GPRS' PDCH idle filling

Hi all, Does anyone know how should we fill PDCH when no downlink transmission is present? I can't find any mention about this in the standard. If someone have traces from existing networks, I would very much appreciate this. Some background - we've recently started working on GPRS implementation for OpenBTS, and we're stuck at the very beginning - mobile sends RACH, but doesn't respond to our Packet Channel Immediate Assignment. After spending quite some time tweaking Immediate Assignment message, our conclusion is that mobile may want to see some "filler" date at the idle PDCH and we don't do this yet. I know that OpenBSC has GPRS working, so I hope someone here may help us. -- Regards, Alexander Chemeris.