There is no message to signal the start of PC.
It is all up to the BSC, the MS doesn’t have to know anything.
As soon as PC is enabled, the BSC will start sending power control orders to MS and BTS.
The power control orders is not really an order : it is just that instead of telling the MS to transmit at 33dBm (for ex.) it will say “30dB” or “28dB”, etc.
By default, with PC disabled, the BSC always transmit the value 33dBm.
To the BTS, for the DL PC, it says “0dB”, which means the BTS should attenuate its tx power by 0dB.
I hope i was clear… hum…