ok, yeah, 8 USF available, but only 6 for users : one for PRACH (even if not used) and one to say “nobody should transmit during next block”
there is always dedicated gprs channels. PRACH, PBCCH and PAGCH are NOT dedicated channels, but rather “common” channels.
Nobody uses those GPRS common channels, so your situation is very standard.
When a MS has a PDCH in UL, it means that it can send data during this TS when it hears its own USF on the concurrent DL timeslot. But if the BSS asks this MS to send signalling instead of data (for instance, a packet acknowledgment), then the PDCH carries signalling. It’s not like in GSM where there is statically one SACCH every 26 bursts.
If you activte NC2 though, the MS will send packet measurement report at regular interval.
Those signalling is carried physically on the PDCH, but logically it’s not a PDTCH anymore, it’s a PACCH.