DTMF signalling does not work (with internal MNCC)
The correct transmission of DTMF tones is currently not working. I have done a series of tests, here are the results:
- unsuccessful_ms_to_ms_dtmf_tone_(NG40).pcapng: Mobile to mobile DTMF tone signalling Start and stop of the DTMF tone is acknowledged. This looks good to my perspective, probably the DTMF forwarding is not properly configured inside the NG40 tester, otherwise I would expect to see the MSC at least trying to deliver the DTMF tones to the other mobile.
- unsuccessful_ms_to_ms_dtmf_tone_with_external_mncc.pcapng: Here I can see the start command, which gets translated into a SIP message on the osmo-sip-connector. The tone also gets acknowledged and is stopped again. But again we see no response to the other MS whatsoever. This could also still be a problem with my PBX setup, since we would expect to see another SIP-Message that triggers the DTMF tone for the other MS.
- unsuccessful_ms_to_ms_dtmf_tone_with_internal_mncc.pcapng: This trace was recorded with the internal MNCC active and really looks not ok. The DTMF tone gets actively rejected.
- successful_ms_to_sip_dtmf_tone.pcapng: Here I see almost the same as in unsuccessful_ms_to_ms_dtmf_tone_with_external_mncc.pcapng, but the tone gets actually played on the other side. So I think sending DTMF tones should be fine, but receiving definetly has a problem.
- When I press buttons on the sip phone actually nothing, not even a sip-message is transmitted. That also confirms that there may be problems with the PBX.
However. I still wonder why the DTMF tones get rejected when osmo-msc is running on its internal MNCC. Something is definetly problematic there. The DTMF tone should travel back to the other end and not get rejected. I also attached the configuration files of osmo-bsc and osmo-msc. Maybe I am just lacking the right configuration options.
- Checklist item document no DTMF withi nternal mncc in OsmoMSC manual added
- Checklist item document challenges of getting MT-DTMF to work with SIP in osmo-sip-connector manual added
- Checklist item document we need DTMF inside the media (not signalling) for MT-DTMF from SIP added
- Checklist item document we provide MO-DTMF as signalling on SIP added
- Checklist item investigate if this could be done in the MGW (are there MGCP message for tone playback?) added
- Priority changed from Normal to Low
AFAIK the DTMF related signalling in GSM L3 Call Control only caters for mobile originated DTMF.
This means that the MSC/MGW or some other entity higher up in the network would have to convert those DTMF signals into actual tones, and substitute some audio samples with those tones before sending it back.
I guess the rationale is that one wants users to be able to use DTMF (e.g. for voice mailbox systems, call center applications, etc.) but as such applications are all connected to the wired telephony network, it's not needed to signal DTMF to a GSM subscriber.
So all in all, it's probably ok for the internal MNCC to reject the DTMF playing, as it doesn't have the media playback ability. In some distant future we should be able to add this to OsmoMGW, but not now.
As for the interworking with external SIP: as long as we properly report the DTMF tones on the SIP trunk, everything else is up to the SIP switch and its media handling. We should document in the osmo-sip-connector manual that this is apparently not working from statch.