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GSM Architecture

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GSM-900 System Specifications

Frequency Range : 890 MHz to 915 MHz for Uplink

935 MHz to 960 MHz for Downlink.

Uses TDM / TDMA technology for Downlink / Uplink.

124 Reusable Spot frequencies of 200 kHz bandwidth each.

Each Spot Frequency carries 8 Time slots for Traffic/Signalling.

Separate Logical Signalling & Traffic channels.

Compatible to ISDN & PSPDN.

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GGlobal SSystem forMMobile

New rapidly expanding & successful technology.

Fully Digitized technology for better speech quality.

Available in almost every part of the world.

Fully compatible with existing Fixed Line Network.

Single number operation with World wide Roaming.

Very well defined interfaces makes truly open system

Encryption of user information

Available versions, GSM 900, 1800 & 1900

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GSM Network Architecture

BTS 3

BTS 2

BTS 1

BSC

PSTN N/W

HLR

VLR

AUC

EIR

SMSC

VMSC

MSC

OMC

NSS

BSS

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Mobile Station (MS)

Mobile Station consist of two unitsMobile Hand set Subscriber Identity Module

Mobile Hand set is one of the most complicated

GSM device. It provides user the access to the

Network. Each handset has unique identity no.

called IMEI.

SIM is a removable module goes into the mobile

handset. Each SIM has unique number called

International Mobile Subscriber Identity (IMSI).

It has built in Micro-computer & memory into it.

It contains the ROM of 6 to 16KB,RAM of 128 to

256 bytes and EEPROM of 3 to 8KB

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Base Transceiver Station (BTS)

BTS

Antenna

Arrangement

BTS has a set of Transceivers to talk to MS.

One BTS covers one or more than one cell.

Capacity of BTS depends on no of Transceivers.

BTS is connected to BSC via Abis interface.

Transmission rate on Abis is 2 Mbps (G.703).

Interface between MS & BTS is called Air I/f.

Transmission rate on Air interface is 13 Kbps.

BTS controls RF parameters of MS.

Each TRx has 8 TDMA channels to carry Voice & signalling.

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Base Station Controller (BSC)

BSC controls several BTSs.

BSC manages channel allocation, & Handover

of calls from one BTS to another BTS.

BSC is connected to MSC via A interface.

Transmission rate on A I/f is 2 Mbps (G.703).

Interface between BSC & BTS is called Abis I/f.

BSC has database for all of its BTSs parameters.

BSC provides path from MS to MSC.

BSC

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Mobile Switching Centre (MSC)

MSC is heart of the entire network connecting Fixed line network to

Mobile network.

MSC manages all call related functions and

Billing information.

MSC is connected to HLR & VLR for subscriber

identification & routing incoming calls.

MSC capacity is in terms of no of subscribers.

MSC is connected to BSC at one end and Fixed

Line network on other end.

Call Detail Record (CDR) is generated for each & every

call in the MSC.

MSC

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Visiting Location Register (VLR)

VLR

MSC

Active Subscriber is registered in VLR.

It has temporary data base of all the active

subscribers used for their call routing.

HLR validates subscriber before registration.

MSC ask VLR before routing incoming call.

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Home Location Register (HLR)

HLR

MSC

All Subscribers data is stored in HLR.

It has permanent data base of all the registered

subscribers.

HLR has series of numbers for all subscribers.

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Authentication Centre (AUC)

HLR

MSC

Authentication is a process to verify the subscriber SIM.

Secret data & verification algorithm are stored in to the

AUC.

AUC & HLR combined to authenticate the subscribers.

Subscriber authentication can be done on every call, if required.

AUC

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Equipment Identity Register (EIR)

EIR

MSC

All subscriber's mobile handset data is stored in EIR.

MSC asks mobile to send it IMEI & then checks it with

data available in EIR.

EIR has different classification for mobile handsetslike, White list, Grey list & Black list.

According to category the MS can make calls or can be

stopped from making calls.

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Operation & Maintenance Centre (OMC)

OMC All the network elements are connected to OMC.

OMC monitors health of all network elements &

carry out maintenance operation, if required.

OMC link to BTSs are via parent BSC.

OMC keeps records of all the faults occurred.

OMC can also do Traffic analysis.

OMC may prepares MIS Report for the network.

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VMSC & SMSC

MSC

SMSC VMSC

Voice Mail Service Centre : To provide Voice Mail service.

It has database for all the VMS subscribers & also stores voice

messages for them.

Short Message Service Centre : To provide text message service.

To send short messages from mobile to another mobile subscriber.

Messages can also be sent by Manual Terminal connected to SMSC.

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Call Processing in GSM 1

BTSBSC PSTN N/WMSC

Mobile Originated Call

Mobile originated calls goes to BTS first & then to BSC.

BSC forwards this call to MSC.

MSC does authentication & call routing as per dialed digits.

If it the call to another mobile subscriber then the process for that

call is same as Mobile Terminated Call as shown in next slide.