May I can explain you properly MKT… Let me try….tell me if m wrong.
As you should know there is 26 Frams of TCH in one multi frame.
in this frame one slot is alloted to SDCCH i suppose that is 12th slot. Now this SDCCH is used to carrly all signalling information during call. Like Measurment, Power Management etc. It also includes HO_COMMAND if required.
Now if BSC decided to do HO and in this multiframe slot number 14 is going on and now BSC can not wait for such a long period because call may ge dropped and at that time BSC decided to steal TCH and replaced by FACCH and gives HO_COMMAND.
So Both channels are atually used for carring out HO_COMMAND.