Voice calls over Virtual Um

[Vadim Yanitskiy]

Hello Andreas,

sorry for late response.

> 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.

First of all, it should be noted that voice support in OsmocomBB
is experimental and undocumented. I have been working on GAPK
back-end integration, and it seems you've already found some
branch, but I would recommend to use 'fixeria/audio' instead of
'fixeria/mncc'.

> calls can be established, but GAPK is never initialized and
> again, I never see voice data appearing as an MNCC message.

Please check out the configuration examples [1] for mobile app.
There you should find the new 'audio' section, where:

  io-target hardware

means that audio I/O (by default) should be handled by the
hardware (i.e. by Calypso phone). GAPK is not enabled by default.

In order to use GAPK based back-end, use 'gapk':

  io-target gapk

It probably makes sense to rename 'gapk' to 'alsa', because
at the moment 'gapk' enables both voice playback and capture
using the audio system of the host that running mobile.

> I never seem to receive a GSM_TCHF_FRAME-message on
> the MNCC socket.

This can also be configured:

  io-target socket

In this case GAPK is not used, and all TCH frames are being
forwarded to the MNCC handler 'as-is'.

See [2] commit description for details.

Please also note that audio I/O target != MNCC handler, that
actually implements the Call Control logic. See [3] for details.

[1] https://git.osmocom.org/osmocom-bb/tree/doc/examples/mobile/default.cfg?h=fixeria/audio
[2] https://git.osmocom.org/osmocom-bb/commit/?h=fixeria/audio&id=bdb2854503afde3ee215f24b1b17b4d12e1c3c44
[3] https://git.osmocom.org/osmocom-bb/commit/?h=fixeria/audio&id=22edbf3f846b13a404dd4e49c3ed25d757d3f527

With best regards,
Vadim Yanitskiy.