6 Dec
2022
6 Dec
'22
6:21 p.m.
As I mentioned in previous email, I found out that setting up a
silent-call from osmo-msc makes osmo-bts produce an RTP stream, which is
not surprising given that we activate a TCH channel as if it was a usual
MT call. Once a silent call is established, one can "attach" to the RTP
stream using gapk [1]:
$ osmo-gapk \
--input-rtp 127.0.1.1/4000 \
--input-format={gsm,rtp-efr,rtp-amr,rtp-hr-ietf} \
--output-alsa=default --output-format=rawpcm-s16le
This command will play the Uplink RTP stream via ALSA. I believe it
should also be possible to loop the Uplink frames back via Downlink if
you know the right RTP address/port for that:
$ osmo-gapk \
--input-rtp $UL_RTP_ADDR/$UL_RTP_PORT \
--input-format={gsm,rtp-efr,rtp-amr,rtp-hr-ietf} \
--output-rtp $DL_RTP_ADDR/$DL_RTP_PORT \
--output-format={gsm,rtp-efr,rtp-amr,rtp-hr-ietf}
This approach has a few benefits:
* you can choose a specific codec: HR, FR, EFR, AMR;
* you can play the RTP streams on the host side.
... and a big spoon of tar: none of the real phones I tested so far turn
on the microphone during a silent-call. This would have been a big
security problem if they did so: nobody wants their phone to be turned
into a spy microphone. But I won't be surprised if some phones do ;)
The mobile app (from osmocom-bb.git) currently does enable the
microphone even without the usual MT call establishment signalling (MT
SETUP in this case). This is acceptable for now, as I doubt anybody is
using this app as a real phone for normal every day calls.
If anyone likes this approach, I could implement a 'test-call' command
which would be like a silent call, but sending an MT SETUP message with
a random calling MSISDN. This way it would work for real phones too.
[1] https://gitea.osmocom.org/osmocom/gapk/
Best regards,
Vadim.
--
- Vadim Yanitskiy <vyanitskiy at sysmocom.de> http://www.sysmocom.de/
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