OK Thanks again but let me rephrase my question:
Firstly, yes you are right that the MS measures the received power level of the BCCH.
But in dedicated mode, the MS is locked on to the TCH, so how does it measure the received power level of the BCCH? Do we need to look at any layer 3 message that the MS received from the BCCH timelsot?
In our network, a cell usually runs on 4 TRXs –
1 BCCH TRX 900 band
1 TCH TRX 900 band
2 TRXs 1800 band
And the SDCCH and GPRS timeslots are also located on the BCCH TRX. So logically, most of the calls are established on the non BCCH TRXs. However, whenever there are cases of co BCCH, the call drop rate of a cell shoots up. My question is: most of the calls, once established, move to the TCH, so why are there so many drops just because the BCCH TRX is experiencing interference?