Hi Neels,
In practice, I guess one way is to either have DTX support everywhere, or disable DTX support everywhere?
There is DTXd for DL and DTXu for UL, and they are configured separately in OsmoBSC - but each of the two is global per BTS. Plus this added complication: DTXu is always allowed and almost always desired, but DTXd is only possible if your BTS has more carriers than just C0.
At the CCC congress, we had a transcoding step between GSM and the phone operating center. Makes me wonder, the DL voice probably never saw DTX, because I doubt the conversion from "plain audio" to our GSM codec would feature DTX?
That conversion is called the encoder, and it can operate with DTX enabled or disabled. I don't know how you had yours configured.
I'm also thinking, the ability to do DTX maybe should also be a part of SDP codec negotiation? i.e. enable DTX only when both call legs support it? I wonder if there is a fmtp parameter for DTX, or maybe we should invent one.
Can you please keep Osmocom CN compatible with traditional non-SDP interfaces? I currently connect my PSTN gateway to OsmoMSC via MNCC, I don't use SDP, and I don't want to. Please keep this option available.
SDP aside, if you do what you propose (enable DTX only when both sides support it), the result will be that DTXu will be disabled whenever the far end does not have DTXd, which will be 100% of the time with hobby networks operating in tiny slivers of spectrum. The effect of disabling DTXu will be that the MS will needlessly burn its battery. TL;DR: bad idea.
M~