IuUP, aka voice between 2G and 3G

Thinking ahead to the 35C3, I have tinkered with IuUP a bit. I'm at the point where I have an IuUP FSM which auto-detects IuUP peers and removes IuUP headers coming from a 3G RNC, and inserts IuUP headers in RTP going to a 3G RNC. So far we were merely hacking over the IuUP header to mimick an Initialization ACK, and were otherwise just handing the IuUP headers through from one 3G peer to the other. I realized now that hacking up an Initialization ACK like we currently do on osmo-mgw master produces a header CRC error, which the femto cells we currently tested completely don't care about, thankfully. Ok, so now I am de/encapsulating the IuUP from/to RTP, so that IuUP is present only on the last leg to/from an RNC. 3G to 3G calls still work well with that, now also having correct IuUP header CRCs. And, the naive idea was that now the 2G would simply understand the RTP from the 3G and vice versa. But that's not the case, apparently. The phones at best crackle some random artifacts. I picked a codec-list of fr3 hr3 in osmo-bsc, and tried a few AMR rates (12.2k, 5.9k, 5.2k). So I think there's still some fundamental concept that I'm lacking. Is there anyone more familiar with AMR and/or the way 3G encodes audio, and whether there's a simple way to make them match? Where should I read on? Would a call router like we use in the C3 POC be able to transcode when the IuUP is stripped? (I'm not even sure what the POC side is doing to connect SIP with GSM.) Various pcaps of the status quo are here: http://kleinekatze.de/eeD0ouCo/ My IuUP branch: osmo-mgw neels/iuup http://git.osmocom.org/osmo-mgw/log/?h=neels/iuup IuUP protocol spec: 3GPP TS 25.415 (I've so far ignored most of it) ~N