Traffic Channel: A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). 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 (see Figure 2). 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, thus simplifying the electronics.

In addition to these full-rate TCHs, there are also half-rate TCHs defined, although they are not yet implemented. Half-rate TCHs will effectively double the capacity of a system once half-rate speech coders are specified (i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are also specified, and are used for signalling. In the recommendations, they are called Stand-alone Dedicated Control Channels (SDCCH).

Signalling Channel:

This channel carries control information to enable the system to operate correctly.  The signalling channels on the air interface are used for call establishment, paging, call maintenance, synchronisation etc. The main categories of signalling channels in GSM which are further divided in several categories:

1. BROADCAST CHANNELS (BCH): This channel carry only downlink information and are responsible mainly for synchronization and frequency correction. This is the only channel type enabling point to multi-point communications in which short messages are simultaneously transmitted to the several mobiles. The BCHs include the following channels:

Broadcast Control Channel (BCCH): This channel handle General information, cell specific, e.g. local area code (LAC), network operator, access parameters, list of neighbouring cells, etc. The mobile station (MS) receives signals via the BCCH from many BTSs within the same network and/ or different networks.

Frequency Correction Channel (FCCH):  This channel carry only downlink information and handle the correction of MS frequencies, transmission of frequency standard to MS, it is 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 carry only downlink information and handle

The frame synchronization and identification of base station. The valid reception of one SCH burst will provide the MS with all the information needed to synchronize with a BTS.

2. COMMON CONTROL CHANNELS (CCCH):

This channel carry a group of uplink and downlink channels between the MS card and the BTS. These channels are used to convey information from the network to MSs and provide access to the network. The CCCHs include the following channels:

Paging Channel (PCH): This channel carry only downlink information. MS is informed by the BTS for incoming calls via the PCH.

Random Access Channel (RACH): This channel carry only uplink information. It 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 MS. The PCH and AGCH are transmitted in one channel called the paging and access grant channel (PAGCH). They are separated by time.

Access Grant Channel (AGCH): This channel carry only downlink information. BTS allocates a TCH or SDCCH to the MS, thus allowing the access to the network.

3. DEDICATED CONTROL CHANNELS (DCCH):

These channels are responsible for roaming, handovers, encryption etc. These channels include the following channels:

Standalone Dedicated Control Channel (SDCCH): This channel carry both uplink and downlink information. In response of RACCH, network allocates SDCCH over AGCH for further communication between BTS and MS. This channel is used for the location update, voice call setup and SMS.

Fast Associated Control Channel (FACCH): This channel carry both uplink and downlink information. This channel is used to convey handover information. There is no TS and frame allocation dedicated to this channel. This channel can be associated with SDCCH or TCH and works on the principle of stealing. The burst of TCH is replaced by FACCH signalling when required.

Slow Associated Control Channel (SACCH):This channel carry both uplink and downlink information. This channel is always associated with TCH or SDDCH used for control and supervision of signals associated with the traffic channels. This channel is used to convey the periodic carrier signal strength measurements to the network, transmit power control and timing advance.