Subject: dtmf relay in SIP?

From: Wing Man Kwok <[email-masked]>
Date: Mon, 2 Aug 1999 13:18:54 -0400

All,

If I have a regular telephone connected to a SIP enabled gateway and calls
another telephone with similar connection, is there anyway I can do dtmf
relay between the two phones in the SIP network?  That is, when the gateway
receives a dtmf tone from the connected telephone, how does the gateway send
out the dtmf tone in a SIP message?

Any suggestion or recommendation is greatly appreciated.

Wing Man Kwok

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From: [email-masked]
Date: Mon, 2 Aug 1999 22:12:42 +0300

Hi,

> out the dtmf tone in a SIP message?

You can use either the SIP INFO method or just encode the DTMF tones in RTP
packets...

Regards,

Gonzalo

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From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 02 Aug 1999 16:41:50 -0400

This configuration is a classic case of where RTP encapsulation is best.
I'm personally unconvinced about dtmf transport in SIP in general; here
its definitely not needed.

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg                       Lucent Technologies
Member of Technical Staff                   101 Crawfords Corner Rd.
High Speed Networks Research                Holmdel, NJ 07733
FAX:   (732) 834-5379                       Rm. 4C-526
EMAIL: [email-masked]
URL: http://www.cs.columbia.edu/~jdrosen

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From: "Jacob Avraham" <[email-masked]>
Date: Tue, 3 Aug 1999 10:16:47 +0300

I'm curious about DTMF in-band within the RTP stream. My audio guys tell me
that once the audio is encoded with something like G.723 or some other
vocoder, you loose the tone frequency info and it's almost impossible
to get accurate detection of DTMF.
Possibly, one could change RTP payload mid-stream to something like G.711
but that's not always feasible (and forbidden by other protocols like H323).
Any real world experience with this issue?

Thanks,

Jacob Avraham
Mediagate

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From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 03 Aug 1999 09:09:24 -0400

This is the purpose of the RTP payload format for tone and dtmf
transport:
http://www.ietf.org/internet-drafts/draft-ietf-avt-tones-00.txt

it carries the dtmf either as a named signal, or through a
frequency/modulation representation. This is what I was referring to
below.

-Jonathan R.

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From: "Adam B. Roach" <[email-masked]>
Date: Tue, 3 Aug 1999 09:10:12 -0500 (CDT)

>to get accurate detection of DTMF.

There's an rtp payload format being designed which addresses this issue. 
The short explanation is that you don't encode tones using a normal 
vocoder; you detect them and send a symbolic representation of them
(e.g. frequency1 + frequency2) down the rtp stream instead.

See draft-ietf-avt-tones-01.txt for details.

>Any real world experience with this issue?

Current commercial implementations usually use G.711 for all calls
to solve this issue. Not optimal, but effective.

--
Adam Roach, Ericsson Inc. |  Ph: +1 972 583 7594 | 1010 E. Arapaho, MS L-04
[email-masked]   | Fax: +1 972 669 0154 | Richardson, TX 75081 USA