transmits each signal with sufficient guard time between time slots.

1.4.2.2 Disadvantages of TDMA

1. Precise synchronization is required.

2. It can only operate through a delay system.

1.4.3 CDMA (Code Division Multiple Access)

CDMA employs spread-spectrum technology and a special coding scheme (where each

transmitter is assigned a code) to allow multiple users to be multiplexed over the same

physical channel. By contrast, time division multiple access (TDMA) divides access

by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA

is a form of spread-spectrum ignaling, since the modulated coded signal has a much

higher data bandwidth than the data being communicated.

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Fig:1.4 Code division multiple acess

1.4.3.1. Advantages of CDMA

1. The transmission power is extremely small, so conversation is having not only less

radiation but also the mobiles have longer battery life

2. The call quality is very good even if there is large background noise.

1.4.3.2 Disadvantages of CDMA

1. It does not offer international roaming.

2. The CDMA handsets can only be used with a fixed network provider,it cannot be

changed.

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CHAPTER-2

HISTORY OF WIRELESS COMMUNICATION

2.1 HISTORY 1982-Groupe Spécial Mobile established by the CEPT.

1986 –Reservation of the 900 MHz spectrum band for GSM agreed in the EC –

Telecommunications Council. Trials of different digital radio transmission schemes and

different speech codes in several countries.

1987 –Basic parameters of the GSM standard agreed in February.

1988 –Completion of first set of detailed GSM specifications for infrastructure.

1989- Groupe Spéciale Mobile (transferred to an ETSI technical committee)defines the

GSM standard as the internationally accepted digital cellular telephony standard.

1990 –GSM adaptation work started for the DCS1800 band.

1991- First GSM call made by Radiolinja in Finland.

1992- First international roaming agreement signed between Telecom Finland and

Vodafone (UK).First SMS sent.

1993- Telstra Australia becomes the first non-European operator. Worlds first

DCS1800 (later GSM1800) network opened in the UK.

1994- GSM Phase 2 data/fax bearer services launched.GSM MoU membership

surpasses 100 operators.GSM subscribers hit one million.

1995 -117 GSM networks on air. The number of GSM subscribers worldwide exceeds

10 million. Fax, data and SMS services started, video over GSM demonstrated. The

first North American PCS 1900 (now GSM 1900) network opened.

1996- First GSM networks in Russia and China go live. Number of GSM subscribers

hits 50 million.

1997 –First tri-band handsets launched.

1998- Number of GSM subscribers worldwide over 100 million.

1999- WAP trials begin in France and Italy.

2000-First commercial GPRS services launched.First GPRS handsets enter the

market.Five billion SMS messages sent in one month.

2001 –First 3GSM (W-CDMA) network goes live.Number of GSM subscribers exceed

500 million worldwide.

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2003- First EDGE networks go live.Membership of GSM Association breaks through

200-country barrier.Over half a billion handsets produced in a year.

2008- GSM surpasses three billion customer threshold.

2.2 GSM ARCHITECTURE

Fig:2.1 GSM architecture

It consists of mainly three parts

1. BSS (Base Station System)

2. SS (Switching System)

3. OSS (Operation and Support System)

2.2.1 The Base Station System (BSS)

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

controllers (BSC’s) and base transceiver stations (BTS’s). BSS is responsible for

handling traffic and signaling between a mobile phone and the network switching

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system. It carries out transcoding of speech channels, allocation of radio channels to

mobile phones, paging, transmission and reception over the air interface and many

other tasks related to radio network.

The two main components of BSS are :

2.2.1.1 BTS (Base Transceiver Station)

It is the part of BSS that directly communicates with the Mobile Station (MS) and handles

the radio interface. It is the radio equipment (transceivers and antennas) needed to service

each cell in the network. A group of BTS’s are controlled by a BSC.

2.2.1.2 BSC (Base Station Controller)

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

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

configuration data , and control of RF(radio frequency) power levels in BTS. A number of

BSC’s are controlled by MSC.

Fig:2.2 Base station system

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2.2.2 The Switching System (SS)

The Switching System is responsible for performing call processing and subscriber related

functions. It includes the following functional units :

2.2.2.1 HLR (Home Location Register)

The HLR is the database used for storage and management of subscriptions. It is

considered the most important database, as it stores permanent data about subscribers,

including a subscriber’s service profile, location information and activity status.

2.2.2.2 MSC (Mobile Services Switching Centre)

It performs the telephony switching functions of the system. It controls calls to and from

other telephone and data systems. It also performs such functions like toll ticketing,

network interfacing, common channel signaling and others.

2.2.2.3 VLR (Visitor Location Register)

It is the database that contains the temporary information about the subscribers that is

needed by the MSC in order to service visiting subscribers. The VLR is always integrated

with MSC .

2.2.2.4 AUC (Authentication Centre)

A unit called AUC provides authentication and encryption parameters that verify the

user’s identity and ensure the confidentiality of each call. The AUC protects the network

operators from different types of fraud found in today’s cellular world.

2.2.2.5 EIR (Equipment Identity Register)

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.

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Fig:2.3 Switching system

2.2.3 The Operation and Support System (OSS)

The operations and maintenance centre (OMC) is connected to all equipments in the

switching system and to 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.

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CHAPTER-3

SERVICES,IDENTITES,OPERATIONS,CONTROL & AREA

3.1 GSM Telecommunication Services

The ETSI Standards define the telecommunication services. With D900/D1800 the

GSM telecommunication services offered to the GSM subscriber are subdivided as follows:

Bearer services (for data only)

Tele-services (for voice and data)

Supplementary services

Bearer services and tele-services are also called basic telecommunication services. The

use of GSM telecommunication services is subject to subscription. A basic subscription

permits participation in those GSM telecommunication services that are generally

available.

If a GSM subscriber roams out of the entitled area there is no possibility of establishing

communication (roaming not allowed), except the use of the tele-service emergency call.

3.1.1 Bearer Services

Bearer services are telecommunication services providing the capability of transmission

of signals between access points. The bearer services describe what the network can offer

(e.g. speech, data and fax).

The bearer services are pure transport services for data. Some of the transmission modes and

rates already used in modern data networks are implemented; others are planned. The following,

already implemented, bearer services provide unrestricted information transfer between the

reference points in the mobile stations.

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Fig:3.1 Bearer services

Data CDA (circuit duplex asynchronous) + basic PAD (packet assembler

Disassembler) access

Data CDS (circuit duplex synchronous)

PAD CDA (dedicated PAD access)

Alternate speech/data CDA (circuit duplex asynchronous)

Speech followed by data CDA (circuit duplex asynchronous)

Data compression on the GSM radio interface

3.1.2 Teleservices

Teleservices are telecommunication services including terminal equipment functions,

which provide communication between users according to protocols established by

agreement between network operators. The teleservices are user end-to-end services (e.g.

emergency call and short message service).

Tele-services use both low layer and high layer functions for the control of communication

from terminal to terminal. The following tele-services have already been realized:

Telephony

Emergency call

Short message service (SMS)

Short message cell broadcast

Automatic facsimile (group 3)

Alternative speech and facsimile (group 3)

Fig:3.2 Teleservices

3.1.3 Supplementary Services

Supplementary Services modify or supplement a basic telecommunication service.

Consequently, they cannot be offered to a customer as a stand-alone service. They must be 14


offered together or in association with a basic telecommunication service. The same

supplementary service may be applicable to a number of telecommunication services. Most

supplementary services are directly inherited from a fixed network, with minor

modifications (when needed) to adapt to mobility. Examples of supplementary services are

calling line identification and call waiting.

Supplementary services extend beyond the normal bearer services and teleservices

(basic telecommunication services) and can be subscribed to separately. In the following a

supplementary service is called simply service, in contrast to basic telecommunication

service.

Number Identification Services

Calling line identification presentation (CLIP)

Calling line identification restriction (CLIR)

Call Offering Services

Call forwarding unconditional (CFU)

Call forwarding on mobile subscriber busy (CFB)

Call forwarding on no reply (CFNRy)

Call forwarding on mobile subscriber not reachable (CFNRc)

Call Completion Services

Call hold

Call waiting (CW)

Multi-Party Service

Charging Services

Advice of charge (AOC)

Call Restriction Services

Barring of all outgoing calls (BAOC)

Barring of all outgoing international calls (BOIC)

Barring of all outgoing international calls except to home PLMN country

(BOICexHC)

Barring of all incoming calls (BAIC)

Barring of all incoming calls when roaming outside home PLMN country

(BIC Roam)

Closed User Group (CUG)

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3.2 GSM IDENTITIES

3.2.1 Mobile Station ISDN Number (MSISDN)

The MSISDN is a number, which uniquely identifies a mobile telephone subscription in

the PSTN numbering plan. The MS international number must be ignall after the

international prefix in order to obtain a mobile subscriber in another country. The MSISDN

numbers is composed of the country code (CC) followed by the National Destination Code

(NDC), Subscriber Number (SN), which shall not exceed 15 digits. Here too the first two

digits of the SN identify the HLR where the mobile subscriber is administrated.

In GSM 900/1800, The MSISDN consists of following:

MSISDN = CC + NDC + SN

National Mobile number

International Mobile station ISDN Number

3.2.2 The Mobile Station Roaming Number (MSRN)

The MSRN is allocated on temporary basis when the MS roams into another numbering

area. The MSRN number is used by the HLR for rerouting calls to the MS. It is assigned

upon demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall

have the same structure as international ISDN numbers in the area in which the MSRN is

allocated. The HLR knows in what MSC/VLR service area the subscriber is located. At the

reception of the MSRN, HLR sends it to the GMSC, which can now route the call to the

MSC/VLR exchange where the called subscriber is currently registered.

3.2.3 International Mobile Subscriber Identity (IMSI)

An IMSI is assigned to each authorized GSM user. It consists of a mobile country

code (MCC), mobile network code (MNC) (to identify the PLMN), and a PLMN unique

mobile subscriber identification number (MSIN). The IMSI is the only absolute identity

that a subscriber has within the GSM system. The IMSI consists of the MCC followed by

the MNC and MSIN and shall not exceed 15 digits. It is used in the case of system-internal

ignalling transactions in order to identify a subscriber. The first two digits of the MSIN

identify the HLR where the mobile subscriber is administrated.

The IMSI consists of three parts:

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IMSI = MCC + MNC + MSIN

Where,

MCC = Mobile Country Code

MNC = Mobile Network Code

MISN = Mobile station Identification Code

3.2.4 Temporary mobile subscriber identity (TMSI)

A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality.

It is assigned only after successful subscriber authentication. The correlation of a TMSI to

an IMSI only occurs during a mobile subscriber’s initial transaction with an MSC (for

example, location updating). Under certain condition (such as traffic system disruption and

malfunctioning of the system), the MSC can direct individual TMSIs to provide the MSC

with their IMSI.

3.2.5 International Mobile Equipment Identity (IMEI)

The IMEI is the unique identity of the equipment used by a subscriber by each PLMN

and is used to determine authorized (white), unauthorized (black), and malfunctioning

(gray) GSM hardware. In conjunction with the IMSI, it is used to ensure that only

authorized users are granted access to the system.

The IMEI consists of the following:

IMEI = TAC + FAC + SNR + SVN

Where,

TAC = type approval code

FAC= Final assembly code

SNR = Serial number

SVN = Software Version Number

3.2.6 Location Area Identity (LAI)

The LAI is used for paging and it tells MSC in which location area the MS is located.

LAI = MCC + MNC + LAC

Where,

MCC = Mobile Country Code

MNC = Mobile Network Code

LAC = location Area Code 17


3.2.7 Cell Global identity (CGI):

This is used for cell identification, within the location area. This is done by adding a

cell identity.

CGI = MCC +MNC + LAC +CI

Where,

CI = cell identity (16 digits maximum)

3.2.8 PERSONAL IDENTITY NO.

It is used to unlock the MS. If one enters the wrong PIN three times it will lock the

SIM. The SIM can be protected by use of PIN password.

3.2.9 PIN UNBLOCKING KEY (PUK)

In case of PIN, the PUK is needed for unlocking the SIM again. PUK is numeric only,

with eight digits. If a correct PUK is entered, an indication is given to the user. After 10

consecutive incorrect entries the SIM is blocked. Either the IMSI or the MSISDN Number

may access the subscriber data. Some of the parameters like IAI will be continuously

updated to reflect the current location of the subscriber. The SIM is capable of storing

additional information such as accumulated call charges. This information will be

accessible to the customer via handset key entry.

3.3 OPERATION & MAINTENANCE CENTER

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.

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The OMC provides alarm-handling functions to report and log alarms generated by the

other network entities. The maintenance personnel at the OMC can define that criticality of

the alarm. Maintenance covers both technical and administrative actions to maintain and

correct the system operation, or to restore normal operations after a breakdown, in the

shortest possible time.

The fault management functions of the OMC allow network devices to be manually or

automatically removed from or restored to service. The status of network devices can be

checked, and tests and diagnostics on various devices can be invoked. For example,

diagnostics may be initiated remotely by the OMC. A mobile call trace facility can also be

invoked. The performance management functions included collecting traffic statistics from

the GSM network entities and archiving them in disk files or displaying them for analysis.

Because a potential to collect large amounts of data exists, maintenance personal can select

which of the detailed statistics to be collected based on personal interests and past

experience. As a result of performance analysis, if necessary, an alarm can be set remotely.

The OMC provides system change control for the software revisions and configuration

data bases in the network entities or uploaded to the OMC. The OMC also keeps track of

the different software versions running on different subsystem of the GSM.

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. Each

LA is assigned a location area identity (LAI) number.

3.4 The Radio interface (Um)

The International Telecommunication Union (ITU), which manages the international

allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for

the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station

to mobile station) for mobile networks in Europe. Since this range was already being used

in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve

the top 10 MHz of each band for the GSM network that was still being developed.

Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.

Since radio spectrum is a limited resource shared by all users, a method must be

devised to divide up the bandwidth among as many users as possible. The method chosen

by GSM is a combination of Time and Frequency Division Multiple Access

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(TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz

bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier

frequencies are then assigned to each base station. Each of these carrier frequencies is then

divided in time, using a TDMA scheme, into eight time slots. One time slot is used for

transmission by the mobile and one for reception. They are separated in time so that the

mobile unit does not receive and transmit at the same time, a fact that simplifies the

electronics.

Each timeslot on a TDMA frame is called a physical channel. Therefore, there are 8

physical channels per carrier frequency in GSM. Physical channels can be used to

transmit speech, data or ignalling information.

Fig:3.3 Radio interface

A physical channel may carry different messages, depending on the information that

is to be sent. These messages are called logical channels. For example, on one of the

physical channels used for traffic, the traffic itself is transmitted using a Traffic

Channel (TCH) message, while a handover instruction is transmitted using a Fast

Associated Control Channel (FACCH) message.

Many types of logical channels exists each designed to carry a different message

to or from an MS. All information to and from an MS must be formatted correctly,

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so that the receiving device can understand the meaning of different bits in the

message.

3.5 CONTROL CHANNELS

When an MS is switched on, it searches for a BTS to connect to. The MS scans the entire

frequency band, or, optionally, uses a list containing the allocated carrier frequencies for this

operator. When the MS finds the strongest carrier, it must then determine if it is a control

channel. It does so by searching for a particular logical channel called Broadcast Control

Channel (BCCH).

A frequency carrying BCCH contains important information for an MS, including e.g. the

current LA identity, synchronization information and network identity. Without such

information, an MS cannot work with a network. This information is broadcast at regular

intervals, leading to the term Broadcast Channel (BCH) information.

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Table:1 BROAD CAST CHANNELS

Broadcast Channels (BCH’s)

Logical Channel Direction BTS MS

Frequency

Correction

Channel

(FCCH)

Downlink,

point to

multipoint

Transmits a

carrier

frequency.

Identifies BCCH carrier

by the carrier frequency

and synchronizes with

the frequency.

Synchronization

Channel (SCH)

Downlink,

point to

multipoint

Transmits information

about the TDMA frame

structure in a cell (e.g.

frame number) andthe

BTS identity (Base

Station Identity Code

(BSIC)).

Synchronizes with the

frame structure within a

particular cell, and

ensures that the chosen

BTS is a GSM BTS -

BSIC can only be

decoded by an MS if the Broadcast Control

Channel (BCCH)

Downlink,

point to

multipoint

Broadcasts some

general cell

information such as

Location Area

Identity (LAI),

maximum output

power allowed in the

cell and

the identity of BCCH

carriers for

Receives LAI and will

signal to the network as

part of the Location

Updating procedure if the

LAI is different to the one

already stored on its SIM.

MS sets its output power

level based on the

information received on

the BCCH. Also, the MS

stores a list of BCCH

carriers on which it will

When the MS has finished analyzing the information on a BCH, it then has all the

information required to work with a network. However, if the MS roams to another cell, it

must repeat the process of reading FCCH, SCH and BCCH in the new cell.

If the mobile subscriber then wishes to make or receive a call, the Common Control Channels

(CCCH) must be used.

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Table:2 Common control channel

Common Control Channels (CCCH)


Paging Channel

(PCH)

Downlink,

point to

point

Transmits a paging

message to indicate

an incoming call or

short message. The

paging message

contains the identity

number of the mobile

At certain time intervals

the MS listens to the

PCH. If it identifies its

own mobile subscriber

identity number on the

PCH, it will respond.

Random Access

Channel

Uplink,

point to

point

Receives request from

MS for a signaling

channel (to be used

for call set-up).

Answers paging message

on the RACH by

requesting a signaling

channel.Access Grant

Channel

(AGCH)

Downlink,

point to

point

Assigns a signaling

channel (SDCCH) to

the MS.

Receives signaling

channel assignment

(SDCCH).

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Table:3 Dedicated control channel

Dedicated Control Channels (DCCH)


Stand alone

Dedicated Control

Channel

(SDCCH)

Uplink and

downlink,

point to

point

The BTS switches to

the assigned SDCCH.

The call set-up

procedure is performed

in idle mode. The BSC

assigns a TCH.

(SDCCH is also used to

The MS switches to the

assigned SDCCH. Call set-

up is performed. The MS

receives a TCH assignment

information (carrier and

time slot).Cell Broadcast

Channel

Downlink,

point to

multipoint

Uses this logical channel

to transmit short

message service cell

MS receives cell

broadcast messages.

Slow Associated

Control Channel

(SACCH)

Uplink and

downlink,

point to

point

Instructs the MS

the transmitting power to

use and gives

instructions on timing

advance (TA).

Sends averaged

measurements on its own

BTS (signal strength and

quality) and neighboring

BTS’s (signal strength).

The MS continues to use Fast Associated

Control Channel

(FACCH)

Uplink and

downlink,

point to

point

Transmits

handover

information.

Transmits necessary

handover information in

access burst

Once call set-up procedures have been completed on the control physical channel, the MS

tunes to a traffic physical channel. It uses the Traffic Channel (TCH) logical channel. There

are two types of TCH:

4 Full rate (TCH): transmits full rate speech (13 kbits/s). A full

rate TCH occupies one physical channel.

5 Half rate (TCH/2): transmits half rate speech (6.5 kbits/s).

Two half rate TCH’s can share one physical channel, thus

doubling the capacity of a cell.

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3.6 GSM NETWORK AREAS

The GSM network is made up of geographic areas. As shown in the figure below, these

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

network (PLMN) areas.

Fig:3.5 GSM network area

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. It is the

smallest building block in a mobile network and that’s why it is called as cellular network.

Typically cells are graphically represented by hexagons. There are 2 main types of cells :

3.6.1 Omni-directional cell

An omni-directional cell (or omnicell) is served by a BTS with an antenna which

transmits equally in all directions( 360 deg).

3.6.2 Sector cell

A sector cell is the area of coverage from anantenna, which transmits, in a given

direction only. Forexample, this may be equal to 120 degrees or 180 degrees ofan

equivalent omni- directional cell. One BTS can serve oneof these sector cells with a

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collection of BTS’s at a siteserving more than one, leading to terms such as two-sectored

sites and more commonly three-sectored sites/

Omni-directional cell Sector

cell

Fig:3.6 Type of network

The location area is a group of cells. It is the area in which the subscriber is paged (the

setting up of channels for one to one communication between mobile transceiver and base

station). Each LA is served by one or more base station controllers, yet only by a single

MSC. Each LA is assigned a location area identity (LAI) number.

Fig:3.7 Location Areas

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

MSC and which is reachable, as it is registered in the VLR of the MSC.

Fig: MSC/VLR service areas

The PLMN service area is an area served by one network operator.

Fig:3.8 PLMN network areas

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CHAPTER – 4CALL FLOW,HANDOVER,GPRS,SIM FREQUENCY

4.1 SAMPLE TRAFFIC CASE

4.1.1 CALL TO AN MS

The following traffic case describes a call to an MS and highlights the use of

some logical channels during the call.

Fig:4.1 Call to an MS

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The MSC/VLR knows which LA the M S is located in. A paging message is sent to the

BSC’s controlling the LA.

The BSC’s distribute the paging message to the BTS's in the desired LA. The BTS’s

transmit the message over the air interface using PCH.

When the MS detects a PCH identifying itself, it sends a request for a signaling channel

using RACH.

The BSC uses AGCH to inform the MS of the signaling channel (SDCCH and SACCH)

to use.

SDCCH and SACCH are used for call set-up. A TCH is allocated and the SDCCH is

released.

The MS and BTS switch to the identified TCH frequency and time slot. The MS rings. If

the subscriber answers, the connection is established. During the call, the radio

connection is maintained by information sent and received by the MS using SACCH.

4.1.2 CALL FROM AN MS

This section describes what happens when a mobile subscriber wants to set up a voice

call to a subscriber in the PSTN. Data and text message calls are described separately.

Fig:4.2 Call set-up MS to PSTN

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1. The MS uses RACH to ask for a signaling channel.

2. The BSC/TRC allocates a signaling channel, using AGCH.

3. The MS sends a call set-up request via SDCCH to the MSC/VLR. Over SDCCH all signaling

preceding a call takes place. This includes:

Marking the MS as “active” in the VLR

The authentication procedure

Start ciphering

Equipment identification

Sending the B-subscriber’s number to the network

Checking if the subscriber has the service “Barring of outgoing calls” activated

4. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are

told to tune to the TCH.

5. The MSC/VLR forwards the B–number to an exchange in the PSTN, which establishes a

connection to the subscriber.

6. If the B-subscriber answers, the connection is established.

4.1.3 CALL TO AN MS

The major difference between a call to an MS and a call from an MS is that in a call to an MS

the exact location of the mobile subscriber is unknown. Therefore, the MS must be located

using paging before a connection can be established.

Below is the description of the call set-up procedure for a call from a PSTN subscriber

to a mobile subscriber. A call from an MS to a mobile subscriber operates according to the

same process, the only difference being that the GMSC is contacted by another MSC/VLR

instead of by a PSTN node.

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Fig:4.3 Call to an msn

1. The PSTN subscriber keys in the MS’s telephone number (MSISDN). The MSISDN is

analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. A

connection is established to the MS’s home GMSC.

2. The GMSC analyzes the MSISDN to find out which HLR the MS is registered in, and

queries the HLR for information about how to route the call to the serving MSC/VLR.

3. The HLR translates MSISDN into IMSI, and determines which MSC/VLR is currently

serving the MS. The HLR also checks if the service, “Call forwarding to C–number” is

activated, if so, the call is rerouted by the GMSC to that number.

4. The HLR requests an MSRN from the serving MSC/VLR.

5. The MSC/VLR returns an MSRN via HLR to the GMSC.

6. The GMSC analyses the MSRN and routes the call to the MSC/VLR.

7. The MSC/VLR knows which LA the MS is located in. A paging message is sent to the

BSC’s controlling the LA.

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8. The BSC’s distribute the paging message to the RBS's in the desired LA. The RBS’s

transmit the message over the air interface using PCH. To page the MS, the network uses an

IMSI or TMSI valid only in the current MSC/VLR service area.

9. When the MS detects the paging message, it sends a request on RACH for a SDCCH.

10. The BSC provides a SDCCH, using AGCH.

11. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call

takes place. This includes:

· Marking the MS as “active” in the VLR

· The authentication procedure

· Start ciphering

· Equipment identification

12. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are told

to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is

established.

4.2 FREQUENCY HOPPING

The mobile station already has to be frequency agile, meaning it can move between a

transmit, receive, and monitor time slot within one TDMA frame, which may be on different

frequencies. GSM makes use of this inherent frequency agility to implement slow frequency

hopping, where the mobile and BTS transmit each TDMA frame on a different carrier

frequency. The frequency-hopping algorithm is broadcast on the Broadcast Control Channel.

Since multipath fading is (mildly) dependent on carrier frequency, slow frequency hopping

helps alleviate the problem. In addition, co channel interference is in effect randomized.

FH may be classified as fast or slow. Fast FH occurs if there is frequency hop for each

transmitted symbol. Thus, fast FH implies hat the hopping rate equals or exceeds the

information symbol rate. Slow FH occurs if two or more symbols are transmitted in the time

interval between frequency hops.

FH allows communicators to hop out of frequency channels with interference or to hop out

of fades. To exploit this capability, error-correcting codes, appropriate interleaving, and

disjointed frequency channels are nearly always used.32


4.3 AUTHENTICATION

When a new subscription is registered in GSM, the mobile is given a subscriber

authentication key (Ki) and a telephone number, or international mobile subscriber identity

(IMSI), which are used in the network to identify the mobile. The Ki and IMSI are stored both

in the mobile and in a special network element called AUC. The AUC uses the Ki and IMSI to

calculate an identification parameter called signal response (SRES). SRES is calculated as a

function of Ki and a random number (RAND) generated by the AUC. RAND and SRES are

then stored in the HLR for use in set-up procedures.

Set-up or registration will not be accepted until authentication has been performed. Using the

mobile's IMSI, the MSC fetches the corresponding RAND and SRES from the HLR. RAND is

sent to the mobile, which uses its stored Ki value to calculate SRES. It then returns the

calculated SRES to the MSC, where it is compared with the SRES value received from the

HLR. If the values tally, the set-up is accepted; if not, set-up is rejected.

Fig:4.4 Authentication in GSM

4.4 ENCRYPTION

Since radio communications can be intercepted by practically anyone in the immediate

surroundings, protection against eavesdropping is an important service in a mobile network.

The best solution is an encrypted air interface, for both traffic and control channels. Since

encryption of voice requires digital coding, it cannot be used in analog mobile networks.

Control channels can, in principle, be encrypted in both analog and digital systems, but

encryption is more common in mobile networks that use digital control channels, such as GSM

and D-AMPS.

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In GSM, voice is encrypted as follows: In addition to SRES, the AUC calculates an encryption

key (Kc) based on Ki and RAND. This key is stored in the HLR together with RAND and

SRES. In connection with authentication, the mobile calculates a Kc value based on the RAND

value received from the MSC and on the Ki value stored in the mobile. If the result of the

authentication is approved, the MSC will store the encryption key in the base station (via the

BSC) for use in encryption/decryption operations. The BSC then sends a "test signal"

(encryption mode command) to the mobile. In response, the mobile should generate an

encrypted signal (encryption mode complete) which - if the BSC can interpret it - permits

continued signaling and communication. All signals, including voice signals, are encrypted.

Fig:4.5 Encryption in GSM

4.5 HANDOVER

When a MS is moving away from the area covered by one cell and entering the area covered by

another cell the MS gets connected to another BTS based on some threshold value of the signal.

This is necessary because if we are moving from one cell to another during a call, it also gets

transferred to the new cell’s BTS. In this way the calls do no drop or terminate.

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Fig:4.6 handover

There are two types of handovers:

4.5.1 HARD HANDOVER

A hard handover is one in which the channel in the source cell is released and only then the

channel in the target cell is engaged. Thus the connection to the source is broken before or ‘as’

the connection to the target is made—for this reason such handovers are also known as break-

before-make. Hard handovers are intended to be instantaneous in order to minimize the

disruption to the call. A hard handover is perceived by network engineers as an event during

the call. It requires the least processing by the network providing service.

4.5.2 SOFT HANDOVER

It is the one in which the channel in the source cell is retained and used for a while in parallel

with the channel in the target cell. In this case the connection to the target is established before

the connection to the source is broken, hence this handover is called make-before-break. For

this reason the soft handover is perceived by network engineers as a state of the call. Soft

handovers may involve using connections to more than two cells: connections to three, four or

more cells can be maintained by one phone at the same time.

Handover, or handoff as it is called in North America, is the switching of an ongoing call to

a different channel or cell. There are four different types of handover in the GSM system,

which involve transferring a call between

Channels (time slots) in the same cell,

Cells (Base Transceiver Stations) under the control of the same Base Station Controller

(BSC),

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Cells under the control of different BSCs, but belonging to the same Mobile services

Switching Center (MSC), and

Cells under the control of different MSCs.

The first two types of handover, called internal handovers, involve only one Base Station

Controller (BSC). To save ignalling bandwidth, they are managed by the BSC without

involving the Mobile service Switching Center (MSC), except to notify it at the completion of

the handover. The last two types of handover, called external handovers, are handled by the

MSCs involved. Note that call control, such as provision of supplementary services and

requests for further handoffs, is handled by the original MSC.

Handovers can be initiated by either the mobile or the MSC (as a means of traffic load

balancing). During its idle time slots, the mobile scans the Broadcast Control Channel of up to

16 neighboring cells, and forms a list of the six best candidates for possible handover, based on

the received signal strength. This information is passed to the BSC and MSC, and is used by

the handover algorithm.

The algorithm for when a handover decision should be taken is not specified in the GSM

recommendations. There are two basic algorithms used, both closely tied in with power

control. This is because the BSC usually does not know whether the poor signal quality is due

to multipath fading or to the mobile having moved to another cell. This is especially true in

small urban cells.

The ‘minimum acceptable performance’ algorithm gives precedence to power control over

handover, so that when the signal degrades beyond a certain point, the power level of the

mobile is increased. If further power increases do not improve the signal, then a handover is

considered. This is the simpler and more common method, but it creates ‘smeared’ cell

boundaries when a mobile transmitting at peak power goes some distance beyond its original

cell boundaries into another cell.

The ‘power budget’ method uses handover to try to maintain or improve a certain level of

signal quality at the same or lower power level. It thus gives precedence to handover over

power control. It avoids the ‘smeared’ cell boundary problem and reduces co-channel

interference, but it is quite complicated.

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4.6 GPRS

General Packet Radio Services (GPRS) is a standardized packet switched data service for GSM

enabling mobile use of internet. The GPRS system provides a basic solution for Internet

Protocol (IP) communication between Mobile Stations (MS) and Internet Service Providers

(ISP) or a corporate LAN (Local Area Network). GPRS establishes an end-to-end IP

connection from the mobile terminal to the servers at the ISP. The packet data transmission is

thus carried out on an end-to-end basis, including the air interface.

GPRS users can remain on-line without continuously occupying a specific radio channel.

GPRS will use the common pool of physical resources across the radio interface in co-existence

with the existing circuit switched GSM. The same physical channels will be used but in a more

efficient way since several GPRS users will be able to share one channel. Thus giving a better

channel utilization. In addition, GPRS channels are allocated only when data is sent or

received.

4.6.1 GPRS SYSTEM OVERVIEW

GPRS is an extension of the GSM architecture packet data traffic runs on a new backbone IP

network and is separate from the existing GSM core network

that is used for circuit switched traffic (mainly speech). Two new nodes form the cornerstones

of the GPRS backbone. The Serving GPRS Support Node (SGSN) handles packet data traffic

of users in a geographical area. The Gateway GPRS Support Node (GGSN) connects to outside

data networks. SGSN and GGSN are routers that support mobility of terminals. GPRS uses the

existing GSM radio network. The transmission links between BTSs and BSCs are reused,

which reduces the overall cost for GPRS.

GPRS also reuses other existing GSM network elements such as the HLR and the MSC/VLR.

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Fig:4.7 Block diagram of GPRS

4.7 SIM Card

Fig:4.8

Sim card

GSM subscribers are provided with a SIM (subscriber identity module) card with its unique

identification at the very beginning of the service. By divorcing the subscriber ID from the

equipment ID, the subscriber may never own the GSM mobile equipment set. The subscriber

is identified in the system when he inserts the SIM card in the mobile equipment. This provides

an enormous amount of flexibility to the subscribers since they can now use any GSM-

specified mobile equipment. Thus with a SIM card the idea of “Personalize” the equipment

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currently in use and the respective information used by the network (location information)

needs to be updated. The smart card SIM is portable between Mobile Equipment (ME) units.

The user only needs to take his smart card on a trip. He can then rent a ME unit at the

destination, even in another country, and insert his own SIM. Any calls he makes will be

charged to his home GSM account. Also, the GSM system will be able to reach him at the ME

unit he is currently using. This is the main advantage of GSM over CDMA.

The SIM is a removable, the size of a credit card, and contains an integrated circuit chip

with a microprocessor, random access memory (RAM), and read only memory (ROM). The

subscriber inserts it in the MS unit when he or she wants to use the MS to make or receive a

call. As stated, a SIM also comes in a modular from that can be mounted in the subscriber’s

equipment.

When a mobile subscriber wants to use the system, he or she mounts their SIM card and

provide their Personal Identification Number (PIN), which is compared with a PIN stored

within the SIM. If the user enters three incorrect PIN codes, the SIM is disabled. The service

provider if requested by the subscriber can also permanently bypass the PIN. Disabling the PIN

code simplifies the call setup but reduces the protection of the user’s account in the event of a

stolen SIM.

4.8 FREQUENCY REUSE

The spectrum allocated for a cellular network is limited. As a result there is a limit to the

number of channels or frequencies that can be used. For this reason each frequency is used

simultaneously by multiple base-mobile pairs. This frequency reuse allows a much higher

subscriber density per MHz of spectrum than other system. System capacity can be further

increased by reducing the cell size down to radii as small as 200m.

Fig:4.9 frequency re-use pattern

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4.9 FREQUENCY BANDS

In India GSM is expanded to operate at three frequency bands 900MHz, 1800MHz and

2100MHz.The original frequency band specified for GSM was 900 MHz.

Most GSM networks worldwide use this band. In somecountries and extended version of GSM

900 can be used, whichprovides extra network capacity. This extended version of GSMis called

E-GSM, while the primary version is called P-GSM.In 1990, in order to increase competition

between operators, theUnited Kingdom requested the start of a new version of GSMadapted to

the 1800 MHz frequency band. Licenses have beenissued in several countries and networks are

in full operation.By granting licenses for GSM 1800 in addition to GSM 900, acountry can

increase the number of operators. In this way, dueto increased competition, the service to

subscribers is improved. In our country only Airtel, Vodafone and Idea use the 900 band, other

operators including Aircel use the 1800 band. The 2100 band is used for 3G communications

and is limited to a few operators.

Fig:4.10 Frequency band

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