kuldeep, i’m not sure i understand your question.
BCCH + CCCH = CBCH (combined BCCH), it is a mix of BCCH and SDCCH/4 signalling.
It is not mandatory to have CBCH in a cell.
It can be a possiblity to set a CBCH in a cell in order to replace one ts BCCH and one ts SDCCH. (explanation : If the cell has TCH congestion, you might want to remove the SDCCH/8 and replace it by a TCH timeslot, therefore you need to compact the BCCH and the SDCCH in one timeslot : the CBCH)
Another case : you have a cell with 1 TS BCCH + 1 TS SDCCH, but you notice SDCCH congestion. You don’t want to loose TCH capacity, so you might decide to keep the existing SDCCH and replace the BCCH by a CBCH. So in the end, you have the same TCH capacity but +50% of SDCCH capacity.