Subject : Mobile Communication

Topic : GSM

Difficulty : Medium

Each time slot on a TDMA frame is called a physical channel.There are 8 physical channels per carrier frequency in GSM.Physical channels can be used to transmit speech, data or signalling information.

A physical channel may carry different messages, depending on information that is to be sent.These messages are called logical channels.For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic Channel (TCH) message.While handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message.Many types of logical channel exists, each designed to carry a different message to or from an MS.All information to and from MS must be formatted correctly, so that the receiving device can understand the meaning of different bits in the message.