there is two type of channel configuration:
combined and non combined
now it depends on ur toplogy
As we have multiframe of 51 frame and in 0 timeslot of every frame u r giving signalling and other timeslot will be used for TCh.
so now in combined configuration there will be 4 sdcch block,2 sacch block, 3 cch and one idle frame so it will be used when u have less signalling load.
while in more signalling load u can use noncombined channle configuration in which u have 8 sdcch in 1st timeslot of every frame so tch will be 6 timeslot.so u can use it in less traffic mode.