pari-gsm

GLOBAL SYSTEM FOR MOBILE COMMUNICATION

Acknowledgement

Before getting into thick of things, we would like to add a

few heartfelt words for the people who were part of this seminar in numerous

ways…People who gave unending support from the stage the seminar idea

were conceived. We are overwhelmed with a sense of gratitude of them. We

take this opportunity to make a mention of the few people who made all the

difference.

Firstly, My sincere thanks to my guide respected

Prof. Ms. Suvarna Jadhav for sharing a great confidence in me, who has been

a constant source of inspiration and guiding star in achieving my goal. It is

owing to his boundless help, suggestions and guidance and providing all

necessary facilities and timely help. I am very much thankful to Prof. S. M.

Mukane for his constant support and providing me resources for my thesis

without which success of seminar would be difficult.

I also thankful to all our friends and those who inherently

supported and encouraged us during our framework

Miss. Parvin K. Naikwad i.

(B.E. Electronics &

Telecommunication)

SVERI’S COLLEGE OF ENGINEERING, PANDHARPUR 1


Abstract:

We are experiencing exponential growth rates in wireless system, increasing

awareness about the wireless in society, and deregulation of former wired systems.

While traditional communication systems deals with fixed networks, wireless systems

raises a new set of questions, techniques and solutions. The trend trends in wireless

system create an ever-increasing demand for well-educated communication engineers

who understands the developments and possibilities of wireless systems.

Through the medium of this paper we sincerely try to explain GSM (Global

System for Mobile Communication) wireless system, which is considered as heart of

mobile communication. We have chosen this subject because what we see today is

only the beginning of the mobile communication era. There are many researches

going on around the world in the field of mobile communication.

The first section of the paper focuses on the wireless system and types of

wireless systems existing today. The next section starts with the typical GSM system

with the history of the same and the services offered by GSM. In next section we have

explained the architecture of GSM following the protocols used by GSM system. The

fourth section gives the in depth explanation of calling technique by the same. The

succeeding section contains the different handover mechanisms used. The sixth

section of this paper gives the security concerns present in the GSM system. The final

section looks for the future developments.



INDEX1. Introduction to Wireless systems

2. Global system for mobile communication

3. History of GSM

4. Services provided by GSM

5. GSM cellular system

6. GSM Overview

7. GSM System Architecture

8. GSM Areas

9. GSM Specification

10. GSM call routine

11. Security

12. Comparison of mobile phone standards

13. Advantages of GSM

14. Disadvantages of GSM

15. Future

16. Conclusion

17. References


https://styx.uwaterloo.ca/~jscouria/GSM/gsmreport.html#3%233




Introduction to Wireless systems:

The ability to communicate with the people on the move has evolved

remarkably since Marconi first demonstrated Radio’s ability to

provide continuous contact with ships sailing the English Channel.

That was in 1897, and since then new wireless communication people

throughout the world have enthusiastically adapted methods and

services.

With the development in technology the demand for data rate of transmission

has been increasing rapidly. The wired systems though provide good data

transmission over a communication link its performance degrades due to

characteristics of the wire. Hence to overcome these inadequacies the trend of

wireless systems is taking the boom in the world of telecommunication.

AMPS AND ETACS

In the late 1970s,AT&T Bell Laboratories developed the first US cellular

Telephone System called Advanced Mobile Phone Service (AMPS). The AMPS

system uses a seven-cell reuse pattern with provisions for sectoring and cell splitting

to increase capacity when needed.

The European Total Access (ETACS) was developed in middle 1980’s and is virtually

identical to AMPS, except it is scaled to fit in 25Khz channels. Another difference is

how the telephone number of each subscriber is formatted.

DECT

The Digital European Cordless Telephone (DECT) is a universal cordless telephone

standard. DECT provides a cordless communications framework for high traffic

density, short-range telecommunications, and covers a broad range of applications and

environments.

PACS

The Personal Access Communication System (PACS) is a third generation Personal

Communication System originally developed and proposed by Bellcore in 1992.



PACs is able to support voice, data and video images for indoor and microcell use.

Main objective is to integrate all forms of wireless local loop communications into

one system with full telephone features.

WLANS

The Wireless Local Area Networks (WLAN) is of special interest for wireless,

mobile computer communication on a campus or in buildings. The global goal of

WLANSs is to replace office cabling and, additionally to introduce a higher flexibility

for ad-hoc communication. The IEEE standard 802.11 specifies the most famous

family of WLANs in which many products are already available. It offers time-

bounded and asynchronous services.

GSM (GLOBAL SYSTEM FOR MOBILE)

The Global System for Mobile Communications

(GSM) is the most popular standard for mobile phones in the

world. Over 1.5 billion people use GSM service across more

than 210 countries and territories. The ubiquity of the GSM

standard makes international roaming very common between

mobile phone operators, enabling subscribers to use their phones

in many parts of the world. GSM differs significantly from its predecessors in that

both signaling and speech channels are digital, which means that it is considered a

second generation (2G) mobile phone system. This fact has also meant that data

communication was built into the system from very early on. GSM is an open

standard, which is currently developed by the 3GPP.From the point of view of the

consumer, the key advantage of GSM systems has been higher digital voice quality

and low cost alternatives to making calls such as text messaging. The advantage for

network operators has been the ability to deploy equipment from different vendors

because the open standard allows easy inter-operability. Also, the standards have

allowed network operators to offer roaming services, which mean subscribers can use

their phone all over the world.



History

Once upon a time there was analog cellular communication that didn’t support

encryption, compression, and ISDN compatibility; in addition each country

(company) developed its own system, which was incompatible with everyone else’s in

equipment and operation. So, in early 80s Europeans realized that pan-European

public mobile system should be developed. The new system had to meet certain

criteria:

Good subjective speech quality

Low terminal and service cost

International roaming

ISDN compatibility

In 1989, GSM responsibility was transferred to the European Telecommunication

Standards Institute (ETSI), and phase I of the GSM specifications was published in

1990. Commercial service was started in mid-1991, and by 1993 there were 36 GSM

networks in 22 countries

Services Provided By GSM:

A variety of data services are offered. GSM users can send and receive data, at

rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet

Switched Public Data Networks, and Circuit Switched Public Data Networks using a

variety of access methods and protocols, such as X.25 or X.32. Since GSM is a digital

network, a modem is not required between the user and GSM network, although an

audio modem is required inside the GSM network to interwork with POTS.

Other data services include Group 3 facsimile, which is supported by use of an

appropriate fax adaptor. A unique feature of GSM, not found in older analog systems,

is the Short Message Service (SMS). SMS is a bi-directional service for short

alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-

forward fashion.



GSM Cellular System:

Cellular systems for mobile

communication implement space

division multiplexing. Each

transmitter is called as base-station,

covers a certain area, a cell. Cell radii

can vary from tens of meters in buildings and hundreds of meters in cities. The shapes

of the cells are never perfect circles or hexagons as shown in fig, but depend on

environment. Mobile communication system uses this form in which the cell around a

base station communicates with the base-station and vice versa.

Following are some advantages offered by cellular systems with small cells:

1.Higher Capacity: By implementing the space division multiplexing the frequency

reuse technique can be used. Foe ex. If one transmitter is away from another i.e.

outside the interference range, it can reuse the same frequencies.

2.Less transmission power: While power aspects are not a big problem for base-

station, they are indeed problematic for mobile stations. A receiver far away from a

base station would need much more transmit power the current few Watts.

3.Local interference only: Having long distances between sender and receiver results

in even more interference problems. With small cells, mobile stations and base

stations only have to deal with ‘local interference’.

4.Robustness: Cellular systems are decentralized and, thus, more robust against

failures of single components. If one antenna fails, this defect only influences

communication within a small area.

There are some disadvantages also:

1.Infrastructure needed: Cellular systems needs more complex infrastructure to

connect all base-stations. This infrastructure includes many antennas, switches for call

forwarding, and location registers to find a mobile station.



2.Handover needed: The mobile station has to perform a handover when changing

from one cell to another. Depending upon the cell size and the speed of movement,

this can be happen quite often.

3.Frequency planning: To avoid the interference problem between the two

transmitters using same frequencies, frequencies have to be distributed carefully. On

the other hand interference should be avoided, as only a specific number of

frequencies are available

GSM :

Throughout the evolution of cellular telecommunications various systems have been

developed without the benefit of standardized specifications. This presented many

problems directly related to compatibility, especially with the development of digital

radio technology. The GSM standard is intended to address these problems.



GSM Overview

Before GSM networks there were public mobile radio networks (cellular). They

normally used analog technologies, which varied from country to country and from

manufacturer to another. These networks did not comply with any uniform standard.

There was no way to use a single mobile phone form one country to another. The

speech quality in most networks was not satisfactory.

GSM became popular very quickly because it provided improved speech quality and,

through a uniform international standard, made it possible to use a single telephone

number and mobile unit around the world. The European Telecommunications

Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now

used in 135 countries.

The benefits of GSM include :

Support for international roaming

Distinction between user and device identification

Excellent speech quality

Wide range of services

Interlocking (e.g. with ISDN, DECT)



Extensive security features

GSM also stands out form other technologies with its wide range of services :

Telephony

Asynchronous and synchronous data services ( 2.1/4.8/9.6 dkit/s)

Access to packet data network (S.25)

Telematic services (SMS, fax, videotext, etc.)

Many value-added features (call forwarding, caller ID, voice mailbox)

E-mail and Internet connections.

GSM System Architecture

The best way to create a manageable communications system is to divide it into

various subgroups that are interconnected using standardized interfaces. A GSM

network can be divided into three groups The mobile station (MS), the base station

subsystem (BSS) and the network subsystem.

They are characterized as follows :

The Mobile Station (MS) :

A mobile station may be referred to as a “handset ”, a ‘mobile”, a “portable

terminal” or “mobile equipment” (ME). It also includes a subscriber identity module



(SIM) that is normally removable and comes in two sizes. Each SIM card has unique

identification number called IMSI (international mobile subscriber identity) In

addition, each MS is assigned a unique hardware identification called IMEI

(International mobile equipment identity)

In some of the newer application (data communication in particular) an MS

can also be a terminal that acts as a GSM interface, e.g. for a laptop computer. In this

new application the MS does not look like normal GSM telephone.

The seemingly low price of a mobile phone can five the (false) impression

that the product is not of high quality. Besides providing a transceiver (TRS) for

transmission and reception of voice and data, the mobile also performs a number of

very demanding tasks such as authentication, handover, encoding and channel

encoding.

The base station subsystem (BSS) :

The base station subsystem (BSS) is made up of the base station controller

(BSC) and the base transceiver station (BTS.)

The base transceiver station (BTS) :

GSM uses a series of radio transmitters called BTSs to connect the mobiles to cellular

network. Their tasks include channel coding/decoding and encryption/decryption. A

BTS is comprised of radio transmitters and receivers, antennas, the interface to the

PCM facility, etc. The BTS may contain one or more transceivers to provide the

require call handling capacity. A cell site may be omni directional or split into

typically three directional cells.

The base station controller (BSC) :

A group of BTSs are connected to a particular BSC which manages the radio

resources for them. Today’s new and intelligent BTSs have taken over many tasks

that were previously handled by the BSCs.

The primary function of the BSC is call maintenance. The mobile stations normally

send a report of their received signal strength to the BSC every 480 ms. With this

information the BSC decides to initiate handovers to other cells, change the BTS

transmitter power, etc.

The mobile switching center (MSC) :

Acts like a standard exchange in a fixed network and additionally provides all the

functionality needed to handle a mobile subscriber. The signaling between functional

entities (registers) in the network subsystem uses signaling system 7 (SS7). If the



MSC also has a gateway function for communicating with other networks, it is called

Gateway (GMSC).

The home location register (HLR) :

A database used for management of mobile subscribers. It stores the international

mobile subscriber identity (IMSI), mobiles station ISDN number (MSISDN) and

current visitor location register (VLR) address. The main information stored there

concerns the location of each mobile station in order to be able to route calls to the

mobile subscribers managed by each HLR. The HLR also maintains the services

associated with each MS. One HLR can serve several MSCs.

The visitor location register (VLR) :

Contains the current location of the MS and selected administrative information form

the HLR, necessary for call control and provision of the subscribed services, for each

mobile currently located in the geographical area controlled by the VLR. AVLR is

connected to one MSC and is normally integrated into the MSC’s hardware.

The authentication center (AuC) :

A protected database that holds a copy of the secret key stored in each subscriber’s

SIM card, which is used fro authentication and encryption over the radio channel. The

AuC provides additional security against fraud. It is normally located close to each

HLR within a GSM network.

The equipment identity register (EIR) :

The EIR is a database that contains a list of all valid mobile station equipment within

the network, where each mobile station is identified by its international mobile

equipment identity (IMEI). The EIR has three databases :

White list : for all known, good IMEIs

Black list : for bad or stolen handsets

Grey list : for handsets/IMEIs that are uncertain

Operation and Maintenance Center (OMC)

The OMC is a management system that oversees the GSM functional blocks.

The OMC assists the network operator in maintaining satisfactory operation of the

GSM network. Hardware redundancy and intelligent error detection mechanisms help

prevent network down-time. The OMC is responsible for controlling and maintaining

the MSC. BSC and BTS. It can be in charge of an entire public land mobile network

(PLMN) or just some parts of the PLMN.



The GSM Areas

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

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

network (PLMN) areas.

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

network identifies each cell via the cell global identity (CGI) number assigned to each

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

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

MSC (see Picture 4). Each LA is assigned a location area identity (LAI) number.

Picture 4. Location Areas



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

one MSC and which is reachable, as it is registered in the VLR of the MSC (see

Picture 5).

Picture 5. MSC/VLR Service Areas

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

Picture 6. PLMN Network Areas

GSM Specifications :

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

following basic terms :

Bandwidth : the range of channel’s limits; the broader the bandwidth, the faster data

canbe 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 units of time; frequency is measured in hertz

(Hz)

Kilo (k) : kilo is the designation for 10002; the abbreviation kbps represents 1000 bits

per second.

Megahertz (MHz): 1000000 hertz (cycle per second)

Milliseconds (ms) : one thousand of a second

Watt (W) : a measure of power of a transmitter.

Specific for different personal communication services (PCS) systems vary

among the different PCS networks. Listed below is a description of the specification

and characteristics for GSM.

Frequency band :

The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station

to base station)

Duplex distance :

The duplex distance is 80 Mhz. Duplex distance is the distance between the

uplink and downwind frequencies. A channel has two frequencies, 80 MHz apart.

Channel separation :

The separation between adjacent carrier frequencies. In GSM, this is 200 KHz.

Modulation :

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

of a carrier frequency. This is done in GXM via Gaussian minimum shift keying

(GMSK)

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

Access method :

GSM utilizes the time division multiple access (TDMA) concept. TDMA is a

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

assigned a particular time slot.

Speech coder :

GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the

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

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

13 kbps.



GSM Subscriber Services

There are two basic types of services offered through GSM :

telephony also referred to as teleservices) and data (also referred to as bearer

services)

Telephony services are mainly voice services that provide subscribers with the

complete capability (including necessary terminal equipment) to communicate with

other subscribers. Data services provide the capacity necessary to transmit appropriate

data signals between two access points creating an interface to the network. In

addition to normal telephony and emergency calling, the following subscriber services

are supported by GSM:

Dual-tone multifriquency (DTMF) : DTMF is a tone signaling scheme often

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

an answering machine. GSM supports full originating DTMF.

Facsimile group II :

GSM supports CCITT Group 3 facsimile. As standard fax machines are

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

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

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

Short message services :

A convenient facility of the GSM network is the short message service. A

message consisting of a maximum of 160 alphanumeric characters can be sent to or

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

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

offered back to the subscriber when the mobile is powered on or has tendered the

coverage area of the network. This function ensures that the message will be received.

Cell broadcast :

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

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

a certain geographic area. Typical applications include traffic congestion warnings

and reports on accidents.



Voice mail :

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

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

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

Fax mail :

With this service, the subscriber can receive fax messages at any fax machine.

The messages are stored in a service center form which they can be retrieved by the

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

Supplementary Services

GSM supports a comprehensive set of supplementary services that can

complement and support both telephony and data services. Supplementary services

are defined by GSM and are characterized as revenue-generating features. A partial

listing of

Supplementary services follows :

Call forwarding : This service gives the subscriber the ability to forward incoming

calls to another number if the called mobile unit is not reachable, if it is busy, if there

is no reply, or if call forwarding is allowed unconditionally.

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

prevent all outgoing.

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

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

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

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

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

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

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

measurements,

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

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

telephony.



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

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

incoming call. Call waiting is applicable to all GSM telecommunication services

using a circuit switched connection.

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

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

subscribers. This service is only applicable to normal telephony.

Calling line identification presentation/restriction : These services supply the

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

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

restriction overrides the presentation.

Closed user groups (CUGs) : CUGs are generally comparable to a PBS. They are a

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

Providing voice or data transmission quality over the radio link is only part of the

function of cellular mobile network. A GSM mobile can seamlessly roam nationally

and internationally, requiring standardized call routing and location updating

functions in GSM networks. A public communications system also needs solid

security mechanisms to prevent misuse by third parties. Security functions such as

authentication, encryption and the use of Temporary Mobile Subscriber Identities

(TMSIs) are an absolute must.



Within a GSM network, different protocols are needed to enable the flow of

data and signaling between different GSM subsystems. Figure shows the intervals that

link the different GSM subsystems and the protocols used to communicate on each

interface.

Protocols:

There are three layers present in the GSM protocol architecture.

Physical Layer:-

which handles all radio specific functions, which include the

multiplexing techniques, data transmission in the form of burst and the

synchronization of frames.

data link layer :-

which handles signaling between entities in the GSM network. For this

purpose LAPDm (Link Access procedure for D-channel) has been defined in the

ISDN standard. It offers reliable data transfer over connections, re- sequencing of data

frames, and flow control

network layer, which comprises of several sublayers:

Radio Resources Management

Controls the setup, maintenance, and termination of radio and fixed channels,

including handovers.



Mobility Management

Manages the location updating and registration procedures, as well as security and

authentication.

Connection Management

Provides call control, short messaging service and supplementary service

Signaling between the different entities in the fixed part of the network, such as

between the HLR and VLR, is accomplished through the Mobile Application Part

(MAP). MAP is built on top of the Transaction Capabilities Application Part (TCAP,

the top layer of Signaling System Number.

System Features

The section provides a brief description of the GSM network features :

Roaming : The roaming feature allows a user to make and receive calls in any GSM

network and to use the same user specific services worldwide. This requires a

roaming agreement between the individual operators. With worldwide roaming the

MS is accessible under the same phone number everywhere.

Handover : In a cellular network, the radio and fixed voice connections are not

permanently allocated for the duration of a call. Hanover, or handoff as it is called in

North America, means switching and ongoing call to a different channel or cell. The

execution and measurements require for handover are a basic function of the RR

protocol layer.

Multipath equalization : At the 900 MHz range, radio waves bounce off everything-

buildings, hills, cars, airoplanes, etc. Many reflected signals, each with a different

phase, can reach an antenna (also known as “multipath propagation”) Equalization is

used to extract the desired signal from the unwanted reflections. It works by finding

out how a known transmitted signal is modified by multipath fading, and constructing

an inverse filter to extract the rest of the desired signal. This known signal is the 26-

bit training sequence transmitted in the middle of every time-slot burst. The actual

implementation of the equalizer is not specified in the GS< specifications.



Discontinuous Transmission (DTX) : To reduce the MS’s power consumption and

minimize interference on the air interface, user signal transmission is interrupted

during pauses in speech. “Comfort noise” is artificially generated by the MS to avoid

disruption due to an abrupt interruption in speech.

Discontinuous Reception (DRS) : Another method used to conserve power at the

mobile station is discontinuous reception. The paging channel, used by the base

station to signal an incoming call, is structured into sub-channels. Each mobile station

needs to listen only to its own sub-channel. In the time between successive paging

sub-channels, the mobile can go into sleep mode, when almost no power is used.

Short Message Service(SMS) : SMS offers message delivery (similar to “two-way-

paging”) that is guaranteed to reach the MS. If the GSM telephone is not turned on,

the message is held for later delivery. Each time a message is delivered to an MS, the

network expects to receive an acknowledgement from this MS that the message was

correctly received. Without a positive acknowledgement the network will re-send the

message or store it for later delivery. SMS supports messages up to 160 characters in

length that can be delivered by any GSM network around the world wherever the MS

is able to roam.

Call Waiting(CW) : CW is a network-based feature that must also be supported by

the GSM telephone(MS). With CW, GSM users with a call in progress will receive an

audible beep to alert them that there is an incoming call for the MS. The incoming call

can be accepted, sent to voice mail or rejected. If the incoming call is rejected, the

dealer will receive a busy signal. Once the call is accepted, the original call is put on

hold to allow a connection to the new incoming call.

Call Hold (CH) : CH must be supported by the MS and the network. It allows the

MS to “park” and “in progress call”, to make additional calls or to receive incoming

calls.

Call Forwarding(CF) : This is a network-based feature that can be activated by the

MS. CF allows calls to be sent to other numbers under conditions defined by the user.

Theses conditions can be either unconditional or dependent on certain criteria. (no

answer, busy, not reachable)

Calling Line ID : Calling Line ID must be supported by the GSM network and the

telephone. The GSM telephone displays the originating telephone number of

incoming calls. This feature requires the caller’s network to deliver the calling line ID

(telephone no.) to the GSM network.



GSM Call Routing

Mobile Subscriber Roaming

When a mobile subscriber roams into a new location area (new VLR), the VLR

automatically determines that it must update the HLR with the new location

information, which it does using an SS7 Location Update Request Message. The

Location Update Message is routed to the HLR through the SS7 network, based on

the global title translation of the IMSI that is stored within the SCCP Called Party

Address portion of the message. The HLR responds with a message that informs the

VLR whether the subscriber should be provided service in the new location.

Mobile Subscriber ISDN Number (MSISDN) Call Routing

When a user dials a GSM mobile subscriber's MSISDN, the PSTN routes the call to

the Home MSC based on the dialed telephone number. The MSC must then query the

HLR based on the MSISDN, to attain routing information required to route the call to

the subscribers' current location.

The MSC stores global title translation tables that are used to determine the HLR

associated with the MSISDN. When only one HLR exists, the translation tables are

trivial. When more than one HLR is used however, the translations become extremely

challenging, with one translation record per subscriber (see the example below).

Having determined the appropriate HLR address, the MSC sends a Routing

Information Request to it.

When the HLR receives the Routing Information Request, it maps the MSISDN to the

IMSI, and ascertains the subscribers' profile including the current VLR at which the

subscriber is registered. The HLR then queries the VLR for a Mobile Station Roaming

Number (MSRN). The MSRN is essentially an ISDN telephone number at which the

mobile subscriber can currently be reached. The MSRN is a temporary number that is

valid only for the duration of a single call.

The HLR generates a response message, which includes the MSRN, and sends it back

across the SS7 network to the MSC. Finally, the MSC attempts to complete the call

using the MSRN provided.

Adding a Second HLR to the GSM Network

As a GSM wireless carrier's subscriber base grows, it will eventually become

necessary to add a second HLR to their network. A service subscription record storage



capacity issue might prompt this requirement, or perhaps an SS7 message processing

performance issue. It might possibly be prompted by a need to increase the overall

network reliability.

The new HLR can be populated with service subscription records as new subscribers

are brought into service or existing service subscription records can be ported from

the old HLR to the new HLR to more evenly distribute the SS7 traffic load.

Typically, when new subscribers are brought into service, the second HLR will be

populated with blocks of IMSI numbers that are allocated when new MSE equipment

is ordered. As the following example shows, this grouping of IMSI numbers within a

single HLR simplifies the routing translations that are required within the SS7

network for VLR to HLR Location Update Request transactions. Global Title

Translation (GTT) tables will contain single translation records that translate an entire

range of IMSIs numbers into an HLR address. Even if some individual records are

moved between the HLRs, as shown in the example, the treatment of IMSIs as blocks

results in a significant simplification of the Global Translation tables.

Much more complicated SS7 message routing Global Title Translations are required

for Routing Information Request transactions between the MSCs distributed over the

entire wireless carrier serving area and the two or more HLRs. MSC Routing

Information Requests are routed to the appropriate HLR based on the dialed MSISDN

and not the IMSI. Unlike the IMSI numbers, the MSISDN numbers can not easily be

arranged in groups to reside within a single HLR and therefore, the MSC must contain

an MSISDN to HLR address association record for every mobile subscriber homed on

each of the MSCs. As the example illustrates, the MSC routing tables quickly grow

much more extensive than the STP tables. The network administration becomes

increasingly complex and prone to error.

Example: Simple Network with two

MSCs and two HLRs

The following example illustrates the

issues relating to GSM network

routing table administration with

multiple HLRs. A simple GSM

network is shown, with the various

routing tables following:



Network aspects

Ensuring the transmission of voice or data of a given quality over the radio link is

only part of the function of a cellular mobile network. A GSM mobile can seamlessly

roam nationally and internationally, which requires that registration, authentication,

call routing and location updating functions exist and are standardized in GSM

networks. In addition, the fact that the geographical area covered by the network is

divided into cells necessitates the implementation of a handover mechanism. These

functions are performed by the Network Subsystem, mainly using the Mobile

Application Part (MAP) built on top of the Signalling System No. 7 protocol.

Figure 3. Signalling protocol structure in GSM

The signalling protocol in GSM is structured into three general layers, depending on

the interface, as shown in Figure 3. Layer 1 is the physical layer, which uses the

channel structures discussed above over the air interface. Layer 2 is the data link

layer. Across the Um interface, the data link layer is a modified version of the LAPD

protocol used in ISDN, called LAPDm. Across the A interface, the Message Transfer

Part layer 2 of Signalling System Number 7 is used. Layer 3 of the GSM signalling

protocol is itself divided into 3 sublayers.

Radio Resources Management :- Controls the setup, maintenance, and

termination of radio and fixed channels, including handovers.

Mobility Management :-Manages the location updating and registration

procedures, as well as security and authentication.

Connection Management :- Handles general call control, similar to CCITT

Recommendation Q.931, and manages Supplementary Services and the Short

Message Service.



Signalling between the different entities in the fixed part of the network, such as

between the HLR and VLR, is accomplished throught the Mobile Application Part

(MAP). MAP is built on top of the Transaction Capabilities Application Part (TCAP,

the top layer of Signalling System Number 7. The specification of the MAP is quite

complex, and at over 500 pages, it is one of the longest documents in the GSM

recommendations

Security:

GSM was designed with a moderate level of security. The system was designed to

authenticate the subscriber using shared-secret cryptography. Communications

between the subscriber and the base station can be encrypted. The development of

UMTS introduces an optional USIM, that uses a longer authentication key to give

greater security, as well as mutually authenticating the network and the user - whereas

GSM only authenticated the user to the network (and not vice versa).

Another level of security is performed on the mobile equipment itself, as opposed to

the mobile subscriber. A unique International Mobile Equipment Identity (IMEI)

number identifies each GSM terminal. A list of IMEIs in the network is stored in the

Equipment Identity Register (EIR). The status returned in response to an IMEI query

to the EIR is one of the following:

White-listed :-The terminal is allowed to connect to the network.

Grey-listed :-The terminal is under observation from the network for possible

problems.

Black-listed :-The terminal has either been reported stolen, or is not type approved

(the correct type of terminal for a GSM network). The terminal is not allowed to

connect to the network.



Comparison of mobile phone standards GSM and CDMA are the two most prevalent mobile

communication technologies. Both technologies have the same goal: to divide the

finite RF spectrum between multiple users.

GSM (Time Division Multiple Access - underlying technology used in GSM) does it

by chopping up the channel into sequential time slices. Each user of the channel takes

turns to transmit and receive signals. In reality, only one person is actually using the

channel at a specific moment. This is analogous to time-sharing on a large computer

server.

CDMA (Code Division Multiple Access) on the other hand, uses

special digital modulation Spread Spectrum which spreads the voice over very wide

channel in pseudo random fashion. The receiver undoes the randomization to collect

the bits together and produce the sound.

Imagine that you go to a UN party where couples from different countries are invited.

In GSM, each couple takes turns talking. They talk for a short time and then stop to

let another couple talk. As there is never more than one person speaking in the room,

no one has to worry about being heard over the background noise. In CDMA, each

couple talks at the same time; however they all use different languages. Because (we

assume) that none of the listeners understand any other language than that of their

partner, the background noise does not create any problem.

Advantages of GSM:- GSM is mature, this maturity means a more stable network with robust features. Less signal deterioration inside buildings.

Ability to use repeaters

Talktime is generally higher in GSM phones due to pulse nature of transmission.

The availability of Subscriber Identity Modules allows to users to switch networks

and handsets at will.

GSM covers virtually all parts of world so international roaming is not a problem.

GSM is an open standard, therefore operators do not need to pay royalties to utilize it.

Disadvantages of CDMA

Breathing of base stations, where coverage area shrinks under load.

Most technologies are patented and must be licensed from Qualcomm.


http://en.wikipedia.org/wiki/Qualcomm

http://en.wikipedia.org/wiki/Open_standard

http://en.wikipedia.org/wiki/Roaming

http://en.wikipedia.org/wiki/Subscriber_Identity_Module

http://en.wikipedia.org/wiki/Digital_modulation

http://en.wikipedia.org/wiki/CDMA

http://en.wikipedia.org/wiki/Time-sharing

http://en.wikipedia.org/wiki/GSM

http://en.wikipedia.org/wiki/RF_spectrum


The Future:

Enhanced Data Rate for GSM Evolution

(EDGE):

EDGE (Enhanced Data Rates for Global

Evolution) is a radio signaling technology for 3G

mobile networks. It boosts data transfer rates and volumes on existing

GSM/GPRS networks by significantly increasing data transfer speeds.

EDGE benefits

1. Faster connection

2. Greater data volumes achieved

3. Significant increase in functionality

4. Can interoperate with GSM networks for global

coverage

GPRS (General Packet Radio Service):

GPRS (General Packet Radio Service) brings the possibility of data services and

applications based on Internet Protocol (IP) to GSM mobile networks. In current

network implementations, GPRS data transfer speeds are between 30 to 40 Kbit/s.

GPRS enables fast connections through which information can be sent and

received as the need arises, without having to wait for a dial-up modem to establish a

connection. This is why GPRS devices are sometimes referred to be as being an

'always online' technology. And because pricing is often based on the amount of data

transferred rather than the connection time, this has made GPRS applications such as

email popular with small and large businesses around the world.



Third Generation (3-G) Wireless Systems:

3G systems promise unparalleled wireless access in ways that have never been

possible before. Multi-megabit internet access, communication using Voice Over

Internet Protocol (VoIP), voice-activated calls, unparalleled network capacity and

ubiquitous “always on” access are just some of the advantages touted by 3G

developers. Companies developing 3G equipments envision users having the ability to

receive music, conduct interactive web sessions, and have simultaneous voice and

data access with multiple parties at the same time using a single mobile handset.

Whether driving, walking, or standing still in an office building.



CONCLUSION

In the modern world where faster common is needed the wireless

communication systems are best to use. For mobile communication various systems

are used in which GSM is most reliable and accepted digital system.



References:

1.Wireless Communications- Theodore

Rappaport

2. Mobile Communications- Jochen Schiller

3.Wireless Communications- William Stallings

www.wikipedia.com

www.nokia.com

www.howstuffworks.com

www.shoshin.uwaterloo.ca

www.pt.com

www.seekdonet.net

www.pulsewan.com


http://www.pulsewan.com/

http://www.seekdonet.net/

http://www.pt.com/

http://www.shoshin.uwaterloo.ca/

http://www.howstuffworks.com/

http://www.nokia.com/

http://www.wikipedia.com/

pari-gsm

Documents