GSM Architecture

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Network Components

• Switching System(SS)• Base Station System(BSS)

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BTS

MSC VLR

HLR

PSTNISDN

DataNetworks

Air interface

OSS

BTS

BTS

MSC VLR

BSCBSC

A Interface

A-bis interface

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BGW

SOG

OSS

ERICSSON’S GSM SYSTEM ARCHITECTURE

SCF

MIN SDP

EIR AUC

HLR

Switching System

ILR

MSC/VLRDTI SSF

MC(MXE)GMSC

Other PLMNsz

ISDN

PSTNPublic Data

Networks

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TRC

BSC

RBS

Base Station System

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Basic or Additional

Abbrev.

System

Full component name Platform

Basic

BasicBasicBasicBasicBasicBasicBasicBasicBasicBasicBasicAdditional Additional Additional Additional Additional Additional

MSC/VLR

GMSCHLRILRAUCEIRDTITRCBSCBTSOMCNMCMCSSPSCPSDPSOGBGW

SS

SSSSSSSSSSSSBSSBSSBSSOSSOSSSSSSSSSS

Mobile service Switching Center/Visitor Location RegisterGateway MSCHome Location Register Interworking Location RegisterAuthentication CenterEquipment Identity RegisterData Transmission InterfaceTRanscoder Controller Base Station Controller Base Transceiver Station Operation and Maintenance CenterNetwork Management CenterMessage CenterService Switching Point Service Control Point Service Data Point Service Order GatewayBilling Gateway

AXE

AXEAXEAXEUnix/AXEUnixAXEAXEAXERBSTMOS TMOSMXEAXEAXEUnix UnixUnix

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Network Structure• Cell A cell is the basic unit of a

cellular system and is defined as the radio coverage given by one BS antenna system.

Each cell is assigned a unique CGI.

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LOCATION AREA

A LA is defined as a group of cells.Within the network, a subsriber’s location is known by the LA which they are in.

The identity of the LA in which an MS is currently located is stored in the VLR. (LAI)

Network Structure

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Cells & LA

MSC

VLR

LA1

LA2

LA3

LA6LA4

LA5

C1C2 C3

C6 C5C4

C=CELL

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Network Structure

• MSC Service Area An MSC Service Area is made

up of LAs and represents the geographical part of the network controlled by one MSC.

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MSC Service Area

MSC

VLRLA1

LA2LA3

LA6LA4

LA5

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Network Structure• PLMN SERVICE AREA A PLMN service area is the entire

set of cells served by one network operator

and is defined as the area in which an operator offers radio coverage and access to its network.

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PLMN Service Area

V

MSC

MSC

MSC

MSC

VLR

VLRVLR

I

II

IVIII

I

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Network Structure

• GSM SERVICE AREA The GSM service area is the entire

geographical area in which a subscriber can gain access to a GSM network.

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Relation between areas in GSM

Location AreaCell

Location AreaMSC Service AreaPLMN Service Area

GSM Service Area

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Mobile Station

GSM MSs consist of:

• Mobile Equipment

• Subscriber Identity Module

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Functions of Mobile Station• Voice and data transmission&

receipt• Frequency and time

synchronization• Monitoring of power and signal

quality of the surrounding cells • Provision of location updates even

during inactive state

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Mobile Station

• Discontinuous Transmission(DTX)• Discontinuous reception(DRX)• MS identified by unique IMEI • STAR#06#

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SIM

Fixed data stored for the subscription:

• IMSI,• Authentication Key, Ki• Security Algorithms:kc,A3,A8 • PIN&PUK

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SIM•Temporary network data:

Location area of subscriber and forbidden PLMNs

•Service data:

language preference, advice of charge

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KEY TERMSAn MS can have one of the following states :

• Idle: the MS is ON but a call is not in progress.

• Active: the MS is ON and a call is in progress.

• Detached: the MS is OFF.

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The following table defined the key terms used to describe GSM mobile traffic cases (there are no traffic cases in detached mode):

Mode

Term Description

Idle Registration

Roaming

International Roaming

This is the process in which an MS informs a network that it is attached.

When an MS moves around a network in idle mode, it is referred to as roaming.

When an MS moves into a network which is not its home network, it is referred to as international roaming. MSs can only roam into networks with which the home network has a roaming agreement.

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Mode Term Description

Active

Location Updating

Paging

Handover

An MS roaming around the network must inform the network when it enters a new LA. This is called location updating.

This is the process whereby a network attempts to contact a particular MS. This is achieved by broadcasting a paging message containing the identity of that MS.

This is the process in which control of a call is passed from one cell to another while the MS moves between cells.

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Network Identities• MSISDN• IMSI• TMSI• MSRN• IMEI

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MSISDN• Mobile Station ISDN Number• The MSISDN is registered in the telephone directory and

used by the calling party for dialing.• MSISDN shall not exceed 15 digits.• NDC--National Destination Code• SN--Subscriber Number

CC NDC SN

1 to 3 digits Variable Variable

MSISDN : not more than 15 digits

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IMSI• International mobile subscriber

Identity• The IMSI is an unique identity which is

used internationally and used within the network to identify the mobile subscribers.

• The IMSI is stored in the subscriber identity module (SIM), the HLR, VLR database.

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IMSI3 digits

MCC MNC MSIN

3 digits Not more than 9 digits

NMSI

IMSI : Max. 15 digits

MCC--Mobile Country Code, MNC--Mobile N/W Code, MSIN--Mobile Station Identification Number

NMSI--National Mobile Station Identity,assigned by Individual Administration.

Mobile station Identification Number. It identifies the subs. In a PLMN. First 3 digit identifies the Logical HLR-id of Mobile subs.

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Temporary Mobile subscriber Identity

• TMSI is a temporary IMSI no. made known to an MS at registration.

• The VLR assigns a TMSI to each mobile subscribers entering the VLR area.

• Assigned only after successful authentication.

• TMSI has only local significance i.e. within VLR area & controlled by the VLR.

• TMSI changes on location updation.• TMSI is less than 8 digit.

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MSRNMobile Station Roaming Number

• The MSRN is used in the GMSC to set up a connection to the visited MSC/VLR.

• MSRN--is a temporary identity which is assigned during the establishment of a call to a roaming subs.

CC NDC SN

CC--Country Code, NDC--National Destination Code, SN– Servicing Node

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IMEI• International Mobile Equipment

Identity• The IMEI is an unique code allocated to

each mobile equipment. It is checked in the EIR.

• IMEI check List White List Grey List Black List

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BASE STATION SYSTEM (BSS)BASE STATION SYSTEM (BSS)

n BTS n BTS

BSC

BSC

BSC

MSC/VLR

BSSBSS

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FUNCTIONS OF BTS

• Radio resources• Signal Processing• Signaling link management• Synchronization• Local maintenance handling• Functional supervision and

Testing

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FUNCTIONS OF BSC

• Radio Network management • RBS Management• TRC Handling• Tx. Network Management• Internal BSC O&M• Handling of MS connections

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MSC-BSS Configurations

BTSBTS

BTS

BTS BTS

BTS

A-bisBSC

BSSMSC AA-bis

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Switching System (SS)

MSC

(PSTN)

VLR

HLR AUC

EIR

D

C

SS7 Signalling

Traffic Path

F

(BSS)

A

EOtherMSC

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MSC Functions • Switcing and call routing• Charging• Service provisioning• Communication with HLR• Communication with VLR• Communication with other MSCs• Control of connected BSCs

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MSC Functions • Echo canceller operation control

• Signaling interface to databases like HLR, VLR.

• Gateway to SMS between SMS centers and subscribers

• Handle interworking function while working as GMSC

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VISITOR LOCATION REGISTER (VLR)

• It controls those mobiles roaming in its area.

• VLR reduces the number of queries to HLR

• One VLR may be incharge of one or more LA.

• VLR is updated by HLR on entry of MS its area.

• VLR assigns TMSI which keeps on changing.

• IMSI detach and attach operation

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Data in VLR• IMSI & TMSI• MSISDN • MSRN.• Location Area• Supplementary service parameters• MS category• Authentication Key

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Home Location Register(HLR)

• Reference store for subscriber’s parameters, numbers, authentication & Encryption values.

• Current subscriber status and associated VLR.

• Both VLR and HLR can be implemented in the same equipment in an MSC.

• one PLMN may contain one or several HLR.

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Home Location Register(HLR)

• Permanent data in HLR• Data stored is changed only by commands.

• IMSI, MS-ISDN number.

• Category of MS ( whether pay phone or not )

• Roaming restriction ( allowed or not ).

• Supplementary services like call forwarding

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Home Location Register(HLR)

• Temporary data in HLR• The data changes from call to call & is

dynamic

• MSRN

• RAND /SRES and Kc

• VLR address , MSC address.

• Messages waiting data used for SMS

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EQUIPMENT IDENTITY REGISTER ( EIR )

• This data base stores IMEI for all registered mobile equipments and is unique to every ME.

• Only one EIR per PLMN.

• White list : IMEI, assigned to valid ME.• Black list : IMEI reported stolen• Gray list : IMEI having problems like faulty

software, wrong make of equipment etc.

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AUthentication Center (AUC)

To authenticate the subs. attempting to use a network.

AUC is connected to HLR which provides it with authentication parameters and ciphering keys used to ensure network security.

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AUC Functions

To perform subscriber authentication and to establish ciphering procedures on the radio link between the network and MS.

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Information provided is called a TRIPLET consists of:

1. RAND(non predictable random number)

2. SRES(Signed response)

3. Kc(ciphering key)

AUC Functions

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The centralized operation of the various units in the system and functions needed to maintain the subsystems.

Dynamic monitoring and controlling of the network

Operations and Maintenance Centre

OMC

Operations and Maintenance Centre

OMC

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-O&M data function

-Configuration management

--Fault report and alarm handling

-Performance supervision/management

-Storage of system software and data

Functions Of OMC

Functions Of OMC