BASIC CONCEPT OF GSM

NETWORK………………

Before discuss the require topic I want to discuss some basic concepts of GSM.

Basic concepts:

GSM Subsystems:GSM architecture is mainly divided into three Subsystems

1. Base Station Subsystem (BSS)

2. Network & Switching Subsystem (NSS)

3. Operations & Support Subsystem (OSS)

Mobile Station (MS)The MS consists of two parts

1.Mobile Equipment (ME) 2. Subscriber Identity module (SIM)

The ME is the only part of the GSM network which the subscriber will really see.

The SIM is a card which plugs into the ME.

This card identifies the MS subscriber and also provides other information regarding the services that subscriber should

receive.

The SIM card, and the high degree of inbuilt system security, provides protection of the subscriber’s information and

protection of networks against fraudulent access.

The SIM can be protected by use of Personal Identity Number (PIN) password, similar to bank/credit charge cards, to

prevent unauthorized use of the card.

SIM cards are designed to be difficult to duplicate.

The SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI)

Temporary Mobile Subscriber Identity (TMSI)

Location Area Identity (LAI)

Subscriber Authentication Key (Ki)

Mobile Station Integrated Services Digital Network (MSISDN)

The SIM also executes the Authentication Algorithm.

Base Station Subsystem (BSS) The BSS consists of two parts

1-Base Station Controller (BSC ) 2- Base Transceiver Station (BTS)

The BSC provides the control for the BSS.

Any operational information required by the BTS will be received via the BSC.

Likewise any information required about the BTS (by the OMC for example) will be obtained by the BSC.

The BTS provides the air interface connection with the MS.

It also has a limited amount of Control functionality which reduces the amount of traffic passing between the BTS and

BSC.

Where the BSC and BTS are both shown to control a function, the control is divided between the two, or may be located

wholly at one.

Network Switching System (NSS)

• The Network Switching System includes the main switching functions of the GSM network.

• It also contains the databases required for subscriber data and mobility management.

• The components of the Network Switching System are listed below:

1. Mobile Services Switching Centre – MSC

2. Home Location Register – HLR

3. Visitor Location Register – VLR

4. Equipment Identity Register – EIR

5. Authentication Centre – AUC

6. Interworking Function – IWF (Interface between the GSM network and the PSTN.)

• MSC is the heart of the system, controlling the Switching.

• The MSC can carry out different functions depending upon its position in the network.

• When provides interface between PSTN & BSS in GSM network then known as a Gateway MSC.

Operations & Support System (OSS)

• The OSS provides the capability to manage the GSM network remotely.

Channels on Air Interface:

There are 124 pairs of carriers in the GSM 900 band. Using Time Division Multiple Access (TDMA) each of these

carriers is divided into eight Time Slots (TS). One TS on a TDMA frame is called a physical channel i.e. on each duplex

pair of carriers there are eight physical channels. The logical channels consist of the information carried over the physical

channel. There are two major categories of GSM Logical channels

1. Traffic channels

2. Control channels.

Traffic channel Full rate: Used for speech at 13 Kbits/s, or sending data at 9.6 Kbits/s

Half rate: Used for speech at 6.5 Kbits/s, or sending data at 4.8 Kbits/s

Enhanced Full rate: Used for speech at 13 Kbits/s, or sending data at 9.6 Kbits/s.

Note: I know how our voice is converting into

13kbps, during the call setup.

But this is not related to my require topics so that

Why I am not discussing here.

Control channels or signaling channels Control channels are further divided into three categories

1- Broadcast Channels

FCCH = FREQUENCY CORRECTION CHANNEL (Downlink only)

To tell the Mobile that this is the BCCH carrier, to able the Mobile to synchronize to the frequency.

SCH = SYNCHRONISATION CHANNEL (Downlink only)

Used for sending BSIC (Base station Identity Code), Give TDMA frame number to the Mobile.

BCCH = BROADCAST CONTROL CHANNEL (Downlink only)

Used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity),

CBCH = CELL BROADCAST CHANNEL (Downlink only)

Used for sending short messages to all the mobiles within a geographic area.

2- Common Control Channels

PCH = PAGING CHANNEL (Downlink only)

Used for paging the Mobile. Reason could be an incoming call or an incoming Short Message.

RACH = RANDOM ACCESS CHANNEL (Uplink only)

Used for responding to the paging (terminating), Location updating or to make call access (originating) by

asking for a signaling channel.

AGCH = ACCESS GRANT CHANNEL (Downlink only)

Used to allocate SDCCH to the mobile.

3- Dedicated Control Channels

SDCCH = STAND ALONE DEDICATED CONTROL CHANNEL (Uplink & Downlink)

Used for allocating voice channel (TCH) to the mobile (call setup) and location updating.

Send short Text message to Idle Mobile.

SACCH = SLOW ASSOCIATED CONTROL CHANNEL (Uplink & Downlink)

Used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Are Identity), BCCH

of all the neighbors and TA (Timing Advance).

FACCH = FAST ASSOCIATED CONTROL CHANNEL ( Uplink & Downlink)

Used for handover.

TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7

200 kHz4.615 ms0.577 ms

FCCH,SCH,BCCHPCH,RACH,AGCH

SDCCH,SACCHCBCH

TCH,SACCHFACCH

TCH,SACCHFACCH

TCH,SACCHFACCH

TCH,SACCHFACCH

TCH,SACCHFACCH

TCH,SACCHFACCH

1 TDMA FRAME