The GSM Network Architecture can be divided into 4 parts:

• The Mobile Station (MS)

• The Base Station Subsystem (BSS)

• The Network and Switching Subsystem (NSS)

• The Operation and Support Subsystem (OSS)

GSM Network Architecture

MSC

VLR

AuC

EIR

HLR

MSBTS

OSS

BSC

PSDN

ISDN

PSTN

PLMN

BSSNSS

Abis interface A interfaceUm interface

Mobile Station

A Mobile Station consists of two main elements:

• The Mobile Equipment or terminal

• The Subscriber Identity Module (SIM)

The Terminal

• Portable, vehicle mounted or fixed, hand helddevice

• Uniquely identified by an IMEI (InternationalMobile Equipment Identity)

• Voice and data transmission

• Monitoring power and signal quality ofsurrounding cells for optimum handover

• Power level : 0.8W – 20 W

The SIM

• Smart card contains the International Mobile Subscriber Identity (IMSI)

• Allows user to send and receive calls and receive other subscribed services

• Normally 25 by 15 millimeters

• Protected by a password or PIN

• Can be moved from phone to phone –contains key information to activate the phone

The Base Station Subsystem

• The BSS connects the MS and the NSS.

It is in charge of the transmission and

reception. The BSS can be divided into

2 parts:

a. The Base Transceiver Station (BTS)or

Base Station

b. The Base Station Controller (BSC)

The Base Transceiver Station

• Contains the equipment for

transmitting and receiving of radio

signals (transceivers), antennas, and

equipment for encrypting and

decrypting communications with the

BSC

• Usually placed in the center of the cell

The Base Station Controller

• Manages Radio resources for BTS

• Assigns Frequency and time slots for all MS’s in its area

• Handles call set up

• Handover for each MS

• Radio Power control

• It communicates with MSC and BTS

• Frequency hopping

The Network and Switching Subsystem

The Network and Switching Subsystem

• manage the communications between the

mobile users and other users, such as

mobile users, ISDN (Integrated Services

Digital Network) users, fixed telephony

users, etc.

• it also includes data bases needed in

order to store information about the

subscribers and to manage their mobility.

Components of the NSS

Mobile Services Switching Center (MSC)

• Heart of the network

• Manages communication between GSM and other networks

• Call setup function and basic switching

• Call routing

• Billing information and collection

• Mobility management

- Registration

- Location Updating

- Inter BSS and inter MSC call handoff

• MSC does gateway function while its customer roams to other network by using HLR/VLR.

Gateway Mobile Services and Switching Center (GMSC)

• Gateway – is a node connecting two

networks.

• The interface between the mobile

cellular network and the PSTN

• Is in charge of routing calls

Home Location Register (HLR)

• Permanent database about mobile subscribers in a large service area (generally one per GSM network operator)

• Database contains:

IMSI (International Mobile Subscriber Identity)

MSISDN (Mobile Subscriber Integrated Services Digital Network Number)

prepaid/postpaid roaming restrictions

supplementary services

Visitor Location Register (VLR)

• Temporary database which updates whenever new MS enters its area, by HLR database

• Controls those mobiles roaming in its area

• Reduces number of queries to HLR

Authentication Center (AuC)

• Protects against intruders in air interface

• Maintains authentication keys and algorithms and provides security triplets. (Ki, Kc, etc.)

• Generally associated with HLR

Equipment Identity Register (EIR)

• Also used for security purposes

• Is a register containing information

about the mobile equipments

• A terminal is identified by its

International Mobile Equipment Identity

(IMEI)

• Forbids calls from unauthorized

terminals

GSM Interworking Unit

• Corresponds to an interface to various

data communications

• During these communications, the

transmission of speech and data can

be alternated

The Operation and Support Subsystem

Operation and Support Subsystem

• Is connected to the different components

of the NSS and BSC, in order to control

and monitor the GSM system

• Is also in charge of controlling the traffic

load in the BSS

• However, the increasing number of base

stations has provoked that some of the

maintenance tasks be transferred to the

BTS