architecture of gsm

8/9/2019 Architecture of GSM

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Report By: Huzaif Zahoor

Architecture of GSM:

Some information on GSMs basic terminologies

Mobile station. The GSM mobile station (or mobile phone) communicates with other parts ofsystem through the base-station system.Base transceiver station (BTS). The base transceiver station (BTS) handles the radio in

to the mobile station. The base transceiver station is the radio equipment (transceivers

antennas)

Base station controller (BSC). The BSC provides the control functions and physical links bethe MSC and BTS. It provides functions such as handover, cell configuration data and cof RF power levels in base transceiver stations. A number of BSCs are served by a MSC


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Mobile services switching center (MSC). The MSC performs the telephony switching functionthe system. It also performs such functions as toll ticketing, network interfacing, commchannel signaling, and others.

Home location register (HLR). The HLR database is used for storage and management of

subscriptions. The home location register stores permanent data about subscribers,including a subscriber's service profile, location information, and activity status.

Visitor location register (VLR). The VLR database contains temporary information aboutsubscribers that is needed by the mobile services switching center (MSC) in order to sevisiting subscribers. When a mobile station roams into a new mobile services switchingcenter (MSC) area, the visitor location register (VLR) connected to that MSC will requesabout the mobile station from the HLR, reducing the need for interrogation of the homelocation register (HLR).

Authentication center (AUC). The AUC provides authentication and encryption parameters thverify the user's identity and ensure the confidentiality of each call. The authentication (AUC) also protects network operators from fraud.

Equipment identity register (EIR). The EIR database contains information on the identity of mequipment to prevent calls from stolen, unauthorized or defective mobile stations.

Message center (MXE). The MXE is a node that provides integrated voice, fax, and datamessaging.

Mobile service node (MSN). The MSN is the node that handles the mobile intelligent network (services.

Gateway mobile services switching center (GMSC). A gateway mobile services switching cen(GMSC) is a node used to interconnect two networks.

GSM interworking unit (GIWU). The GIWU consists of both hardware and software that providinterface to various networks for data communications. Through the GSM interworking

(GIWU), users can alternate between speech and data during the same call.

Operation and support system (OSS). The OSS is the functional entity from which the networoperator monitors and controls the system. The purpose of operation and support systto offer support for centralized, regional, and local operational and maintenance activitiare required for a GSM network.


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Function of these above terminologies:-

This below figure shows the layout of a generic GSM network. The GSM network can be diviinto three broad parts. The Mobile Station is carried by the subscriber, the Base StationSubsystem controls the radio link with the Mobile Station. The Network Subsystem, thepart of which is the Mobile services Switching Center, performs the switching of callsbetween the mobile and other fixed or mobile network users, as well as management ofmobile services, such as authentication. Not shown is the Operations and Maintenancecenter, which oversees the proper operation and setup of the network. The Mobile Statand the Base Station Subsystem communicate across the Um interface, also known as interface or radio link. The Base Station Subsystem communicates with the Mobile serSwitching Center across the A interface.

,---------------------------------------------------------------------------,

| Um A |

| Interface A-bis Interface |

| | Interface | ,--------------------, || | ,-----, ,-----, | |

| | ,----------|----------, | | | VLR | | HLR | | ,-------, |

| ,-----, | ,-----, | | `-----' `-----' | | Other | |

| | SIM | | | | BTS | | ,-----, | | | ,-----, | | MSCs | |

| `--,--' | `-----'---| | | | | |--------------'-------' |

| | , | : | | BSC |-----------| MSC | | |

| ,-----, /| | ,-----,---| | | | | |--------------,------, |

| | MS |' | ,---| BTS | | `-----' | | | `-----' | / PSTN / \ |

| `-----' |/ | `-----' | | ,-----, ,-----, | \ ISDN / |

| ' `----------|----------' | | | EIR | | AC | | `------' |

| | `-----' `-----' | |

| | Base Station Subsystem `--------------------' |

| Network Subsystem |

`----------------------------------------------------------------------------'

SIM Subscriber Identity Module HLR Home Location Register

MS Mobile Station VLR Visitor Location Register

BTS Base Transceiver Station EIR Equipment Identity Register

BSC Base Station Controller AC Authentication Center

MSC Mobile services Switching Center PSTN Public Switched Telecomm Networ

VLR Visitor Location Register ISDN Integrated Services Digital Network

FIGURE 1

Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, diand digital signal processors, and a smart card called the Subscriber Identity Module (SThe SIM provides personal mobility, so that the user can have access to all subscribedservices irrespective of both the location of the terminal and the use of a specific termiinserting the SIM card into another GSM cellular phone, the user is able to receive callsphone, make calls from that phone, or receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IThe SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the


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subscriber, a secret key for authentication, and other user information. The IMEI and thare independent, thereby providing personal mobility. The SIM card may be protected aunauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) aBase Station Controller (BSC). These communicate across the specified Abis interfaceallowing (as in the rest of the system) operation between components made by differensuppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the link protocols with the Mobile Station. In a large urban area, there will potentially be a l

number of BTSs deployed. The requirements for a BTS are ruggedness, reliability, portand minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles rchannel setup, frequency hopping, and handovers, as described below. The BSC is theconnection between the mobile and the Mobile service Switching Center (MSC). The BStranslates the 13 kbps voice channel used over the radio link to the standard 64 kbps cused by the Public Switched Telephone Network or ISDN.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (M

It acts like a normal switching node of the PSTN or ISDN, and in addition provides all thfunctionality needed to handle a mobile subscriber, such as registration, authenticationlocation updating, handovers, and call routing to a roaming subscriber. These servicesprovided in conjuction with several functional entities, which together form the NetworkSubsystem. The MSC provides the connection to the public fixed network (PSTN or ISDand signalling between functional entities uses the ITUT Signalling System Number 7 (Sused in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSprovide the callrouting and (possibly international) roaming capabilities of GSM. The Hcontains all the administrative information of each subscriber registered in the correspGSM network, along with the current location of the mobile. The current location of the

mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISnumber used to route a call to the MSC where the mobile is currently located. There islogically one HLR per GSM network, although it may be implemented as a distributeddatabase.

The Visitor Location Register contains selected administrative information from the HLR,necessary for call control and provision of the subscribed services, for each mobile culocated in the geographical area controlled by the VLR. Although each functional entitybe implemented as an independent unit, most manufacturers of switching equipment


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implement one VLR together with one MSC, so that the geographical area controlled byMSC corresponds to that controlled by the VLR, simplifying the signalling required. Nothe MSC contains no information about particular mobile stations - this information is sin the location registers.

The other two registers are used for authentication and security purposes. The Equipment Id

Register (EIR) is a database that contains a list of all valid mobile equipment on the netwwhere each mobile station is identified by its International Mobile Equipment Identity (IMAn IMEI is marked as invalid if it has been reported stolen or is not type approved. TheAuthentication Center is a protected database that stores a copy of the secret key storeeach subscriber's SIM card, which is used for authentication and ciphering of the radiochannel.

Interfaces

UmThe air interface is used for exchanges between a MS and a BTS. LAPDm, a modified version oISDN LAPD, is used for signalling.

AbisThis is a BSS internal interface linking the BSC and a BTS, and it has not been standardised. Th

interface allows control of the radio equipment and radio frequency allocation in the BTS.

AThe A interface is between the BTS and the MSC. The A interface manages the allocation of suradio resources to the MSs and mobility management.

BThe B interface between the MSC and the VLR uses the MAP/B protocol. Most MSCs are asso

with a VLR, making the B interface "internal". Whenever the MSC needs access to data regardi

MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.

CThe C interface is between the HLR and a GMSC or a SMS-G. Each call originating outside of

(i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing

information required to complete the call, and the MAP/C protocol over the C interface is used purpose. Also, the MSC may optionally forward billing information to the HLR after call clearin


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DThe D interface is between the VLR and HLR, and uses the MAP/D protocol to exchange the d

related to the location of the MS and to the management of the subscriber.

EThe E interface interconnects two MSCs. The E interface exchanges data related to handover bethe anchor and relay MSCs using the MAP/E protocol.

FThe F interface connects the MSC to the EIR, and uses the MAP/F protocol to verify the status

IMEI that the MSC has retrieved from the MS.GThe G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to tra

subscriber information, during e.g. a location update procedure.

HThe H interface is between the MSC and the SMS-G, and uses the MAP/H protocol to support t

transfer of short messages.

IThe I interface (not shown in Figure 1) is the interface between the MSC and the MS. Messages

exchanged over the I interface are relayed transparently through the BSS.

Protocols over the A, A-Bis and Um interfaces

below shows the signaling protocols between the MS and BTS, between the BTS and BSC, and betwBSC and the MSC.

The CM, MM and RR layers together correspond to layer three in the ISO OSI protocol suite, and layis composed of LAPD and LAPDm. Customarily, the lower three layers terminate in the same node.

in GSM, where the functionality is spread over distinct functional entities with standardised interface

between them. For instance, the RR part of layer three is spread over the MS, BTS, BSC, and MSC.

CMThe Communication Management (CM) layer consists of setting up calls at the users' reque

functions are divided in three: Call control, which manages the circuit oriented services;

Supplementary services management, which allows modifications and checking of thesupplementary services configuration; Short Message Services, which provides point-to-po

message services.

MMThe Mobility Management (MM) layer is in charge of maintaining the location data, in add


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the authentication and ciphering procedures.

RRThe Radio Resource (RR) Management layer is in charge of establishing and maintaining a

uninterrupted communications path between the MSC and MS over which signalling and uscan be conveyed. Handovers are part of the RR layers responsibility. Most of the functions

controlled by the BSC, BTS, and MS, though some are performed by the MSC (in particula

inter-MSC handovers.).RR'The RR' layer is the part of the RR functionality which is managed by the BTS.

LAPDmThe layer two protocol is provided for by LAPDm over the air-interface. This protocol is a

modified version of the LAPD (Link Access Protocol for the ISDN D-channel) protocol. Th

modifications are due to the tight synchronisation required in TDMA and bit error protectiomechanism required over the air-interface (and in GSM handled by layer 1), making the

corresponding functionality of the LAPD protocol redundant (and thus wasteful over the air

interface). The LAPD frame flags are replaced by a length indicator, and the FEC field is re

BTSMThe Base Transceiver Station Management (BTSM) is responsible for transferring the RR

information (not provided for in the BTS by the RR' protocol) to the BSC.

LAPDThis is the ISDN LAPD protocol (Link Access Protocol for the ISDN D-channel) providing

free transmission between the BSC and MSC.BSSAPThe Base Station System Application Part (BSSAP) is split into two parts, the BSSMAP an

DTAP (not shown in the above figure). The message exchanges are handled by SS7. Messa

which are not transparent to the BSC are carried by the Base Station System ManagementApplication Part (BSSMAP), which supports all of the procedures between the MSC and th

that require interpretation and processing of information related to single calls, and resource

management. The messages between the MSC and MS which are transparent to the BSC (MCM messages) are catered for by the Direct Transfer Application Part (DTAP).

SCCPThe Signalling Connection Control Part (SCCP) from SS7.

MTPThe Message Transport Part (MTP) of SS7.

architecture of gsm

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