As we know, every cell has several TRX and every TRX includes 8 timeslots (i.e. providing 8 basic physical channels). In the radio subsystem, the physical channel supports the logical channel based on the type of message transmitted . In this way, the physical channels are mapped as different logical channels. In the GSM system, the logical channel is classified as the dedicated channel (DCH) and the common channel (CCH). Sometimes, it can also be classified as the traffic channel and control channel.
The traffic channel (TCH) carries voice or data, which are the full-rate traffic channel (TCH/F) and half-rate traffic channel (TCH/H). These two types of channels carry information at the rates of 13 kbit/s and 6.5 kbit/s respectively. The channel using half of the time slots of a full-rate channel is the half-rate channel. Therefore, a carrier can provide 8 full-rate or 16 half-rate traffic channels.
The frequency correction channel (FCCH) carries the information for frequency correction of MS and BTS.
The control channel (CCH) is used to transmit signaling or synchronous data. There are mainly 3 types of control channels: Broadcast Channel (BCCH), Common Control Channel (CCCH) and Dedicated Control Channel (DCCH).