gsm traffic channels

Traffic Channels (TCH) in GSM

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are 

defined using a 26-frame multi frame, or group of 26 TDMA frames. The length of a 

26-frame multi frame is 120ms. Out of the 26 frames, 24 are used for traffic, 1 is 

used for the slow associated control channel

(SACCH) and 1 is currently unused.

Full Rate & Half Rate TCH

They can be defined as full-rate TCHs (TCH/F, 22.8 kbps) and half-rate TCHs 

(TCH/H, 11.4 kbps). Half-rate TCHs double the capacity of a system effectively 

by making it possible to transmit two calls in a single channel. If a TCH/F is used for 

data communications, the usable data rate drops to 9.6 kbps (in TCH/H: max. 4.8 kbps) 

due to the enhanced security algorithms. Eighth-rate TCHs are also specified, and are 

used for signaling. In the GSM Recommendations, they are called stand-alone dedicated

 control channels (SDCCH)

Signaling channels

The signaling channels on the air interface are used for call establishment, paging, 

call maintenance, synchronization, etc.

There are three type of signaling channels

Broadcast Channels

Common Control Channels

Dedicated Control Channel

Broadcast Channels (BCH)

Carry only downlink information and are responsible mainly for synchronization and 

frequency correction. This is the only channel type enabling 

point-to-multipoint communications in which short messages are simultaneously 

transmitted to several mobiles

BCH Characteristics

1.     • Each cell has a designated BCH carrier

2.     • All BCH timeslots transmit continuously on full power

3.     • TS 0 contains logical control channels

4.     • TS1-7 optionally carries traffic

5.     • BCCH block occur once each 51-frame multiframe

6.     • Each block comprises 4 frames carrying 1 message

The BCHs include the following channels

7.     Broadcast Control Channel (BCCH): General information, cell specific (local area 

code (LAC), network operator, access parameters, list of neighboring cells, etc). The MS 

receives signals via the BCCH from many BTSs within the same network and/or different 

networks.

8.     Frequency Correction Channel (FCCH): Downlink only; correction of MS frequencies; 

transmission of frequency standard to MS; it is also used for synchronization of an 

acquisition by providing the boundaries between timeslots and the position of the first 

timeslot of a TDMA frame.

9.     Synchronization Channel (SCH): Downlink only; frame synchronization 

(TDMA frame number) and identification of base station. The valid reception of one 

SCH burst will provide the MS with all the information needed to synchronize with a BTS