GSM channel concept

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Channel concepts:

There are two types of channel in GSM Network - Control channel and Traffic channel: 

The traffic Channels:  are used to transport encoded speech and data information. Full rate traffic channels TCH/F are defined using a group of 26 TDMA frames called a 26 frame multi-frame. The 26 frame multi-frame lasts 120ms and the traffic channels for the downlink and uplink are separated by three bursts. The TCH/F consists of one time slot in each TDMA frame i.e., one slot every 4.615ms.

The control Channels: are used for network management messages (call set up, control signaling etc.) and some channel maintenance tasks. These can be subdivided into BCH (Broadcast Channel), CCCH (Common Control Channel), and DCH (Dedicated Channel)

BCH: BCCH, FCCH, SCH / CCCH: RACH, AGCH, PCH / DCH: SDCCH, SACCH, FACCH, CBCH

BCCH (Broadcast channel): Its downlink channel is used for transmitting of system information unique BCCH define for entire cell and it is transmitted on Timeslot zero of BCCH carrier and Read only by idle mobile at least once every 30 secs

SCH (Synchronization Channel): It’s a Downlink channel and Carries information for frame synchronization and Contains TDMA frame number and BSIC.

FCCH (Frequency Correction Channel) it is a Downlink only and Enables MS to synchronize to the frequency and also helps mobiles of the ncells to locate TS 0 of BCCH carrier

RACH (Random Access Channel) It is Uplink only and Used by the MS to access the Network.

AGCH (Access Grant Channel) It is Downlink only and Used by the network to assign a signaling channel upon successful decoding of access bursts.

PCH (Paging Channel): Downlink only and Used by the Network to contact the MS.

SDCCH (Standalone Dedicated Control Channel): Its Uplink and Downlink and Used for call setup, location update and SMS.

SACCH (Slow Associated Control Channel): Used on Uplink and Downlink only in dedicated mode and Uplink SACCH messages - Measurement reports. Downlink SACCH messages - control info.

FACCH (Fast Associated Control Channel): Uplink and Downlink and Associated with TCH only and it are used to send fast messages like handover messages. Works by stealing traffic bursts

CBCH (Cell broadcast channel): It is used transmitting of broadcasting msg

TCH/f and TCH/H : gross rate 22.8kbps, speech 13kbps, data 9.6kbps     (gross rate = 57*2*24/120ms = 22.8KBPS)

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