PDCH preemption occurs when a CS call is established on the cell by the BSC, on a timeslot that is used by GPRS.
PDCH preemption is divided in two types:
soft preemption is performed when the preempted PDCH carries a PACCH, or if the radio PDCH itself is not preempted, but one Abis nibble of its MEGCH will be.
during soft preemption the MFS has 4 seconds to reallocate this PDCH to another timeslot, in order to save the PACCH (save the PACCH = avoid a TBF drop), or reallocate in order to save the current MCS.
after 4 seconds, if nothing was done, fast preemption is done. If the reallocation (T1) was succesful, then nothing more has to be done.
fast preemption means that the PDCH is removed from the timeslot, without any delay. It will happen if the PDCH doesn’t carry a PACCH for instance.
MFS and BSC just exchange a bitmap of timeslot “status”. The MFS informs the BSC about which TS are allocated as PDCH or not, and the BSC tells the MFS which PDCH timeslot shall be deallocated ASAP.
With DTM, there are now a little bit more info on this bitmap (about DTM timeslots).