“Once a time slot is assigned to a user means, whether the same time slot will be associated with the user till he ends his call or any possibilities for change in time slot??
The timeslot can be changed, and will most probably be changed during the average 2mn lifetime of a call.
1/ user is mobile and needs the call to be handed over to a neighbour cell : the TCH will be allocated to another cell, and this is managed by the BSC.
2/ traffic condition is the cell requires the call rate to be modified, from Full Rate to Half Rate (or vice versa). In this case the call will most likely be allocated to another physical timeslot.
3/ quality is poor and the BSC thinks it is caused by frequency interference. In this case the BSC moves the call to a timeslot located on another TRX, with a different frequency.
Those are 3 basic scenarios in which the timeslot on which the call is set is modified during the call. There are some other scenarios as well.