gsm network components

7/28/2019 GSM Network Components

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GSM Network Components


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Contents

GSM system architecture.

Mobile Station or MS including:

Base Station Subsystem or BSS including:

Mobile-service Switching Centre or MSC.

Registers: SIM card.

Mobile equipment.

Base Transceiver Station or BTS.

Base Station Controller or BSC.

Home Location Register.

Visitor Location Register.

Equipment Identity Register.

Authentication center.


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Base Transceiver Station (BTS)

Base Transceiver Station (BTS) is a fixed radio stationthat communicates with the mobile telephones usingelectromagnetic waves. The Base Transceiver Stationincludes transmitters, receivers, antennas and control

and communication equipment to interconnect with theBSC via optical cable or microwave link.

There is one BTS per cell, usually housed in a 2m heightrack as showed herein. This rack together with theappropriate power system and communicationequipment towards the BSC are located typically in acontainer next to the antenna tower.

The power system provides the necessary operatingvoltages for the equipment and backup from batteriesin case of power failure.Transmission to BSC is mostly performed via opticalcable or point-to-point microwave link


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Base Transceiver Station (BTS)

Base stations use mostly directional antennas in order to increase capacity and

therefore frequency band utilization (see lesson 1). The directional antennas are

assembled on antenna mast of variable height in order to secure full coverage of

the territory. In the drawing below a typical cellular 120 degrees directional

antenna is represented and its radiation patterns. As can be seen the antenna is

actually an array of dipoles providing a very narrow radiation pattern in thevertical direction and the required 120 degrees in the horizontal direction


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Base Station Controller (BSC)

Base Station Controller or BSC is the first

intelligent component of the GSM network

we meet, needed in order to control and

manage all the BTSs it is connected with.

The BSC is typically housed in a 2m height

rack as showed in the picture herein.

The BSC site includes also an uninterruptible

power supply with battery back-up in case of

power failure (similar to BTS) and

communication equipment to BTSs on one

side and to the MSC on the other side.


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Mobile Switching Center

(MSC) Mobile-service Switching Centre (MSC) is essentially an ISDN switch with

significantly enhanced processing capability to cater for the special needs of GSM.An MSC will parent (support) a number of BSCs. The primary responsibility of anMSC is voice and data call handling for the mobile subscribers within its domain.Through the MSC mobile subscribers can communicate each-other and withtelephones connected to fix PSTN network. MSC is the core of the GSM networktherefore each cellular operator will have its own MSC(s) to handle the calls of its

users. The following figure shows MSC main switching activities. We can see the fix

telephones connected to PSTN (fix telephone network). Multiple cellular operatorshaving one or more MSCs are also connected to PSTN. The remaining part of thedrawing including BSCs and BTSs was already learned. We can distinguish betweenthree types of connections:

mobile-to-mobile within same operator in which case MSCs of one operator areinvolved

mobile-to-fix that involves MSCs of one operator and the PSTN

mobile-to-mobile between operators involving MSCs of two operators and thePSTN


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MSC

The following figure shows MSC main switching activities.We can see the fix telephones connected to PSTN (fixtelephone network). Multiple cellular operators having oneor more MSCs are also connected to PSTN. The remainingpart of the drawing including BSCs and BTSs was already

learned. We can distinguish between three types ofconnections:

1. mobile-to-mobile within same operator in which caseMSCs of one operator are involved.

2. mobile-to-fix that involves MSCs of one operator and thePSTN.

3. mobile-to-mobile between operators involving MSCs oftwo operators and the PSTN.


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MSC

But MSC has also other than voice switching functions -that will all be learned in future lessons - as follows:

Delivering SMS (Short Message Service - one of theimportant features of GSM that will be learned later) fromsubscribers to SMS Center (SMSC) and vice versa

Switching inband fax and modem calls.

Supporting handovers between BSCs and from one MSC toanother

Supporting other services per specific vendors

implementation like conference calls, call hold, call forward,etc.

Collect billing information, namely generate recordsincluding all necessary information to charge the user


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Gateway MSC (GMSC)

Gateway MSC (GMSC)

MSC is the switching equipment we

learned till now. The network of a

cellular operator will include few

such units

Gateway MSC or GMSC is the switch that connects a cellular

operator to the PSTN and also the one that determines

which visited MSC the subscriber who is being called is

currently located. Typically an operator will have one such

switch yet very large operators covering large surfaces may

have more.


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Home Location Register (HLR)

Home Location Register or HLR is an intelligentdatabase and service control function responsible formanagement of each individual subscribers records. Itcontains details of each mobile phone subscriber -more precisely of its SIM card - that is authorized to

use the GSM core network.


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Home Location Register (HLR)Home Location Register (HLR)

HLR includes for all SIM cards issued by a cellular operator a profile including:

The unique IMSI identifier of the SIM which is one of the primary keys to each HLRrecord. Primary key means that IMSI is one of the search parameters for the database

Telephone numbers associated to the SIM used to make and receive calls to the

mobile phone, known as Mobile Subscriber ISDN (MSISDN) number. An MSISDN

consists of a country code + a national destination code + a subscriber number.

Current location of the subscriber meaning which Location Area is he in. It includes

the BTSs managed by same BSC and is identified by its Location Area Code or LAC

The main MSISDN is the number used for making and receiving voice calls and SMS.

This is the cellular phone number we all know and use. But it is possible for a SIM to

have other secondary MSISDNs associated with it for fax and data calls (well learn

more about this later)

Each MSISDN is also a primary key to the HLR record.

GSM services that the subscriber has requested or been given

Call divert to data services MSISDN and/or GPRS settings to allow the subscriber to

access packet services


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Visitor Location Register (VLR)

Visitor Location Register (VLR) is an intelligent database and service control function. It stores

on a temporary basis the information needed to handle calls set up or received by MSsregistered with it. VLR is responsible for a group of location areas, typically associated with an

MSC. This is why most equipment vendors either integrate the VLR with MSC or at least

connect them tightly via a proprietary interface.

The data stored in the VLR has either been received from the HLR, or collected from the MS.

The main functions of the VLR are as follows:

Inform HLR that a subscriber has arrived to its area

Track which LA the various MSs are located

Allow or deny service to MS

Delete subscriber record when moves to other VLR under HLR control

Delete subscriber record if inactive for longer than a preset time


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Equipment Identity Register (EIR)

The EIR is a database that contains information about the identity of MEs that prevents calls from

stolen, unauthorized, or defective mobile stations.

The EIR (Equipment Identity Register) is often integrated to the HLR. The EIR keeps a list of mobile

phones (identified by their IMEI) which are to be banned from the network or monitored. This is

designed to allow tracking of stolen mobile phones. In theory all data about stolen mobile phones

should be distributed to all EIRs in the world through a Central EIR. It is clear, however, that there

are operators where EIR is not operational.

The EIR data does not have to change in real time, which means that this function can be lessdistributed than the function of the HLR.


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Equipment Identity Register (EIR)

The Equipment Identity Register (EIR) is accessed during the

equipment validation procedure when a mobile station accesses

the system.

EIR Includes a list of MEs identified by their IMEI divided into:

White or Valid list - List of valid MS equipment identities

Grey or Monitored list - List of suspected mobiles under

observation

Black or prohibited list - List of mobiles for which service is

barred


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The Authentication Centre (AUC)

The Authentication Centre or AUC is the function that authenticates each SIM

card that attempts to connect to the GSM core network (typically when thephone is powered on). Once the authentication is successful, the HLR is

allowed to manage the SIM and services described above. An encryption key

is also generated that is subsequently used to encrypt all wireless

communications (voice, SMS, etc.) between the mobile phone and the GSM

core network.

The scope of AuC is to prevent SIM cloning.

Authentication is based on a secret, individual to each user number called Ki.


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The Authentication Centre (AUC)Authentication process:

When a particular IMSI requests access to the GSM core network, the AUC

generates a specific random number - RAND - that is sent to the SIM. The SIM

then feeds this number and the Ki (which is burned onto the SIM) into the A3

or A8 proprietary algorithm as appropriate and an SRES is calculated and sent

back to the MSC. In parallel the AUC performs the same operations and getsits own SRES*. If the SRES matches with SRES* (which it should if it is a valid

SIM), then the mobile is allowed to attach and proceed with GSM services.


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SIMThe Subscriber Identity Module or SIM is a removable smart card

located inside the MS that carries all the subscriber specificinformation. The SIM is one of the important innovations

introduced by the GSM network.

The SIM is actually a tiny computer - microcomputer - inside the

phone having 16 to 128 Kbit memory.


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SIM

SIM contents :

Unique identification of the SIM based on a number called the InternationalMobile Subscriber Identity (IMSI). Changing the SIM means changing your phonenumber/identity

Authentication and encryption based on a secret key for the purpose of preventingeavesdropping

Personal settings and services that can be moved from one handset to another for

example: alarms, information services, login to different services etc Personal phone book, abbreviated dialling codes, and text messages

Rate plans, billing information, account information. (not active in Turkey)

Location information for the purpose of accelerating the connection to thenetwork when the phone is switched on. We already know that in GSM there are124 FDMA channels. When a handset is switched on it has to search allfrequencies in order to find the operating one in its specific cell. To shorten thisprocess the phone has in the SIM the information regarding the last cell it wasconnected with before was switched off. Of course if meantime was moved toanother cell this info is not anymore helpful.


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Mobile Station (MS)

Mobile Equipment (ME) provides the radio and processing needed to access the GSM

network, plus a friendly Man Machine Interface or MMI including: display, keyboard andtones to enable the user to access services.

The ME is uniquely identified by the International Mobile Equipment Identity (IMEI) for the

purpose of preventing use of stolen/lost MEs. Service to such an ME will be denied by the

network.

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