Cell reselection

While camped on a cell of the registered PLMN ("camped normally"), the MS may need to select a different cell ("normal cell reselection" state). The following events trigger a cell reselection:
i) The path loss criterion parameter C1 (see subclause 3.6) indicates that the path loss to the cell has become too high;
ii) There is a downlink signalling failure (subclause 3.6);
iii) The cell camped on (current serving cell) has become barred;
iv) There is a better cell (in terms of the path loss criterion C2) in the same registration area, or a much better cell in another registration area of an equivalent PLMN (using the CRH parameter, subclause 3.4);
v) A random access attempt is still unsuccessful after "Max retrans" repetitions; "Max retrans" being a parameter broadcast on control channel;
vi) Upper layers have determined that the network has failed an authentication check (see subclause 3.5.5).

Ref: 03.22 - sec 4.5

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.