if you set RRBP = 3, it means that the MS is going to send the PDAN (packet dl ack/nack) in the the third UL block, starting from now.
“3” is the only available value in alcatel system, so it’s a safe bet to use this value.
On your side (MFS to MS), it means that the USF specified in the third DL block from now should be set to … errr… 0 ? now i’m not sure anymore… is there a special usf value to allow the MS to send a PDAN ? I’ll let you check it out.