Megha Training Report mobile communication

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A

PRACTICAL TRAINING SEMINAR REPORTON

MOBILE COMMUNICATION

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR

THE AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGY

(ELECTRONICS AND COMMUNICATION ENGINEERING)

Session- 2010-11

SUBMITTED TO: SUBMITTED BY:

Mr. GAJANAND GUPTA MEGHA GOYAL

(E&C) V SEM

ROLL NO.-52

( A VENTURE OF SEEDLING GROUP OF INSTITUTIONS)

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JAIPUR NATIONAL UNIVERSITY, JAIPUR(A VENTURE OF SEEDLING GROUP OF INSTITUTIONS)

CANDIDATES DECLARATION

I hereby certify that the work which is being presented in the

report entitled MOBILE COMMUNICATION by MEGHA

GOYAL in partial fulfillment of requirements for the award of

degree of B.Tech. (2010&E&C) submitted in the department of

electronics and communication at jaipur national university, jaipur is

an authentic record of my own work carried out during a periodfrom 10.06.2010 to 25.07.2010 under the supervision of MR.ANKIT

SANI .

SIGNATURE OF THE STUDENT:

The B.tech 3rd year E&C seminar presentation of MEGHAGOYAL has been held on and accepted.

Signature of examiner 1 Signature ofexaminer 2

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ACKNOWLEDGEMENT

It has been a wonderful and enlightening for me as an undergraduate

student take training at NITS EDUCATION, Jaipur.

I thank my Head Of Department, Mr. Lakshminarayan and Mr. Gajanand

for giving support to my training subject.

I express my sincere thanks to Mr. Ankit Sani for his inexplicable

guidance, incessant encouragement and constant support.

Above all, I would like to thank all those members of NITSEDUCATION who helped me to complete my training work.

MEGHA GOYAL

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ABSTRACT

Mobile telephony adoption is on the rise. wireless subscribers will reach 1billion worldwide by 2002 . Recent technological innovations have also

dramatically enhanced the capabilities of the wireless telephone . No

longer restricted to voice communications, wireless devices are now also

able to transmit and manipulate data. Leveraging the power of these new

technologies, various business sectors are working together to offer a

wide array of services, including voice communications, short messaging,

information services, web surfing, location-based services, and e-

commerce. Each sector is looking for the next "killer application," yet we

are still learning about people's information and communication needs

while "on the go" . Additionally, it is difficult to anticipate what new

possibilities or challenges are created for the user upon the introduction

of these new computational capabilities.

The GSM system has become the most popular system for mobilecommunication in the world. Criminals commonly use GSM phones, and

it is therefore a need for forensic investigators to understand which

evidence can be obtained from the GSM system. This reportbriefly explains the basics of the GSM system.With GSM, systems for mobile communication reached a global scale. In

the western world, it seems everyone has their own mobile phone, and

GSM has taken more and more of the market. GSM allows users to roam

seamlessly between networks, and separate the user identity from the

phone equipment.

A GSM network is composed of several functional entities, whose

functions and interfaces are defined. GSM system defines 2 truly open

interfaces. The first one is between the mobile station & the base

station named AIR INTERFACE. The second one is between themobile service switching centre MSC & the Base Station Controller

(BSC) called A INTERFACE.

Intelligence between the interfaces increases with complications in the

interface. Hence in GSM, the decentralized intelligence is implemented

by dividing the whole network into three separate subsystems.

BSS

NSS

NMS

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Table Of Contents

1. Introduction to Mobile Communication

1.1 First Generation cellular system1.2 Second Generation (2G) cellular system

1.3 Third Generation cellular (3G) system

2. Global System for Mobile

2.1 Introduction

2.2 GSM Architecture

2.3 GSM Subsystem

3. Cellular System Design in GSM

3.1 Introduction

3.2 Frequency Reuse

3.3 Channel Assignment strategies

3.4 Handover for successful Mobility

3.5 Interference

4. GSM Call Processing

4.1 Location Update

4.2 Call set-up in a GSM network

4.3 Transmission of Radio & communication channels

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Introduction to Mobile Communication

The technology of communication & connecting people has evolveddramatically since Gugliemo Marconi first demonstrated radios abilityto provide continuous contact with ships sailing the English Channelin 1897. The Fixed Wire line phone available in almost each & everyhome or office in the world is now considered obsolete. To keep upwith the ever increasing pace of life, people need something thathelps them stay connected to the world while they are mobile. Mobiletelephony is no longer a luxury but a necessity.

The ability to provide wireless communications to an entirepopulation was considered impossible until Bell Laboratoriesdeveloped the cellular concept in 1960s & 1970s. Wirelesscommunications era was born in 1970s with the development ofhighly reliable, miniature, solid-state radio frequency hardware. Theworlds first cellular system was implemented by Nippon Telephone &telegraph company (NTT) in Japan in 1979. This system uses 600 FM

duplex channels (25 kHz for each one-way link) in the 800 MHz band.

1.1 First Generation Cellular System

Since the mid 1990s, the cellular communications industry haswitnessed explosive growth. Mobile radio transmission systems maybe classified as simplex, half duplex or full duplex. In simplexsystems, communication is possible in only one direction.Full-duplexsystems allow simultaneous radio transmission & reception betweena subscriber & a base station, by providing two simultaneous butseparate channels (FDD) or adjacent time slots on a single radiochannel (TDD) for communication to & from user.

1st Generation Cellular Characteristics

Widespread Introduction in early 1980s Analogue modulation Frequency Division Multiple Access Voice traffic only No inter-network roaming possible

Insecure air interface

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T he First Generation Problems

Problems with the analog systems included: Limited capacity could not cope with increase in subscribers

Bulky equipment

Poor reliability

Lack of security air interface analogue signals could be intercepted

Incompatibility between systems in different countries - no roaming

To improve on the analog systems, the European Conference of Posts a

Telecommunications Administrations (CEPT) established Groupe Specia

Mobile (GSM) to set a new standard

The system developed became the Global System for Mobile

Communications (also GSM)

1.2 Second Generation (2G) Cellular Networks

Second generation standards use digital modulation formats &TDMA/FDD & CDMA/FDD multiple access techniques. Time DivisionDuplexing (TDD) is based on the possibility of sharing a single radiochannel in time, so that a portion of time is used to transmit from thebase station to the mobile, & the remaining time is used to transmit

from mobile to the base station.2nd Generation Characteristics

Widespread Introduction in 1990s Uses digital modulation Variety of multiple access strategies More efficient use of radio spectrum Voice and low rate circuit switched data International roaming capability Secure air interface Compatibility with ISDN

1.2.1 2.5 G Mobile Networks

2.5G Characteristics 2.5G Characteristics

Available now...

Digital modulation Voice and intermediate rate circuit/packet

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switched data 2G technology roaming Secure air interface Based upon existing dominant 2Gstandards such as GSM and cdmaOne

Enhanced data rates

EDGE, a revision by the 3GPP organization to the older2GGSM based

transmission methods, utilizing the same switching nodes, basestation sites and

frequencies as GPRS, but new basestation and cellphone RF circuits. It is based on

the three times as efficient 8PSKmodulation scheme as supplement to the

originalGMSKmodulation scheme. EDGE is still used extensively due to its ease

of upgrade from existing 2G GSM infrastructure and cell-phones.

Chapter 9

1. 2.5 G:2.5G is a stepping stone between 2Gand 3Gcellular wireless technologies. Theterm "second and a half generation" is used to describe 2G-systems that have implemented

apacket switched domain in addition to thecircuit switched domain. It does not necessarily

provide faster services because bundling of timeslots is used forcircuit switcheddata services

(HSCSD) as well. The first major step in the evolution of GSM networks to 3G occurred with

the introduction of General Packet Radio Service (GPRS). CDMA2000 networks similarly

evolved through the introduction of1xRTT. The combination of these capabilities came to beknown as 2.5G.

GPRS could provide data rates from 56 kbit/s up to 115 kbit/s. It can be used for services such

as Wireless Application Protocol (WAP) access, Multimedia Messaging Service (MMS), and

for Internet communication services such as email and World Wide Web access. GPRS data

transfer is typically charged per megabyte of traffic transferred, while data communication via

traditional circuit switching is billed per minute of connection time, independent of whether

the user actually is utilizing the capacity or is in an idle state.

1xRTT supports bi-directional (up and downlink) peak data rates up to 153.6 kbit/s,

delivering an average user data throughput of 80-100 kbit/s in commercial networks.[3]It can

also be used for WAP, SMS & MMS services, as well as Internet access

1.3 Third Generation (3G) wireless networks

3G is the technology for multi-megabit Internet access,communications using Voice over Internet protocol (VoIP), voiceactivated calls or unparalleled network capacity. They aim to receivelive music, conduct interactive web sessions & have simultaneousvoice & data access. Wideband CDMA is a 3G technology also calledUniversal mobile telecommunications Services (UMTS).

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http://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/GSMhttp://en.wikipedia.org/wiki/8PSKhttp://en.wikipedia.org/wiki/GMSKhttp://en.wikipedia.org/wiki/GMSKhttp://en.wikipedia.org/wiki/GSMhttp://itlaw.wikia.com/wiki/2Ghttp://itlaw.wikia.com/wiki/2Ghttp://itlaw.wikia.com/wiki/3Ghttp://itlaw.wikia.com/wiki/Cellular_wirelesshttp://itlaw.wikia.com/wiki/Packet_switched_domainhttp://itlaw.wikia.com/wiki/Circuit_switched_domainhttp://itlaw.wikia.com/wiki/Circuit_switched_domainhttp://itlaw.wikia.com/wiki/Circuit_switchedhttp://itlaw.wikia.com/wiki/Circuit_switchedhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/1xRTThttp://en.wikipedia.org/wiki/2G#cite_note-2http://en.wikipedia.org/wiki/2G#cite_note-2http://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/GSMhttp://en.wikipedia.org/wiki/8PSKhttp://en.wikipedia.org/wiki/GMSKhttp://en.wikipedia.org/wiki/GSMhttp://itlaw.wikia.com/wiki/2Ghttp://itlaw.wikia.com/wiki/3Ghttp://itlaw.wikia.com/wiki/Cellular_wirelesshttp://itlaw.wikia.com/wiki/Packet_switched_domainhttp://itlaw.wikia.com/wiki/Circuit_switched_domainhttp://itlaw.wikia.com/wiki/Circuit_switchedhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/1xRTThttp://en.wikipedia.org/wiki/2G#cite_note-2


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3G or3rd generation mobile telecommunications, is a generation of standards formobile

phonesand mobile telecommunication services fulfilling the International Mobile

Telecommunications-2000 (IMT 2000) specifications by the International Telecommunication

Union.[1] Application services include wide-area wireless voicetelephone,mobile Internet access, video

callsand mobile TV, all in a mobile environment. To meet the IMT-2000 standards, a system is

required to provide peak data rates of at least 200 kbit/s. Recent 3G releases, often

denoted3.5G and3.75G, also provide mobile broadbandaccess of

several Mbit/s to smartphonesand mobile modems in laptop computers.

The following standards are typically branded 3G:

:

The original and most widespread radio interface is called W-CDMA.

The TD-SCDMAradio interface, was commercialised in 2009 and is only offered in

China.

EDGE, a revision by the3GPP organization to the older2GGSMbased transmission

methods, utilizing the same switching nodes, basestation sites and frequencies as GPRS, but new

basestation and cellphone RF circuits. It is based on the three times as efficient 8PSKmodulation

scheme as supplement to the originalGMSKmodulation scheme. EDGE is still used extensively

due to its ease of upgrade from existing 2GGSM infrastructure and cell-phones.

EDGE combined with theGPRS2.5G technology is calledEGPRS, and allows peak

data rates in the order of 200 kbit/s, just as the original UMTSWCDMA versions, and thus

formally fullfills the IMT2000 requirements on 3G systems. However, in practice EDGE is

seldom marketed as a 3G system, but a2.9Gsystem. EDGE shows slightly bettersystem

spectral efficiencythan the original UMTS andCDMA2000systems, but it is difficult to

reach much higher peak data rates due to the limited GSM spectral bandwidth of 200 kHz, and

it is thus a dead end.

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Global System for Mobile

2.1 INTRODUCTION

At the beginning of the 1980s it was realized that the Europeancountries were using many different, incompatible mobile phone

systems. At the same time, the need for telecommunication systemwas remarkably increasing. Due to this, CEPT (Conference Europeandes Postes Et Telecommunications) founded a group to specifycommon mobile system for Western Europe. This was namedGROUPE SPECIALE MOBILE & the system name GSM arose. Themost common expression nowadays is Global System for Mobilecommunications.

The specifications for this system are as follows:

There should be several network operators for each country.

This would lead to competition in tariffs & service provisioning& it was assumed to be the best way to ensure the rapidexpansion of the GSM system; The price of the equipment willfall & the users would find the price reducing.

The GSM system must be an open system, meaning that itshould contain well-defined interfaces between system parts.This enables the equipments from several manufacturers to co-exist & hence improves the cost efficiency of the system fromthe operator point of view.

GSM networks must be built without causing any majorchanges to the already existing Public Switched Telephone

Networks (PSTN).

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In addition to the commercial demands above, other main objectiveswere defined as:

The system must be Pan European.

The system must maintain a good speech quality.

The system must use radio frequencies as efficiently as

possible. The system must have high/adequate capacity.

The system must be compatible with the ISDN & other datacommunication specifications.

The system must maintain security concerning both subscriber& transmitted information.

Advantages of GSM

GSM uses radio frequencies efficiently, and due to digital radiopath, the system tolerates more intercell disturbances.

The average quality of speech achieved is better than inanalogue cellular systems.

Data transmission is supported throughout the GSM system

Speech is encrypted & subscriber information security isguaranteed.

Due to the ISDN compatibility, new services are offeredcompared to the analogue systems.

International roaming is technically possible within all countriesusing GSM system.

The large market increases competition & lowers tariffs both forinvestment & usage.

2.2 GSM Architecture

Open Interfaces of GSM

The purpose behind the GSM specifications is to define several openinterfaces, which then are limiting certain parts of the GSM system.Because of this interface openness, the operator maintaining thenetwork may obtain different parts of the network from different GSMnetwork suppliers, which helps the economic aspects of implantation.

A GSM network is composed of several functional entities, whose functions and

interfaces are defined. GSM system defines 2 truly open interfaces. The

first one is between the mobile station & the base station namedAIR INTERFACE. The second one is between the mobile service

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switching centre MSC & the Base Station Controller (BSC)called A INTERFACE.

Intelligence between the interfaces increases with complications inthe interface. Hence in GSM, the decentralized intelligence is

implemented by dividing the whole network into three separatesubsystems:

Network Switching Subsystem (NSS): The actual networkneeded for establishing calls is composed of the NSS & BSS.NSS takes care of call control functions. Calls are connected byand through NSS

Base Station Subsystem (BSS): It is responsible for radio pathcontrol & every call is connected through the BSS.

Network Management Subsystem (NMS): it is the operation &maintenance related part of the network & it is needed for the

control of the whole GSM network.

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Fig. 2.1 GSM Architecture

2.3 GSM Subsystems

The functions of the various subsystems NSS, BSS & NMS are asfollows:

BTS

BTS

BTS

BTS

BSC

BSC

BSS

HLR VLR AUC

MSC

NSS

OMC

NMS

PSTN

ISDN

DataNW

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2.3.1 Network Switching Subsystem(NSS):

Its elements are

MSC (Mobile Services Switching Subsystem) VLR (Visitor Location Register) HLR (Home Location Register)

The functions of NSS are: Call Control- It identifies the subscriber, establishes a call& clear the connection at the end Charging- This collects the charging information about acall such as the number of caller & called subscriber, the time &type of transaction & transfer it to the billing centre Mobility Management- It maintains the information aboutthe location of subscriber. Signaling with other networks & BSS- This provides theinterfacing with the BSS & PSTN Subscriber Data Handling- This is the permanent datastorage in the HLR & Temporary data storage in the VLR Locating the subscriber- This locates a subscriber beforeestablishing a call

2.3.2 Base Station Subsystem

Its elements are BSC (Base Station Controller)- This central elementcontrols radio network. It handles connection between MSC &NSS, Mobility management, raw data collection, Air & Ainterfacing signaling support BTS (Base transceiver Station )- it maintains AIRinterface signaling & speech processing TC (Transcoder)- It takes care of trans coding. It is capableof converting speech from 1 digital coding format to another &vice versa.

The functions are: Radio Path Control- BSS controls radio resources. GSMspecifications define 120 different parameters for each BTSwhich handle kind of handovers, paging organization, radiopower level control & BTS Identification. BTS & TC control- BSC is capable of separating a BTS fromnetwork & collecting ALARM information. Synchronization- BSS uses hierarchical synchronizationmeaning MSC synchronizes BSC, which synchronizes BTS. Ifthere is synchronization error, calls may be cut or call quality

may not be best possible. AIR & A interfacing- it implements various protocols for

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interfacing. Connection Establishment between MS & NSS- BSS islocated between AIR & A interface. If a call has to beestablished, MS must have a connection through these twointerfaces.

Mobility Management & Speech transcoding-Handovers & speech coding are managed. Collection of Statistical Data- BSS collects a lot of shortterm statistical data that is further sent to NMS for callprocessing.

2.3.3 Network Management Subsystem(NMS):

It monitors various functions & elements of the network. Itcommunicates with these elements over a Data CommunicationNetwork (DCN). Its functions are divided into three categories:

Fault Management- Ensures smooth functioning & rapidfault repairs. It informs the operator about current status aboutthe alarms & their history. Configuration Management- It maintains up to date infoabout the operation & configuration status about the networkelements. It takes care of the radio network, software &hardware management of the network elements, timesynchronization & security operations.

Performance Management- NMS collects measurementdata from individual elements & store them in a data base.Hence it helps to compare the real & planned performance.

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Cellular System Design in GSM

3.1 Introduction

Consider a Mobile system which uses a pair of simplex channels oftwo different frequencies for communication with a mobile phone.Thus each single subscriber would require a set of two differentfrequencies. If a city has even one lac subscriber, then the bandwidthrequired would be in the order of several MHz (100,000*2). If a circlecontains several such cites, then the BW required will be extremelyhigh. Unfortunately, there is a limit to everything & BW allotted for

mobile communication by CEPT is no exception. This also makes theprovision for more than one service provider in a circle impossible.The BW allotted for mobile communication is only 50 MHz. Hence asolution should be found out to accommodate ever increasingnumber of subscribers & service providers in the allotted BW.Reusing a frequency was an obvious solution.

Providing a good coverage for uninterrupted service to the customerswas another important issue. The design objective of the early mobileradio systems was to achieve a large coverage area by using a single,high powered transmitter with a tall antenna. While this approach

achieved very good coverage, it also meant that it was impossible toreuse those same frequencies throughout the system, since anyattempt to achieve frequency reuse would result in interference.

The cellular concept was a major breakthrough in solving theproblem of spectral congestion & user capacity. It offered very highcapacity in a limited spectrum allocation without any majortechnological changes. The cellular concept allows to replace a single,high powered transmitter (large cell) with many low poweredtransmitters (small cells), each providing a coverage to only a smallportion of the service area. Each base station is allocated a portion ofthe total number of channels available to the entire system, & nearbybase stations are assigned different groups of channels so that all theavailable channels to a relatively small number of neighboring basestations. Hence the interference between neighboring cells is alsominimized.

As the demand for services increases (i.e. as more channels areneeded within a particular market), the number of base stations maybe increased (along with the corresponding decrease in thetransmitted power to avoid added interference), thereby providing

additional capacity with no additional increase in radio spectrums.This fundamental principle is the foundation for all modern wireless

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communication systems, since it enables fixed number of channels toserve an arbitrarily large number of subscribers by reusing thechannels throughout the coverage area. The cellular concept also hasthe advantage of all the mobile handsets anywhere in the world to bemanufactured with the same set of channels.

3.2 Frequency Reuse

Intelligent allocations & frequency reuse forms the basis of cellularradio system. Each cellular base station is allocated a group of radiochannels to be used within a small geographical area called a cell.Base stations in adjacent cells are assigned channel groups whichcontain completely different channels than neighboring cells. Bylimiting the coverage area within the boundaries of a cell, the samegroup of channels may be used to cover different cells that are

separated from one another by large distances enough to keepinterference levels within tolerable limits. The design process ofselecting & allocating channel groups for all of the cellular basestations within a system is called frequency reuse or frequency

planning.

Fig. 3.1 Cellular frequency reuse where same letter indicate

same frequency

Figure 3.1 indicates frequency reuse concept, where cells labeledwith the same letter use the same group of channels. The frequencyreuse plan is overlaid upon a map to indicate where differentfrequency channels are used. The hexagonal cell shape isconceptual & simplistic model of the radio coverage for each basestation, but it has been universally adopted since the hexagonpermits easy & manageable analysis of a cellular system. The actualradio coverage of a cell is known as footprint& is determined fromfield measurements or propagation prediction models.

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A circular shape for representing a cell structure is not accepted(though a circle can accommodate largest area of all the basicsymmetrical geometric shapes) because the area between adjacentcircles cannot be overlaid upon a map without leaving gaps orcreating overlapping regions, as is visible in the following figure:

Fig 3.2 A failed circular shape cell concept

Thus, when considering geometric shapes which cover entire regionwithout overlap & with equal area, the choices are a square, anequilateral triangle & a hexagon. However any engineering problemmust be considered taking into account the worst case. The worstcases in a cell are weakest mobiles within a footprint, & these aretypically located at the edge of a cell. For a given distance betweenthe center of a polygon & its farthest perimeter points, the hexagonhas the largest area of the three (closest shape resembling a circle).Thus the hexagon geometry approximates a circular pattern whichwould occur for an omni directional base station antenna & free space

propagation.

3.3 Handover for successful mobility

Maintaining the traffic connection with a moving subscriber is madepossible with handover function. When a mobile moves into adifferent cell while a conversation is in progress, the MSCautomatically transfers the call to a new channel belonging to thenew base station. These handover operations involve identifying anew base station & require the voice & control signals be allocated to

channels associated with the new base station.

Processing handoffs is an important task in mobile system becausewithout it mobility cannot be implemented. Handovers must beperformed successfully & as infrequently as possible, & beimperceptible to the users.

In order to meet these requirements, system designers must specifyan optimum signal level at which to initiate a handover. Once aparticular signal level is specified as the minimum usable signal foracceptable voice quality at the base station receiver (normally taken

as between -90 dBm & -100 dBm), a slightly stronger signal level is

Coverage lessarea

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used as a threshold at which a handover is made. This margin givenby:

Delta D = P R HANDOFF - P R MINIMUMThis margin cannot be too large or too small. If D is too large,

unnecessary handovers which burdens MSC may occur. IfD istoo small, there may be insufficient time to complete ahandover before a call is lost due to weak signal. Hence D ischosen carefully to meet these conflicting requirements. Let usanalyze this thing graphically. Consider a mobile is moving from placeA (in cell coverage 1) to place B (in cell coverage 2) as shown below.To maintain this moving call, handover of the call has to be highlyprecise in time.

Fig 3.3a A call requiring a handoff

3.4 Interference

Interference is the major limiting factor in the performance of cellularsystem. Sources of interference include another mobile phone in thesame cell, a call in progress in a neighboring cell, other base stationsoperating in the same frequency band, or any non cellular system

which inadvertently, leaks energy into the cellular band.

Interference on voice channels causes cross talk, where thesubscriber hears interference in the background due to anundesired transmission.

Interference on control channels leads to missed & blockedcalls due to errors in the digital signaling.

Interference is more severe in urban areas, due to greater RF noisefloor & the large number of base station mobiles. It is a major culpritfor being a bottleneck in increasing capacity & is often responsible for

A

B

Cell 1Cell 2

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dropped calls. The two major types of system-generated cellularinterference are

Co Channel Interference

Adjacent Channel Interference

Controlling interference from out of band users, which arises withoutwarning due to front end overload of subscriber equipment orintermittent intermodulation products, is most difficult to control. Amajor source of out-of-band interference is a transmitter fromcompeting cellular providers carriers, since competitors often locatetheir base stations in close proximity to one another in order toprovide comparable coverage to customers.

3.5.1 Co-Channel Interference.

Frequency reuse applies that in a given coverage area there areseveral cells that use the same set of frequencies. These cells arecalled co-channel cells, & the interference between signals fromthese cells is called co-channel interference. Unlike thermal noisewhich can be overcome by increasing the signal-to-noise ratio (SNR),co channel interference cannot be combated by simply increasingcarrier power of a transmitter. This is because an increase incarrier power increases the interference to neighboring co-channel cells. To reduce co-channel interference, co-channels

must be physically separated by a minimum distance to providesufficient isolation due to propagation.

3.5.2 Adjacent channel interference

Interference resulting from signals which are adjacent in frequency tothe desired signal is called adjacent channel interference. Itresults from imperfect receiver filters, which allow nearby frequenciesto leak into the passband. In the near-far effect, an adjacent

channel user is transmitting in very close range to subscribers basestation on a desired channel. Alternatively, the near-far effect occurswhen a mobile close to a base station transmits on a channel close toone being used by a weak mobile. The base station may havedifficulty in discriminating the desired mobile user from thebleedover caused by the close adjacent channel. Mobile.

Adjacent channel interference can be minimized through carefulfiltering & channel assignments. Since each cell s given only afraction of the available channels, a cell need not be assignedchannels which are all adjacent in frequency. Hence by keeping the

frequency separation as large as possible adjacent channelinterference can be minimized. Base station receivers are

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preceded by a high Q cavity filter in order to reject adjacent channelinterference.

GSM Call Processing

Connecting People is the basic service of all telephonenetworks. Hence a network must be able to set up & maintain a callwhich involves a number of tasks:

Identifying the called person

Determining the location

Routing the call to him Ensuring that the connection is sustained as long as the

conversation lasts.

After the transaction, the connection is terminated & callinguser is charged for the service he has used.

In a fixed telephone network, providing & managing connections is arelatively easy process because telephones are connected by wires tothe network & their location is permanent. In a mobile network, theestablishment of call is a far more complex task as the wirelessconnection enables the user to move at their own free will providing

they stay within the service area network.

4.1 Registration & Database

To establish a call in GSM system, the network has to find outsolutions to three problems:

Where is the subscriber

Who is the subscriber

What does the subscriber want

The subscriber has to be located & identified to provide him with therequested service. Consider the following example to understand theprocess:

A well known engineer is traveling around the world. He decides tospend a night in a hotel in Surat. The first thing he will do is tocontact at the reception desk for registration. Basically, thereception desk is an office that supports registration. The

receptionists records the registration in a database which we call thevisitors register.

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The receptionist carefully checks the passport of engineer. Thepassport is also a Database. She finds the basic facts such ascitizenship, identification & his name & name of the issuing authority.

The receptionist contacts the embassy issuing this passport & informsthem about the engineer. Then he is admitted into the hotel & theembassy of the engineers country notes down the latest informationabout his location.

In other words, this is a transaction between the two offices, & as aresult, Identification & Localization of the customer takes place inboth the databases. In this example, the embassy maintains thedatabase which contains the basic data of all citizens who aretraveling around the world & record their movements.

When the registration is completed, the engineer goes to his room.We can say that he is using the service provided by the hotel. As allthe hotels in the world give this type of service which is called basicservice & the additional service provided is called supplementaryservices.

Now the engineer leaves this place & goes to another place. Heregisters in the hotel & once again the receptionist informs theembassy in the home country.

Registration in Surat is cancelled & new registration is made & thelocation data in the home register is brought up to date. We havethus made a successful location update.

The register of the embassy in engineers home countryresembles the HOME LOCATION REGISTER (HLR) in the GSMsystem.

The hotels register resembles the VISITOR LOCATIONREGISTER (VLR) in the GSM system.

The network keeps track of the subscribers location with the help ofdatabases. Let us study the various databases of GSM system indetail.

4.1.1 The SUBSCRIBER IDENTITY MODULE (SIM card)

From the users point of view, the first & most important database isinside the mobile phone. This is known as the SIM Subscriber

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Identity Module. It is a small memory device mounted on a card &contains user specific identification. The SIM card be taken out of onemobile equipment & inserted into another. In the GSM network, SIMcard identifies the user just like the passport in the above example.

The SIM card contains

the Identification number of user.

a list of service that the user has subscribed to .

a list of available network.

Tools needed for authentication & ciphering depending on thetype card.

Storage space for messages such as phone number & SMS.

A home operator issues a SIM card when user joins the network by

making a service subscription. The home operator of the subscribercan be anywhere in the world but for practical reasons, the subscriberchose one of the operator in the country where he spends most of histime.

4.1.2 The HOME LOCATION REGISTER (HLR)

The home operator of the subscriber also needs to know the locationof the subscriber & so it maintains another register just as theembassy in above example which is called a Home Location

Register (HLR).

The HLR stores the basic data of the subscriber on permanentbasis. The only variable data in the HLR is the current location (VLRaddress) of subscriber.

4.1.3 The VISITOR LOCATION REGISTER (VLR)

A complete coverage area of a service area is provided is divided

further into smaller area each maintaining its own Visiting LocationRegister (VLR). The VLR keeps track of the subscribers in thatparticular area. The VLR is integrated into Mobile Switching Centre(MSC). However in the VLR, the subscriber data is stored temporarily.When the subscriber moves to another VLR area, its data is erasedfrom old VLR & stored in the new VLR.

4.2 Location Update:

No matter how often or how quickly a mobile user moves, thenetwork must be able to locate him continuously. The transaction

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that enables the network to keep track of subscriber is calledlocation update.The mobile constantly receives the information sent by the network.This information includes Identification of the current VLR area which

it stores in its memory. Every time the network broadcasts the ID ofthe area, mobile compares this information to the area ID stored in itsmemory. When the 2 IDs are no longer same, the mobile sends arequest to the network i.e. a registration enquiry to the area it hasjust entered. The network on receiving this, registers the mobile innew VLR area. Simultaneously the subscribers HLR is informed aboutthe new VLR location & the data concerning the subscriber is clearedfrom the previous VLR.

Fig 4.1 Elements involved in location update.

HLR

VLR

MSC (OLD)

VLR

MSC (NEW)

SIM

Mobilemoves

Locationupdate

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Fig. 4.2 Location Update procedures

In this way network can keep track of the subscriber all the time,however, that is only a part of the job!

GSM Identifiers

IMEI International Mobile Equipment Identifier

IMSI International Mobile Subscriber Identifier

TMSI Temporary Mobile Subscriber Identity

MSISDN Mobile Subscriber ISDN number

MSRN Mobile Station Roaming Number

LAI Location Area Identity

CI Cell Identifier

BSIC Base Station Identity Cod

MSBSS MSC

VLRHLR

Location Update Request

Request Subscriber ID

Send Subscriber ID

Request Subscriber INFO

Send Subscriber INFO

Authentication

Authentication Response

All OK, HLR UPDATE

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IMEI International Mobile Equipment Identifier.

The IMEI is an internationally-unique serial number allocated to the MS hardware at

the time of manufacture. It is registered by the network operaand (optionally) stored in

the AuC for validation purposes.

IMSI International Mobile Subscriber Identifier

When a subscriber registers with a network operator, a unique subscriber

IMSI identifier is issued and stored in the SIM of the MS. An MS can only

function fully if it is operated with a valid SIM inserted into an


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4.3 Call Set up in a GSM network

Let us study the process involved in setting up calls to & from themobile phones. There are various cases.

4.3.1 Call from fixed landline phone to a GSMphone

The steps when a call is made from fixed landline network to GSMphone are:

1. A subscriber in a fixed network dials to mobile station. The

dialed no. is called MSISDN (Mobile Subscriber InternationalISDNNumber) which is

MSISDN = CC + NDC + SN

CC - Country Code which is ++91 for IndiaNDC - National Destination Code like 9898x for Airtel in GujaratSN - Subscriber Number which is a five digit

2. The PSTN exchange analyses the dialed no. for finding themobile network in which the called subscriber has made hissubscription. The PSTN identifies the mobile network on thebasis of NDC after which it accesses the mobile network via thenearest Gateway Mobile Switching Centre (GMSC).

3. The GMSC analyses the MSISDN in the same way as the PSTNexchange did. Hence it obtains the HLR address in which thesubscriber is permanently registered. This can only bedetermined by the two databases HLR & VLR. At this stage,the GMSC only knows the HLR address & so sends a message

(MSISDN) to the HLR. This is called HLR Enquiry.

4. The HLR identifies the called subscriber & checks its databaseto determine the subscriber location. HLR knows in which VLRthe subscriber is currently registered. HLR is regularly updatedabout the mobiles present VLR through location updateprocedure.

A speech connection is a network service & it can be handled by an

MSC only. Therefore to enable a traffic connection, may be 2 MSCswill have to be connected GMSC (contacted by PSTN network) &

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destination MSC. HLR acts as a coordinator between the GMSC &Destination MSC.

The HLR database has following fields:

HLR

MSISDN : ++91 98980 59784IMSI : 404 98 9898059784VLR address : xyzSubscriber data : services

Here identification involved is International Mobile EquipmentIdentity (IMSI) to identify the subscriber in the mobile network(length 15 digits)

MCC : Mobile Country Code (three digits e.g. 404 for INDIA)MNC : Mobile Network Code (two digits e.g. 98 for AIRTEL)MSIN : Mobile subscriber Identification Identity (Ten digits)

Why is IMSI used for network registration rather thanMSISDN alone??

--- The length of the country code is different for each no.

during international roaming. If the MSISDN number wereused for registration, then we would also need a length indicatorfor each field to prevent the various parts of number from gettingmixed up. Hence if the length of the field is fixed, no extra field isneeded. Also, MSISDN identifies the service used as speech, datafax etc. Therefore one subscriber may have several MSISDNsdepending on the services he uses, but only one IMSI.

5. Now the HLR interrogates the MSC/VLR that is currently servingthe called subscriber. This step is required because the current

status of the mobile station is stored in the VLR database & weneed to know the status to avoid setting up a call to asubscriber whose phone is switched off. Also, we need to havesome sort of information that enables the GMSC to route thecall to the target MSC, wherever it is.

6. After receiving the message from HLR, the serving MSC/VLRgenerates temporary Mobile Station Roaming Number(MSRN) & associates with the IMSI. The roaming number is

used in initiating the connection:

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MSRN = CC + NDC + SN

Though the MSRN & the MSISDN have same structure, theMSISDN is used to interrogate the HLR, whereas the MSRN is

the response given by the serving MSC/VLR & it is used for

routing the call. The SN field of MSRN is actually an internalnumber that is temporarily associated with the IMSI. The MSRNdoesnt merely identify the subscriber, it also points to exchangeitself so that all intermediate exchanges, if there are any, knowwhere the call is to be routed. Since the roaming no. istemporary, it is available for establishing other traffic connectionsafter the call has been set up. Hence the SN field in the MSISDNpoints to the database entry in the HLR & the SN field in MSRNpoints to the database entry in VLR.

7. The MSC/VLR sends the roaming number to HLR but it doesntanalyze it because MSRN is used for traffic transactions only &the HLR doesnt handle traffic. It is only a database that helpsin locating subscribers & coordinates call set up. Therefore HLRsimply sends the MSRN forward to the GMSC that originallyinitiated the process.

8. When the GMSC receives the message containing the MSRN, itanalyses the message. The roaming number identifies location

of the subscriber- this routing process identifies thedestination of the call- the serving MSC/VLR.

9. The final phase of routing process is taken care of by theserving MSC/VLR. In fact, the serving MSC/VLR also has toreceive the roaming number so that it knows that it is not anew call but one that is going to terminate here. By checkingthe VLR it recognizes the number & so it is able to trace thecalled subscriber

10. Once the location area LA is found out, the destinationMSC/VLR initiates a paging process to locate the subscriber.

The GMSC/VLR & MSC/VLR has now been connected via a traffic &signaling channel & the call set up has almost been completed. Thecaller is connected to the PSTN exchange, PSTN is connected to theGMSC & the GMSC is connected to the MSC/VLR that is serving thecalled subscriber but we have not yet established the connection tothe called subscriber. In order to set up the connection we have to

understand how the subscriber is located.

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As we do not know the exact location of the subscriber, it seemsinevitable that we have to search for him in the entire VLR servicearea. This could be a wide geographical area & so finding asubscriber requires a lot of work for MSC/VLR. Hence dividing the areain smaller sections is a better option. This is called a location area

managed by MSC/VLR. Each VLR contains several location areas. Wecan find the location area in which we search for the subscriber incase there is a call addressed to a mobile station.

VLR contains following data:

IMSI - 404 98 9898059784LAC - 404 98 8976DATA - abcMSRN - 123 45 678901

Each LA is defined by a location area identity

LAI = MCC + MNC + LAC

MCC Mobile Country CodeMNC - Mobile Network CodeLAC - Location Area Code

4.3.2 Mobile originated call

The mobile subscriber dials a number & sends a request to network inwhich he is currently registered as a visitor. After receiving therequest, the network analyses the data of the calling subscriber inorder to do three things:

Authorize the use of the network

Activate the requested service

Route the call

Call may have 2 types of destination: Mobile stations or A fixed linephone. If the call is addressed to a fixed line network, it is routed toPSTN which in turn routes to the destination. If the call is to a mobilein the same network, the MSC starts the HLR enquiry procedure asabove. The following diagram

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Fig 4.3 Mobile originated call procedure

1.Channel assignment

2.Security procedures

3 Call Set up

4 check services

5 all ok

6. call is proceeding

7 traffic channel allocated

8. set up the call

9.call set up complete

10.alert

11 voice transfer

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PSTN GMSC HLR MSC VLR BSS MS


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4.4 Transmission of radio & terrestrialchannels

In a mobile communication network, part of the transmissionconnection network uses a radio link& another part uses 2 Mbit/sPCM (E1 carrier system). Radio Transmission is used between themobile station & the base transceiver station & the information mustto be adapted to be carried over 2Mbit/s PCM transmission to theremainder of the network.

The frequency ranges of GSM 900, GSM 1900 & GSM 1800 areindicated below:

BAND UPLINK DOWNLINK

GSM 900:

GSM 1800

GSM 1900

890 915 MHz

1710 1785 MHz

1850 1910 MHz

935 960 MHz

1805 1880 MHz

1930 1990 MHz

Bharti Cellular Ltd. (Airtel) has been assigned the GSM 1800 Bandfor its operations in Gujarat.

The radio carrier frequencies are arranged in pairs . In GSM 900 it is45 MHz & in 1800 it is 95 MHz.

The number of channels can be found out by:

GSM 900: BW = 25 MHz, Channel width = 200 KHz

Channels = 25 M / 200 k = 125But 1 is used for channelingHence total channels =124

GSM 1800: BW = 75 MHz, Channel width = 200 KHzChannels = 75 M / 200 k = 375But 1 is used for channelingHence total channels =374

GSM 1900 : BW = 60 MHz, Channel width = 200 KHz

Channels = 60 M / 200 k = 300But 1 is used for channeling

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Hence total channels =299

Why is Downlink higher than Uplink frequency ??-. The higher the frequency, higher is the attenuation & hence

higher power is required. Since BTS has big electrical powersource while MS has a small battery, BTS is made to operate athigher frequency.

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CONCLUSION:-

We have come a way ahead in terms of telecommunication starting from

1G to 3G.Now communicating people through mobile has become an

easy task and a part of one life.Every new technology bring with its new

facilities & improvements to mobile world.In this roject I have studied

about the moble communication and the basic funadamentals of GSM

system which is the most widespread moble communication system.

GSM has taken more and more of the market.GSM allows users to roam

seamlessly between networks and seprate the user identity from phone

equipment

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References:-

BOOKS:-

B.P.Lathi(Modern Digital & Analog Comunicaton System)

Theodore S. Rappaport(Wireless Communication)

Megha Training Report mobile communication

Documents

Transcript of Megha Training Report mobile communication