Let us consider a particular handover scenario; inter-cell and intra-BSC handover. Currently our MS is in a dedicated call served by cell ‘A’. Handover has to take place from cell ‘A’ to cell ‘B’. Both cell A and cell B belongs to same BSC ‘Z’.
MS sends an Access Burst on FACCH to cell ‘B’ and at the same time starts timer T3124. Now suppose this access burst never reach cell ‘B’ and is lost. In this case T3124 will eventually expire after which the MS will try to revert to old channel on cell ‘A’.
BSC holds onto the channel that MS was using on cell ‘A’ (i.e. resource is not released to be used by other calls for some time. Note that MS will send the access burst several times (until T3124 expires). If the access is successful and MS is handed over to cell ‘B’ the BSC sends release command to the held channel on cell ‘A’; in such a case reversion to old channel is not needed.