What are different GSM Channels?


GSM Channels are categorized in 2 types:

  • Traffic Channels(TCH)
  • Signaling Channels


Traffic Channels (TCH):

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are basicallygroup of 26 TDMA framesThe 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.

TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thereby simplifying the electronic circuitry. This method permits complex antenna duplex filters to be avoided and thus helps to cut power consumption.

TCH is mainly 2 types.

  • Full Rate(TCH/F)
  • Half Rate (TCH/H)

TCH/F defined as 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.

Signalling channels:

Signalling channels also called Control channels. These are basically used on the air interface for the purpose of call establishment, paging, call maintenance, synchronization, etc.

There are three type of signalling channels

  • Broadcast Channels
  • Common Control Channels
  • Dedicated Control Channel


Broadcast Channels (BCH):

This channel carry only downlink information and it is 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 is categorised in 3 types.

Broadcast Control Channel (BCCH): It carry information like cell specific (local area code (LAC), network operator, access parameters, list of neighbouring cells, etc). The MS receives signals via the BCCH from many BTSs within the same network and/or different networks.


Frequency Correction Channel (FCCH): This channel used for correction of MS frequencies; transmission of frequency standard to MS.Also used for synchronization of an acquisition by providing the boundaries between timeslots and the position of the first timeslot of a TDMA frame.


Synchronization Channel (SCH): This channel used for frame synchronization (TDMA frame number) and identification of base station.


Common Control Channels (CCCH):

CCCH is group of uplink and downlink channels between the MS and the BTS. These are used to carry information from the network to MSs and provide access to the network. The CCCHs contain the following channels;

  • Paging Channel (PCH): This channel used to search the MS under BTS coverage for incoming calls or mobile terminating (MT) calls
  •  Access Grant Channel (AGCH): BTS used for allocating a TCH or SDCCH to the MS, thus allowing the MS access to the network.
  •  Random Access Channel (RACH): Within this channel, BTS allows the MS to request an SDCCH in response to a page or due to a call; the MS chooses a random time to send on this channel. This creates a possibility of collisions with transmissions from other MSs

Dedicated Control Channels (DCCH):

DCCH is responsible for GSM service like roaming, handovers, encryption, etc. DCCHs contains the following channels;

  • Stand-alone Dedicated Control Channel (SDCCH):This channel used as a communications channel between MS and the BTS.During call setup before a traffic channel (TCH) is allocated, SDCCH is used.
  • Slow Associated Control Channel (SACCH):ItTransmits continuous measurement reports in parallel to operation of a TCH or SDCCH
  • Fast Associated Control Channel (FACCH): It operates similar to SDCCH, but used in parallel to operation of the TCH; if the data rate of the SACCH is insufficient, “borrowing mode” is used: Additional bandwidth is borrowed from the TCH; this happens for messages associated with call establishment authentication of the subscriber, handover decisions, etc.







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