Hi Pix,
you make reference to SID frames (SID FIRST, SID UPDATE and SID ONSET, sent respectively at the beginning of silent period, refreshed every 8-9 frames, and at the end of the silent period) containing information about comfort noise. They are transmitted with the standard power commanded by PC algorithm. During the silence period however empty frames (NO DATA, NO SPEECH frames) are sent with a very low power, near to 0 dBm (on BTS side).
I refer to Siemens BSS.