GSM logical channels and their functions

This page on GSM tutorial covers GSM logical channels and their functions. It covers FCCH, SCH, BCCH, CCCH, RACH, AGCH, PCH, CBCH, SACCH, SDCCH and FACCH.

As shown in the figure there are two main types of channels in the GSM. Traffic channels and control channels. Different bursts are mapped to these channels uniquely as per GSM TECHNICAL SPECIFICATION 05.02.

Traffic channels carry speech or data. There are two main categories here, Full rate (13 kpbs) and Half rate.

Control channels used to for control/command/signaling. Control channels are divided into three categories.
Category 1: Broadcast channels
As the name suggests they are point-to-multipoint and downlink only channels.
FCCH: Frequency correction control channel, this is transmitted by BTS to MS. This helps MS tune its local oscillator to exact RF carrier frequency of the BTS cell. All zero sequences are transmitted here which will produce fixed tone at the output of GMSK modulator. The frequency value will be about 67.7075 KHz.

SCH:synchronization channel, this carry BSIC(Base transceiver station identity code) and Frame number which helps MS tune to specific (Frequency,Ts) physical slot on TDMA frame in GSM network.
BCCH: Broadcast control channel, carry CGI,MNC,MCC which is received by MS. It is compared with SIM information, once varified OK connection is established with the network.
Category 2: Common Control channels
They are point-to-multipoint and downlink only channels except RACH which is used in uplink.
PCH:Paging channel,When someone is calling mobile phone, this channel sent information on downlink to alert called mobile phone.This is known as mobile phone terminated call.

RACH:Random Access channel, used in mobile originated call. When mobile wants to call some other mobile phone, control information is sent on this channel.
AGCH:Access Grant Channel,transmitted by BTS to MS once network approves request of mobile by RACH.
CBCH:Cell Broadcast channel, Used to carry the short message service cell broadcast.
Category 3: Dedicated Control channels
They are bidirectional and point-to-point Channels.
SDCCH: Stand alone dedicated control channel, used for call setup.
SACCH: Slow associated control channel, is used for control and supervisory signals associated with the traffic channels.
FACCH: Fast associated control channel, is used for control requirements such as handoff/handovers.

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