GSM-GPRS

A GSM physical channel is defined by a timeslot number (TN) and a frequency or set of
frequencies (called MA, mobile allocation in the specification) if the physical channel is frequency hopping.
The physical channels in GSM are derived from the basic characteristics of the radio
channel and the original voice data rate of 13 kb/s is raised to 22.8 kb/s with added
protective redundancy. These characteristics enable a radio channel to carry eight
physical channels.
A physical channel is a burst of radio energy. There are eight bursts of radio energy
in 4.615 ms and each burst is labelled, from TN0 to TN7.

GSM radio and physical channels. This diagram shows, at the top, the radio channels with their ARFCN (absolute radio channel frequency number) for the GSM900,1800 and 1900 systems. To the left is shown an extra allocation of frequencies for the 900 band, called the EGSM (extended GSM band). To the right is shown a cell transmitting and receiving on ARFCN 118. This radio channel is allocated, in this example, as a BCCH radio carrier. Timeslots 0 and 1 are reserved as control channels. It is seen that there is a three burst
offset between the MS receiving on Tn0 and transmitting on Tn0.