Tuesday, 1 July 2014


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

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.

iiRandom Access Channel ( RACH):-

•  It is a uplink Channel.
•  It is used by MS to request for SDCCH.

iiiAccess 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.
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)