There are mainly two types of GSM logical channels.

(i) Traffic channels (TCHs).

(ii) Control channels (CCHs).

Traffic channels carry digitally encoded user voice or user data and have identical formats of both forward link and reverse link. Control channels carry signal and synchronization commands between the base station and mobile station. Other control channels are used only for forward and reverse link. GSM traffic channel carry digital voice and user data either at half rate or at full rate.

Full Rate Traffic Channels (TCH):

(i) Full-rate speech channel (TCH/FS): This channel carries user speech in digitized form at a raw data rate of 13 kbps. GSM channel coding is added to digitized speech then the full rate speech channel carries 22.8 kbps.

(ii) Full-rate data channel for 9600 bps (TCH/F9.6): This channel carries raw user data which is transfers at 9600 bps with additional forward error correction applied by GSM, the 9600 bps data sent at 22.8 kbps.

(iii) Full-rate data channels for 4800 bps (TCH/F4.8): This channel carries raw user data which is transferred at 4800 bps with additional forward error correction applied by GSM, the 4800 bps is sent at 22.8 kbps.

(iv) Full rate data channel for 2400 bps (TCH/F2.4): This channel carries raw user data which is transferred at 2400 bps with additional forward error correction coding by GSM, the 2400 bps is sent at 22.8 bps.

Half-rate Traffic Channels:

(i) Half-rate speech channel (TCH/HS): This channel carries digitized speech which is sampled at a half rate then the full rate GSM channel coding added to digitized speech and half rate speech channel carry 11.4 kbps.

(ii) Half-rate data channels for 4800 bps (TCH/H4.8): This channel carries raw user data which is to be transferred at 4800 bps. With additional forward error correction applied by GSM, the 4800 bps data sent at 11.4 kbps.

(iii) Half-rate data channels for 2400 bps (TCH/H2.4): This channel carries raw user data which is to be transferred at 2400 bps with additional forward error correction by the GSM, the 2400 bps data sent at 11.4 kbps.

GSM Control Channels (CCH):

There are three control channels in GSM:

1. Broadcast control channels.

2. Common control channels.

3. Dedicated control channels.

1. Broadcast control channels (BCH) :

The BTS uses this channel to give information to all MSs within a cell. Information uses by this channel is cell and network identity, current control channel structure, channel availability and congestion. The broadcast control channel also sends the list of channels that are currently used within cell.

2. Common Control Channels (CCCH): All the information regarding setting up a connection between MS and BS is exchanged via the CCCH. The common control channel occupies TSO (frame) of GSM frame and that is not used by BCH and ideal channels.

3. Dedicated Control Channels (DCCH): They have same format and function on both forward and reverse links.