GSM Channels



During this study we will learn about

  • What is channel ?
  • Classification of channel ?
  • What are Traffic channel and Control channel ?
  • Different type of Logical channels such as :-
  • Common control channel
  • Dedicated control channel
  • Broadcast control channel


  • In telecommunications a channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel.
  • The channel used in the air interface is divided into two types: Physical channel and Logical channel.
  • Physical channel : It is the medium over which the information is carried.
  • Logical channel : It consist of information carried over a physical channel.

Physical Channel

When an MS and a BTS communicate, they do so on a specific pair of radio frequency (RF) carriers, one for the up-link and the other for the down-link transmissions, and within a given time slot. This combination of time slot and carrier frequency forms what is termed a physical channel .
One RF channel will support eight physical channels in time slots zero through seven.

Logical channels

  • It transports either user data during a call or signalling information for MS or base station.
  • The data, whether user traffic or signalling information, are mapped onto the physical channels by defining a number of logical channels .
  • Two groups of logical channels:
    • Traffic Channels - for call data
    • Control channels - To communicate service data between network equipment nodes.

From Physical Channel to Logical


Logical channels

  1. Traffic Channels (TCHs) - The traffic channels are intended to carry encoded speech or user data.
  2. Control Channels (CCHs) - The control channels are intended to carry signalling and Synchronization data between the base station and the Mobile station. Logical are used by the system and the MS for different purposes

Classification of logical channel

  1. Traffic Channel

  2. Traffic channel (TCH) : Traffic channels are intended to carry encoded speech and user data.

    • Full rate traffic channels at a net bit rate of 22.8 Kb/s (TCH/F)
    • Half rate traffic channels at a net bit rate of 11.4 Kb/s (TCH/H)
    Speech Channels : Speech channels are defined for both full rate and half rate traffic channels. Data Channels : Data channels support a variety of data rates (2.4, 4.8 and 9.6 Kb/s) on both half and full rate traffic channels. The 9.6 Kb/s data rate is only for full rate application

  3. Control channel

  4. Control channels carry signalling information between an MS and a BTS. There are several forms of control channels in GSM, and they can generally be divided into three categories according to the manner in which they are supported on the radio interface and the type of signalling information they carry.

    1. Broadcast control channel
    2. Common control channel
    3. Dedicated control channel

Broadcast control channels

  • Downlink Only
  • Broadcast control channels are transmitted in downlink direction only i.e. only transmitted by BTS.
  • The broadcast channels are used to broadcast synchronization and general network information to all the MSs within a cell. Such as Location Area Identity (LAI) and maximum output power.

It has three types


  1. Frequency Correction Channel (FCCH)

    • Used for the frequency correction / synchronization of a mobile station.
    • The repeated (every 10 sec) transmission of Frequency Bursts is called FCCH.
    • FCCH is transmitted on the downlink, point-to-multipoint.
  2. Synchronization Channel (SCH)

    • Downlink Only
    • Allows the mobile station to synchronize time wise with the BTS.
    • Repeated broadcast (every 10 frames) of Synchronization Bursts is called (SCH)
    • SCH is transmitted on the downlink, point to multipoint.
  3. Broadcast control channel(BCCH)

    • The broadcast control channel(BCCH) is used to broadcast control information to every MS within a cell.
    • This information includes details of the control channel configuration used at the BTS, a list of the BCCH carrier frequencies used at the neighbouring BTSs and a number of parameters that are used by the MS when accessing the BTS.
    • BCCH is transmitted On the downlink, point-to-multipoint.

Common Control Channel

The common control channels are used by an MS during the paging and access procedures.

Common control channels are of following types
  1. Random Access Control Channel (RACH)
  2. Paging Channel (PCH)
  3. Access Grant Control Channel (AGCH)
  4. Cell Broadcast Channel (CBCH)

  1. Random Access Control Channel
    • Uplink Only
    • Transmitted by the mobile when it wishes to access to the system
    • This occurs when mobile initiates a call or responds to a page.
  2. Paging Channel
    • Downlink Only
    • Transmitted by the BTS when it wishes to contact a mobile.
    • The reason for contact may be an incoming call or short message.
  3. Access Grant Control Channel
    • Downlink Only
    • It carries data which instructs the mobile to operate in a particular physical channel (Time slot).
    • The AGCH is used by the network to grant, or deny, an MS access to the network by supplying it with details of a dedicated channel, i.e. TCH or SDCCH, to be used for subsequent communications
  4. Cell Broadcast Channel
    • Downlink Only
    • This channel is used to transmit messages to be broadcast to all mobiles within a cell e.g. traffic info.
    • The CBCH will steal time from SDCCH.

Dedicated Control Channel

Signalling information is carried between an MS and a BTS using associated and dedicated control channels during or not during a call.

The are of following type :-

  1. Standalone Dedicated Control Channel (SDCCH)
  2. Associated Control Channel (ACCH)
  3. Slow Associated Control Channel (SACCH)
  4. Fast Associated Control Channel (FACCH)

  1. Standalone Dedicated Control Channel (SDCCH)
    • Downlink Only
    • The MS is on the SDCCH informed about which physical channel (frequency and time slot) to use for traffic (TCH).
    • It also carries information for call forwarding and Transmission of short message.
  2. Associated Control Channel (ACCH)
    • These Channel Could be associated with either a SDCCH or a TCH.
    • They are used for carrying out information associated with the process being carried out on either SDCCH or TCH.

    They are of two type

    1. Fast ACCH
    2. Slow ACCH
  3. Slow Associated Control Channel (SACCH)
    • Downlink Only
    • Conveys power control and timing information in the downlink direction.
    • Receive signal strength Indicator and link quality report in uplink direction.
    • It occupies one timeslot in every 26. SACCH messages may be sent once every 480ms, i.e. approximately every 2 s.
  4. Fast Associated Control Channel (FACCH)
    • Downlink Only
    • The FACCH steal the TCH bust and inserts its own information.
    • The FACCH is used to carry out user authentication and handover.
    • A complete FACCH message may be sent once in every 20 ms.