GSM Study Document

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INTRODUCTION TO GSM

GSM History

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

applications ever. Today it represents a large and continuously increasing percentage of all new

telephone subscriptions around the world. In many cases, cellular solutions successfully compete

with traditional wire networks and cordless telephones. In the long term perspective cellular systems

using a digital technology, will become the universal method of telecommunication.

In 1982 the Nordic PTTs sent a proposal to CEPT (Conference Europenne de Postal e

Tlcommunications) to specify a common European telecommunication service at 900 MHz. A

standardization group for GSM Global System for Mobile communication was established in order

to formulate the specifications for this panEuropean mobile cellular radio system.

During 19821985 there were discussions of whether to build an analogue or a digital system but in

1985 it was decided to specify a digital system. The next step was to choose between a narrow band

and a broadband solution. In 1986 there was a field test in Paris where different companies, with

different solutions, competed. In May 1987 the narrow band TDMA (Time Division Multiple Access)

solution was chosen. At the same time, the first 13 countries (in United Kingdom two operators)

signed the MoU (Memorandum of Understanding) committing themselves to fulfill the specifications

and promised to have a GSM system running by July 1, 1991, thus opening a very large potentia

market.

The specifications have later been extended to include an air interface also for the 1800 MHz

frequency range, DCS 1800 Digital Communication System.

The use of digital radio transmission and the advanced handover algorithms between radio cells in

GSM networks allow for significantly better frequency usage than in analogue cellular systems, thus

increasing the number of subscribers that can be served. Since GSM provides a common standard,

cellular subscribers will also be able to use their telephones over the entire GSM service area.

Roaming is fully automatic between all countries covered by GSM systems, on condition that the

operators have roaming agreements among themselves.

In addition to international roaming, GSM provides new user services, such as high speed data

communication, facsimile and short message service. The GSM technical specifications are designed

to work together with other standards, e.g. ISDN (Integrated Services Digital Network). Interworking

between the standards is in this way assured.

GSM system specification

The system specifications for the GSM network are:


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Frequency band (Uplink: 890 MHz 915 MHz; Downlink: 935 MHz 960 MHz)

Extended GSM (Including 880 - 890 MHz on uplink and 925 - 935 MHz on downlink)

Duplex distance 45 MHz

Carrier separation 200 kHz, (the first carrier at 890.2 MHz)

Modulation GMSK

Air transmission rate 270 kbit/s

Access method TDMA

Speech coder : RPE-LTP-LPC (Regular Pulse Excitation-Long Term Prediction-Linea

Predictive Coder)

Diversity

Channel coding

Interleaving

Frequency hopping

Adaptive equalization

For DCS networks the system specification is the same, except for the frequency band and duplex

distance. The uplink is specified between 17101785 MHz and the downlink 18051880 MHz. Theduplex distance is 95 MHz. After it was decided that a digital system should be designed, the GSM

group had to deal with the access method and the bandwidth.

Access method

The digital GSM system uses TDMA Time Division Multiple Access where each carrier is divided

into eight time slots. The mobile station sends and receives in the same time slot. This means that

eight simultaneous conversations can take place on the same carrier.

Narrow band versus broadband

The GSM specification decided on narrow band TDMA. Why was the broadband alternative rejected?

In summary, some of the disadvantages with broadband compared to narrow band are described

below:

Inflexible frequency planning


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Each cell will be equipped with a large number of carriers and each site can use the same set of radio

channels, providing simple planning for the individual operator. This will lower the initial costs per site,

but sets a limit for expansion of the networks.

Overdimensioned base stations in rural areas

Due to the broadband width, we have even more channels per carrier, generating a high cost per

traffic channel in less dense areas where the need for channels might not be as high.

High peak power in the transmitters

More complex, fast signal processing due to higher bit rate

This leads to a lower degree of transmission reliability and a higher power consumption.

Modulation

The chosen modulation method Gaussian Minimum Shift Keying is state of the art concerningmodulation of digital signals. Further information on modulation methods is referred to other literature.

The work of GSM

In the GSM recommendations the hardware is not specified, instead the functions and interfaces are

specified in detail. The reason for this is to limit the designers as little as possible, and to make it

possible for the operators to buy equipment from different suppliers. The GSM recommendations

consists of 12 series, see table below.

These series have been written by different working parties and a number of expert groups. A

permanent nucleus was also established in order to support the working groups activities, to

constitute a point of reference for industries and to perform special tasks directly. All these groups are

organized by ETSI (European Telecommunication Standards Institute).

The DCS 1800 part of the GSM specification is written as a delta part within the GSM

recommendations, which means that it only describes the differences between GSM 900 and DCS

1800.


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GSM and its phases

Originally GSM was seen as a panEuropean only system, i.e. a single version with a fixed path

towards the future. But in the late 1980s the GSM community realized it would not be possible to

finalize the originally planned range of services and features of GSM in its entirety, in time. That is

how phase 1, with a limited set of services and features, was born.

Phase 2 and 2+ were introduced in order to finalize the remaining features (see Figure 1).

With phase 1 the GSM specifications were closed for further modifications or enhancements. The

phase 1 standard is not only the platform for further GSM development, it also contains a variety of

services and features, although advanced analogue cellular systems like NMT and TACS, still, in

many respects, offer more.

But GSM offers features such as international roaming, SIM cards, ciphering, short message service

and data services up to 9.6 kbit/s. Even more important, GSM has a high potential in terms of

features and services.

GSM also implies capacity, ISDN conformity, openness for network specific services, termina

innovations, and built-in mechanisms for future enhancements.

The ETSI and GSM group have designed an alternative to the very strong, first created, ciphering

algorithm A5, which is generally forbidden to export outside NATO. The phase 2 standard caters for

multiple ciphering algorithms.

When creating phase 2, a proper interworking between phase1 and phase 2 equipment has to be

ensured. This is called cross phase compatibility. The phase 2 standard encompasses a far larger set

of services and features than phase 1, such as extended frequency band for GSM 900 and DCS

1800 specifications, and an optimization of the phase 1 standard in general.
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Regarding services, close cooperation between ETSI and the operators association has made sure

that the standard covers what is expectable and feasible.

Most new services are to be found in the Supplementary Services category. Example of these

services are (they will be more thoroughly explained later on in the book):

Call Forwarding

Call Barring

Calling and called line identification display of calling and called number and prevention against

this.

Call waiting and Call hold indication of an incoming call during conversation and the ability to

switch to this without dropping the first one.

Multiparty call with up to six parties.

Phase 1 SIMs contain all the necessary network control information, while phase 2 SIMs will imply a

large number of extra features, (most of them described on pages 81 - 86) such as:

Language identifier and Preferred language for network messages in the display

Advice of charge, storage, control and security mechanisms

Fixed number dialling

Second PINcode

Storage of supplementary service control information

Enhanced control of Short Message Service parameters.

The standardization groups have already started the process towards a life after phase 2: the phase

2+ program. It covers among others:

DECT (standard for digital cordless phones) access to GSM infrastructure

Multiple subscriber number

Business oriented features

General improvements to public networks

Priorities and time scales for new features and functions depend primarily on the interest shown by

GSM operating companies and manufacturers, and also on developments in related areas.


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Accessing a GSM network

In order to gain access to GSM services, a user needs three things:

A billing relationship with amobile phone operator. This is usually either where services are paid for in

advance of them being consumed (prepaid), or where bills are issued and settled after the service has beenconsumed (postpaid).

Amobile phone which is GSM compliant and operates at thesame frequency as the operator. Most phone

companies sell phones from third-party manufacturers.

ASIM ("Subscriber Identity Module") cardwhich is activated by the operator once the billing relationship

is established. After activation the card is then programmed with the subscriber'sMSISDN ("Mobile

Subscriber Integrated Services Digital Network Number") (the telephone number). Personal informationsuch as contact numbers of friends and family can also be stored on the SIM by the subscriber.

After subscribers sign up, information about their identity (telephone number) and what services they are allowedto access are stored in a "SIM record" in the Home Location Register(HLR).

Once the SIM card is loaded into the phone and the phone is powered on, it will search for the nearest mobile

phone mast (also called a Base Transceiver Station (BTS)) with the strongest signal in the operator'sfrequencyband. If a mast can be successfully contacted, then there is said to be coverage in the area. The phone then

identifies itself to the network through the control channel. Once this is successfully completed, the phone is said

to be attached to the network.

The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available.This is generally called roaming from a customer perspective, but also called visiting when describing the

underlying technical process. Each geographic area has a database called the Visitor Location Register(VLR)which contains details of all the mobiles currently in that area. Whenever a phone attaches, or visits, a new area,the VisitorLocation Register must contact theHome Location Register to obtain the details for that phone. The

current cellular location of the phone (i.e. which BTS it is at) is entered into the VLR record and will be used

during a process calledpaging when the GSM network wishes to locate the mobile phone.

Every SIM card contains a secret key, called the Ki, which is used to provide authentication and encryptionservices. This is useful to prevent theft of service, and also to prevent "over the air" snooping of a user's activity.

The network does this by utilising the Authentication Centerand is accomplished without transmitting the key

directly.
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Every GSM phone contains a unique identifier (different from the phone number), called theInternational Mobile

Equipment Identity (IMEI). This can be found by dialling " *#06# ". When a phone contacts the network, its IMEImay be checked against the Equipment Identity Registerto locate stolen phones and facilitate monitoring.

Voice calls

Outgoing

Once a mobile phonehas successfully attached to a GSM network as described above, calls may be made from the

phone to any other phone on the global Public Switched Telephone Network.

The user dials the telephone number, presses thesendortalkkey, and the mobile phonesends a call setup requestmessage to the mobile phone networkvia the nearest mobile phone mast (BTS).

The call setup request message is handled next by the Mobile Switching Center, which checks the subscriber's

record held in theVisitor Location Registerto see if the outgoing call is allowed. If so, the MSC then routes the

call in the same way that a telephone exchange does in a fixed network.

If the subscriber is on aPay As You Go tariff (sometimes known as Prepaid (for example, in Australia and India)),then an additional check is made to see if the subscriber has enough credit to proceed. If not, the call is rejected. If

the call is allowed to continue, then it is continually monitored and the appropriate amount is decremented fromthe subscriber's account. When the credit reaches zero, the call is cut off by the network. The systems that monitor

and provide the prepaid services are not part of the GSMstandard services, but instead an example ofintelligent

networkservices that a mobile phone operatormay decide to implement in addition to the standard GSM ones.

Incoming

Gateway MSC contact

When someone places a call to amobile phone, they dial the telephone number(also called a MSISDN) associatedwith the phone user and the call is routed to the mobile phone operator's Gateway Mobile Switching Centre. TheGateway MSC, as the name suggests, acts as the "entrance" from exterior portions of the Public Switched

Telephone Networkonto the provider's network.

As noted above, the phone is free to roam anywhere in the operator's network or on the networks of roaming

partners, including in other countries. So the first job of the Gateway MSC is to determine the current location ofthemobile phone in order to connect the call. It does this by consulting the Home Location Register (HLR),

which, as described above, knows which Visitor Location Register (VLR) the phone is associated with, if any.

Routing the call

When the HLR receives this query message, it determines whether the call should be routed to another number(called a divert), or if it is to be routed directly to the mobile.

If the owner of the phone has previously requested that all incoming calls be diverted to another number,

known as the Call Forward Unconditional (CFU) Number, then this number is stored in the Home

Location Register. If that is the case, then the CFU number is returned to the Gateway MSC for immediaterouting to that destination.

If the mobile phone is not currently associated with a Visited Location Register (because the phone has

been turned off) then the Home Location Register returns a number known as the Call Forward NotReachable (CFNRc) number to the Gateway MSC, and the call is forwarded there. Many operators may
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set this value automatically to the phone's voice mail number, so that callers may leave a message. The

mobile phone may sometimes override the default setting.

Finally, if the Home Location Register knows that the phone is roaming in a particularVisited Location

Registerarea, then it will request a temporary number (called an MSRN) from that VLR. This number is

relayed back to the Gateway MSC, and then used to route the call to the MSC where the called phone isroaming.

Ringing the phone

When the call arrives at the Visiting MSC, the MSRN is used to determine which phone is being called. The MSC

thenpages all the mobile phone masts in the area in order to inform the phone that there is an incoming call for it.If the subscriber answers, a speech path is created through the Visiting MSC and Gateway MSC back to the

network of the person making the call, and a normaltelephonecall follows.

It is also possible that the phone call is not answered. If the subscriber is busy on another call (and call waiting is

not being used) the Visited MSC routes the call to a pre-determined Call Forward Busy (CFB) number. Similarly,if the subscriber does not answer the call after a period of time (typically 30 seconds) then the Visited MSC routes

the call to a pre-determined Call Forward No Reply (CFNRy) number. Once again, the operator may decide to set

this value by default to the voice mailof the mobile so that callers can leave a message.

If the subscriber does not respond to the paging request, either due to being out of coverage, or their battery hasgone flat/removed, then the Visited MSC routes the call to a pre-determined Call Forward Not Reachable

(CFNRc) number. Once again, the operator may decide to set this value by default to the voice mailof the mobile

so that callers can leave a message.

Voice charges

See also:Mobile telephony#Tariff models

In the United States and Canada, callers pay the cost of connecting to the Gateway MSC of the subscriber's phone

company, regardless of the actual location of the phone. As mobile numbers are given standard geographicnumbers according to theNorth American Numbering Plan, callers pay the same to reach fixed phones and mobile

phones in a given geographic area. Mobile subscribers pay for the connection time (typically using in-plan orprepaid minutes) for both incoming and outgoing calls. For outgoing calls, any long distance charges are billed as

if they originate at the GMSC, even though it is the Visiting MSC which completes the connection to the PSTN.

Plans that include nationwide long distance and/or nationwide roaming at no additional charge over "local"

outgoing calls are popular.

Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia & Argentina

only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber; however, callers

typically pay a higher rate when calling mobile phones. Special prefixes are used to designate mobile numbers so

that callers are aware they are calling a mobile phone and therefore will be charged a higher rate.

From the caller's point of view, it does not matter where the mobile subscriber is, as the technical process of

connecting the call is the same. If a subscriber is roaming on a different company's network, the subscriber,

instead of the caller, may pay a surchargefor the connection time. International roaming calls are often quiteexpensive, and as a result some companies require subscribers to grant explicit permission to receive calls while

roaming to certain countries.

When a subscriber is roaming internationally and a call is forwarded to his or her voice mail, such as when his or

her phone is off, busy, or not answered, he or she may actually be charged for two simultaneous international
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phone callsthe first to get from the GMSC to the VMSC and the second to get from the VMSC to the Call

Forward Busy or Call Forward No Reply number (typically the voice mailbox) in the subscriber's country.However, some networks' GMSCs connect unanswered calls directly, keeping the voice signal entirely within the

home country and thus avoiding the double charge.[citation needed]

How speech is encoded during mobile phone calls

During a GSM call, speech is converted fromanalogue sound waves todigital data by the phone itself, and

transmitted through the mobile phone network by digital means. (Though older parts of the fixedPublic SwitchedTelephone Networkmay use analog transmission.)

The digital algorithm used to encode speech signals is called a codec. The speech codecs used in GSM are called

Half-Rate (HR),Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except

AMR operate with a fixed data rate and error correction level.

Data transmission

The GSM standard also provides separate facilities for transmitting digital data. This allows a mobile phone to act

like any other computer on the Internet, sending and receiving data via the Internet Protocol.

The mobile may also be connected to a desktop computer,laptop, orPDA, for use as a network interface (just likea modem orEthernet card, but using one of the GSM data protocols described below instead of a PSTN-

compatible audio channel or an Ethernet link to transmit data). Some GSM phones can also be controlled by a

standardised Hayes AT command set through a serial cable or a wireless link (using IrDA orBluetooth). The AT

commands can control anything from ring tones to data compression algorithms.

In addition to general Internet access, other special services may be provided by the mobile phone operator, such

as SMS.

Circuit-switched data protocols

Acircuit-switcheddata connection reserves a certain amount of bandwidth between two points for the life of a

connection, just as a traditional phone call allocates an audio channel of a certain quality between two phones for

the duration of the call.

Two circuit-switched data protocols are defined in the GSM standard: Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD). These types of connections are typically charged on a per-second basis,

regardless of the amount of data sent over the link. This is because a certain amount of bandwidth is dedicated to

the connection regardless of whether or not it is needed.

Circuit-switched connections do have the advantage of providing a constant, guaranteedquality of service, whichis useful for real-time applications like video conferencing.

General Packet Radio Service (GPRS)

The General Packet Radio Service (GPRS) is apacket-switched data transmission protocol which was

incorporated into the GSM standard in 1997. It is backwards-compatible with systems that use pre-1997 versions

of the standard. GPRS does this by sending packets to the local mobile phone mast (BTS) on channels not beingused by circuit-switched voice calls or data connections. Multiple GPRS users can share a single unused channel

because each of them uses it only for occasional short bursts.
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The advantage of packet-switched connections is that bandwidth is only used when there is actually data to

transmit. This type of connection is thus generally billed by the kilobyte instead of by the second, and is usually acheaper alternative for applications that only need to send and receive data sporadically, like instant messaging.

GPRS is usually described as a 2.5Gtechnology; see the main article for more information.

Short Message Service (SMS)

Main article:Short message service

Short Messages (more commonly known as text messages) has become the most used data application on mobile

phones, with 74% of all mobile phone users worldwide already as active users of SMS, or 2.4 billion people bythe end of 2007. In many advanced countries, the users have shifted from considering the voice call being the

most desired feature of a mobile phone, to considering SMS text messaging as the most desired feature.

SMS text messages may be sent by mobile phone users to other mobile users or external services that accept SMS.

The messages are usually sent from mobile devices via the Short Message Service Centre using the MAPprotocol.

The SMSC is a central routing hubs for Short Messages. Many mobile service operators use their SMSCs asgateways to external systems, including theInternet, incoming SMS news feeds, and other mobile operators (often

using the de factoSMPP standard for SMS exchange).

The SMS standard is also used outside of the GSMsystem; see the main article for details.

Supplementary Services

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

data services described above. They are all defined in GSM standards. (See GSM codes for supplementary

services) A partial listing of supplementary services follows.

Call forwarding. This service gives the subscriber the ability to forward incoming calls to another number

if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed

unconditionally.

Barring of Outgoing Calls. This service makes it possible for a mobile subscriber to prevent all outgoing

calls.

Barring of Incoming Calls. This function allows the subscriber to prevent incoming calls. The followingtwo conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls

when roaming outside the home PLMN.

Advice of Charge (AoC). The AoC service provides the mobile subscriber with an estimate of the call

charges. There are two types of AoC information: one that provides the subscriber with an estimate of thebill 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 subsequentlyreestablish 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 toall GSM telecommunications 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.
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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 thecalling party to restrict the presentation. The restriction overrides the presentation.

Closed User Groups (CUGs). CUGs are generally comparable to a PBX. They are a group of subscribers

who are capable of only calling themselves and certain numbers.

Explicit Call Transfer (ECT). This service allows a user who has two calls to connect these two calls

together and release its connections to both other parties.

GSM SYSTEM OVERVIEW

The GSM Network

GSM is basically divided into the Switching System (SS) and the Base Station System (BSS), see

Figure 3.

Each of these contains a number of functional units, where all system functions are realized. The

functional units are implemented in various equipment (hardware).

The Switching System (SS) includes the following functional units:

Mobile services Switching Centre (MSC)

Visitor Location Register (VLR)

Home Location Register (HLR)

Authentication Centre (AUC)
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Equipment Identity Register (EIR)

The Base Station System (BSS) includes:

Base Station Controller (BSC)

Base Transceiver Station (BTS)

Cells

The system is realized as a network of neighboring radio cells, together providing a complete

coverage of the Service Area.

Base Transceiver Station (BTS)

Each cell has a Base Transceiver Station (BTS) operating on a set of radio channels. These are

different from the channels used in neighboring cells to avoid interference.

Base Station Controller (BSC)

A group of BTSs is controlled by a Base Station Controller (BSC). BSC controls such functions as

handover and channel assignment.

Mobile services Switching Centre (MSC)

A number of Base Station Controllers are served by a Mobile services Switching Centre (MSC). The

MSC controls calls to and from other telephony and data communication systems, such as the Public

Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), Public LandMobile Network (PLMN), Public Data Networks and, possibly, various private networks.

Databases

The abovementioned units are all involved in carrying speech connections between a Mobile Station

(MS) and, for example, a subscriber in the PSTN, the fixed network. If it were not for the possibility o

making calls to an MS we would not need any further equipment. The problem arises when we want

to make an MS terminated call. The originator hardly ever knows where the called MS is. Due to this

we need a number of databases in the network to keep track of the MS.

The most important of these databases is the Home Location Register (HLR). When someone buys a

subscription from one of the GSM operators, it will be registered in the HLR of that operator. The HLR

contains subscriber information, such as supplementary services and parameters. Furthermore, there

will be information about the location of the MS, i.e. in which MSC area the MS resides presently.

This information changes as the MS moves. The MS will send location information (via the MSC/VLR)

to its HLR, thus providing means to receive a call.


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A unit called Authentication Centre (AUC) is connected to the HLR. The function of the AUC is to

provide the HLR with authentication parameters and ciphering keys, both used for security reasons.

The Visitor Location Register (VLR) is a database containing information about all the MSs currently

located in the MSC area. As soon as an MS roams into a new MSC area, the VLR connected to that

MSC will request data about the MS from the HLR. At the same time the HLR will be informed in

which MSC area the MS resides. If, later on, the MS wants to make a call, the VLR will have all the

information needed for the call setup without having to interrogate the HLR. The VLR can be seen

as a distributed HLR. The VLR will also contain more exact information about the location of the MS

in the MSC area.

The Equipment Identity Register (EIR) contains the unique hardware identity of all the mobile

equipment, the IMEI. The EIR is connected to the MSC over a signalling link, which enables the MSC

to check the validity of the equipment. For example, a nontypeapproved telephone or stolen

equipment can be barred.

Gateway

If someone in the fixed network (PSTN) wants to make a call to a GSM subscriber, the exchange in

the PSTN will connect the call to a gateway. The gateway is often realized in an MSC. This MSC wil

be known as the gateway MSC, GMSC. It can be any of the MSCs in the GSM network (probably

most of them). The GMSC will find the location of the searched MS by interrogating the HLR. The

HLR will reply with the address to the current MSC area. Now the GMSC can reroute the call to the

correct MSC. When the call reaches that MSC, then the VLR will know in more detail where the

Mobile Station, the MS is. The call can be switched through.

Mobile Station (MS)

In GSM there is a difference between the physical equipment and the subscription. The Mobile

Equipment (ME), which can be vehicle installed, portable or handheld is separated from the small

unit called the Subscriber Identity Module (SIM) containing the subscription. The SIM and the mobile

equipment together make up the mobile station. Without SIM, the MS cannot access the GSM

network, except for emergency calls. As the SIMcard is connected to the subscription and not to the

mobile equipment, the subscriber can use other equipments as well as his own. The authentication of

the subscription is done by parameters from AUC. The ME is identified by the International Mobile

Equipment Identity (IMEI).

Operation and Maintenance Centre (OMC)

The Operations and Maintenance Centre (OMC) is connected to all equipment in the Switching

System and to the BSC. The implementation of OMC is called Operation and Support System (OSS).


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The geographical network structure

Every telephone network needs a certain structure in order to route incoming calls to the correct

exchange and finally to the called subscriber. In a mobile network this structure is of great importance

because of the mobility of all its subscribers.

The Network Area and the Gateway MSC

The links between a GSM/PLMN network and the other PSTN, ISDN or PLMN networks will be on thelevel of international or national transit exchanges. All incoming calls for a GSM/PLMN network will be

routed to the Gateway MSC enabling the system to route calls to their final destination: the calledmobile station.

The Service Area

A Service Area is an area in which a mobile subscriber can be reached without the calling partys

knowledge of the MS current location.

The PLMN Service Area

The geographical area in which the operator offers radio coverage and possibility to access the

network.

The MSC/VLR Service Area

A GSM/PLMN service area is divided into one or several MSC/VLR Service Areas.

An MSC service area represents the geographical part of the network that is covered by one MSC. In

order to route a call to a mobile subscriber, the path through the network links to the MSC in the MSC

area where the subscriber is currently located.

The MS is registered in a Visitor Location Register (VLR). Since MSC and VLR always areimplemented in the same node in CME 20, the MSC area and the Service Area cover exactly the
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same part of the network.

The Location Area (LA)

Each MSC/VLR Service Area is divided into several Location Areas.

A Location Area is a part of the MSC/VLR Service Area in which a mobile station may move freely

without updating location information to the MSC/VLR exchange.

A Location Area is the area where a paging message is broadcasted in order to find the called mobile

subscriber. The Location Area can have several cells and depend on one or more BSCs, but it

belongs to only one MSC/VLR.

The Location Area can be identified by the system, using the Location Area Identity (LAI).

The Cell
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A Location Area is divided into a number of cells. The cell is an area of radio coverage which thenetwork identifies with the Cell Global Identity (CGI). The Mobile Station itself distinguishes between

cells using the same carrier frequencies, by use of the Base Station Identity Code (BSIC).
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