I should have been a bit clearer in the original post.
The Immediate Assign is for an Packet Uplink Assignment in response to a single-phase channel request. Once the PDCH is active I get data blocks on the channel, which is good. Once I’ve got all the data blocks then I acknowledge the data, then the MS stops sending data blocks and sends a Packet Control Acknowledge, which is also good.
What I want to do then, while the PDCH is still allocated, is to allocate a downlink TBF. As I understand it I use a Packet Downlink Assignment for this, but the MS never seems to respond to this.
I think there are two possibilities. One is that there is a timing issue, and the Packet D/L Assignment has to go on a specific block, or has to be sent before the uplink TBF is acknowledged, or something of the sort. The other is that the Packet D/L Assignment message is not formatted correctly, and this is possible although I’ve tried adding and removing parameters, etc., with no success.
If I can rule out any timing issue then I can focus on the message format (and maybe configuration issues).