Voice calls over Virtual Um

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Andreas Galauner andreas at galauner.de
Tue Nov 20 09:56:43 UTC 2018


I set up a network with Virtual Um and multiple OsmocomBB phones. They
register, they can send and receive SMS and I wanted to try voice calls.
I looked around and found a repository [1] with an MNCC example
application which sends voice samples. Calls are established and can be
picked up, but on the other end, but I never seem to receive a
GSM_TCHF_FRAME-message on the MNCC socket.

I then checked out the fixeria/mncc branch [2] which includes GAPK
integration which can use ALSA devices for audio, because that's what I
need anyway at some point for my project I'm working on.

Again, calls can be established, but GAPK is never initialized and
again, I never see voice data appearing as an MNCC message.
The GAPK audio backend is supposed to be initialized [3] when a CHANNEL
MODE MODIFY message is received from the network. I never see the
network request a channel mode modify though. I don't know enough about
GSM to find out if that message is always supposed to be sent when a
call is established or not.

Did anybody ever transport audio with two OsmocomBB instances over the
Virtual Um interface?

- Andy

[1]: https://github.com/GerardPinto/call-control-mncc-sap
[2]: https://git.osmocom.org/osmocom-bb/log/?h=fixeria/mncc

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