GSM CHANNELS

Channel Concepts In GSM

Channel concepts:

• In GSM 900 band, there are 124 carriers.
• In GSM 1800 band, there are 374 carriers.
• In GSM 1900 band, there are 299 carriers.
• The carrier separation in the above bands are 200 kHz..
• Each  carrier is divided into 8 timeslots (TS).
• Each timeslot (TS) is referred as physical channel.
• Physical channels are further divided into different logical channels.
• Logical channels are separated into two categories:-

      1. Traffic Channels.

      2. Signaling/Control channels. 

Traffic Channels:-

Traffic channels (TCH) carry speech and data traffic.

There are two types of Traffic channels:

¤. Full Rate Channel (TCH/F) at the gross rate of 22.8 kbits/sec.

¤. Half Rate Channel (TCH/H) at the rate of 11.4 kbits/sec.

Traffic Channel / Full:-

•  Gross rate of 22.8 kbits/sec.

•  Use when load on the network is low.

•  1 Timeslot is given to 1 user.

• TCH/F carries user speech at 13 kbps and data at 9.6  kbps,4.8 kbps and 2.4 kbps.

• User data rates for the TCH/H are 4.8 kbps and 2.4 kbps.

Traffic Channel / Half:-

•  Gross rate of 11.4 kbits/sec.

•  Use when load on the network is high.

•  1 Timeslot is given to 2 user.

Signaling Channels are divided into three

categories:-

1. Broadcast Channels.

2. Common Control Channels.  

3. Dedicated Control Channel.

Broadcast Channels (BCH):-

These channels are transmitted down link only.

Helps the MS to tune always to the current network.

Acts like the light from a light house for the MS.

Three types of Broadcast channels available in GSM:-

i.  Broadcast Control Channels (BCCH)

ii. Frequency Correction Channel  (FCCH).

iii. Synchronization Channel (SCH).

i. Broadcast Control Channel ( BCCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

•  Give the information of LAI ( Location Area Identity) & CGI ( Cell Global Identity).

•   Tell about BSIC ( Base Station Identity Center).

•   Provide neighboring  frequency data.

•   Also deals with power i.e. how much power is given to MS.

ii. Frequency Correction Channel ( FCCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

•  Used for frequency synchronization.

iii. Synchronization Channel ( SCH ):-

•  It is a Downlink Channel.

•  Also Unidirectional.

• Used for time and frame synchronization.

Common Control Channels (CCCH):-

Point to multipoint bi-directional channels. 

Used to establish connections between MS and BS.

Before a DCCH is assigned to a MS.

Three types of Common Control channels available:-

i.  Paging Channel  (PCH).

ii. Random Access Channel (RACH).

iii. Access Grant Channel (AGCH).

i. Paging Channel ( PCH):-

•  It is a Downlink Channel.

•  Also Unidirectional.
•  With the help of PCH Network, MTC sends the information to MS via MSC, BSC & BTS
to immediately  report back to network as there may be any incoming call or SMS.

ii. Random Access Channel ( RACH):-

•  It is a uplink Channel.

•  It is used by MS to request for SDCCH.

iii. Access Granted Channel ( AGCH):-

•  It is a Downlink Channel.

•  Also Unidirectional.
•  Its is an Acknowledgement for RACH.

Dedicated control Channel ( DCCH):-

Used for call set up procedure.

Used for signaling and control after call.

Establishment.

Also used for SMS transmission.

Four types of Dedicated control channel available:-

i.  Stand alone Dedicated Control Channel (SDCCH).

ii.  Slow Associated Control Channel (SACCH).

iii. Fast Associated Control Channel (FACCH).

iv. Cell Broadcast Channel (CBCH).

i. Stand alone Dedicated Control Channel (SDCCH):-

•  It is the most important channel.

•  Its a bi- directional channel.

•   Used for call setup.

•   It is also used in authentication.

•   IMSI number checking is done by SDCCH.

•   Its is also used for SMS transfer in Ideal Mode.

•   Its also used in location updation.

•   Also handles user Encryption Process.

ii. Slow Associated Control Channel (SACCH):-

• Its a bi- directional channel.

• It handle the SMS transfer in Dedicated / Active Mode and also Measurement Report in

    dedicated mode.

• Also deals with power control.

• Also handle call waiting.

• used to convey transmit power level control instructions and timing advance information

     to the GSM mobile.

• In the uplink, SACCH carries received signal strength, TCH quality information and

    measurement report of neighbouring cells etc. 

• SACCH reports neighbour cell information such as signal level to the serving cell 

    which helps make handover decision.

•  SACCH message may be sent once every 480 ms (i.e. 2 Sec) duration.

iii. Fast Associated Control Channel (FACCH):-

•  Its a bi- directional channel.

•  It handle Handover Process.

•  FACCH information message can easily be transmitted in 20 ms duration. 

 (As compare to SACCH it is too much  Fast).

• It is used to send fast message like hand over message.

iv. Cell Broadcast Channel (CBCH):-

•  Its a unidirectional channel.

•  It is only downlink.

•  Used by the Operators to Broadcast SMS (like flash SMS, Activity Promotion through SMS)