The Logical Channel is used in time multiplex in a physical channel, which is categorized according to the types of messages transmitted in the physical channel. Different logical channels are used in transmission of different types of information between BS and MS, such as the signaling or traffic data. In GSM system, five different types of burst sequences are specified for different logical channels, which have different time-amplitude diagrams as shown in the above diagrams.
The training sequence helps to discriminate radio channels with same frequency so as to help to demodulate the signals. However, there is no training sequence for FB and DB; for SB and AB, the training sequence is constant, i.e. the synchronous bit; for NB, there are 8 different training sequences specified in the specifications. These 8 different training sequences of NB are numbered from 0 to 7, which are called training sequence numbers. By allocating training sequences with distinct differences to channels of the same frequency used in cells that are close to and may interfere with each other, the co-frequency interference can be avoided efficiently during modulation.