[SIPForum-discussion] RFC2833 dtmf with inband dtmf

[Gast, Jim]

Hi, Chet -

RFC-2833 has room for confusion.  The danger to be avoided is double-counting.  The recipient may think the regular audio is a DTMF "6" and then also think the RFC-2833 telephone-event "6" is a digit, too.

The RFC encourages us to replace regular audio packets with telephone-events:


RFC-2833 explicitly states "the Internet telephony gateway detects DTMF on the incoming circuits and sends the RTP payload described here instead of regular audio packets."


The RFC states an inconvenient reality: "DTMF digit recognition takes several tens of milliseconds, the first few milliseconds of a digit will arrive as regular audio packets. Thus, careful time and power (volume) alignment between the audio samples and the events is needed to avoid generating spurious digits at the receiver."  If the recipient has a better DSP, it may see enough regular audio to believe it is seeing a DTMF tone.

By RFC, the device that injects RFC-2833 DTMF telephone-events "MUST use the same sequence number and time-stamp base as the regular audio channel".

But the RFC also explicitly allows Simultaneous Generation of Audio and Events:

"A source MAY send events and coded audio packets for the same time instants, using events as the redundant encoding for the audio stream, or it MAY block outgoing audio while event tones are active and only send named events as both the primary and redundant encodings."


So what should we do in practice?  If you aren't designing a DSP and you don't control the source code to a software implementation, the best you can hope for is to configure the recipient to require a larger minimum duration and / or a larger gap between digits.  Failing that, try to configure a device that is early enough in the chain on the sender's side to recognize the DTMF while it is still somewhat clean.

In our network, we require all of the gateways and devices on our edge to replace regular audio with RFC-2833 telephone-events on the way into our network.  We configure these devices (when we can) to not do simultaneous generation.

Sorry to give such an un-satisfying answer,
/ Jim

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Chet Curry
Sent: Tuesday, December 11, 2012 3:14 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] RFC2833 dtmf with inband dtmf

I have searched the RFC's but I have not found the answer to my question.

Is it normal to transmit inband dtmf (audio frequencies) while also using RFC 2833 DTMF events?

I am receiving calls from Level 3 with both being used.

--
Chet Curry
Sr. Network Architect

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