Cellular Network Infrastructure » Radio Access Network » OsmoBTS

The problem is that conversation is uncomfortable bordering on impossible due to random lost words and cut off of initial speech after silence periods.

I've been experimenting sending the AMR RTP stream into decoders or SIP endpoints, (all using libopencore-amr) and also, without using any external MNCC, thereby only having the the audio stream pass through osmo-mgw before going to a B-leg MS. I observe the problem in all cases.

I've put quite some hours into this so far, without being able to achieve any improvements. Rather than leave it all fade from memory, I'll just make a quick note of the main points of what was observed:

It all seems fine as long as you have sequences of speech followed by SID_FIRST_P1, SID_FIRST_P2, then eventually an ONSET, followed by speech frames... rinse and repeat. This is fine.

When it goes wrong is if you have a situation where (because of the timing of the VAD) you get either a SID_FIRST_INH or a SID_UPDATE_INH, in these cases, what I've observed is that after the SID_UPDATE_INH, the L1 does not seem to be sending us any Speech frames, (just TCH/H with no payload) even though I am speaking. Eventually, there will be an ONSET followed by speech frames.

I may be wrong, but I don't think there's anything that osmo-bts is doing or can do about this, so I'm pretty convinced at this point that is is an L1 problem, and therefore will not (cannot?) be fixed. I'm aware that there is code in libosmocore to deal with all this weird interleaving and such that goes on in AMR DTX, but we don't use any of it in osmo-bts-sysmo.

The pcap comes from an osmo-bts with some modifications to logging and RTP marking, (ignore the BAD AMR frames following ONSET - that's a hack but not relevant) the main point would be to observe what is happening around and after packet no 1482 - Note all that PH-DATA.ind TCH/H with no payload. I am speaking then.