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RFC2833 digits not propagated in a SIP <-> TDM call

Symptom

In a SIP <-> TDM (i.e., SS7/ISDN) call, all or some RFC 2833 DTMF digits are not propagated through to the SIP side and therefore no real digits are heard on the TDM side.

Reason for the issue

One possible cause of this could be the use of multiple SSRC identifiers in a single multiplexed stream from the user agent on the SIP side. The SSRC is defined in RFC 3550 but basically is used to identify a particular RTP source in a multiplexed stream. It is possible (and technically allowed by RFC 3550) to have multiple media sources on the same host transmitting simultaneously on the same port. For example, an endpoint that has multiple cameras or microphones or a conferencing solution could potentially do this.

The most common use cases for RTP however are point-to-point Voice over IP or streaming applications where there is usually not more than one media source per end-point. The RTP is usually "mixed" on the endpoint and presented as a single stream.

In a Wireshark trace gathered from either the UA machine or via port monitoring on the Dialogic® Integrated Media Gateway (IMG) itself, it may become apparent that the SSRC is different between the main RTP stream and the RFC 2833 digits and if so, this will be the reason why the DTMF is not heard on the TDM side since the IMG does not support these multiple media source scenarios.

Solution

The solution is to ensure that any devices that connect to the IMG mix the RTP streams and present a single RTP source to the IMG.

Product List:

Dialogic® IMG 1004 Integrated Media Gateway
Dialogic® IMG 1010 Integrated Media Gateway
Dialogic® BorderNet™ 2020 Session Border Controller

Related Documentation: 

[1] RFC 3550 --  RTP: A Transport Protocol for Real-Time Applications

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