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Re: [Linphone-developers] Wait between DTMF tones in linphonec

From: John Wimer
Subject: Re: [Linphone-developers] Wait between DTMF tones in linphonec
Date: Mon, 03 Jan 2011 18:41:40 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101208 Thunderbird/3.1.7

On 01/03/2011 11:55 AM, Guillermo Rodriguez Garcia wrote:
Hello all,

I am writing an application that uses linphone and needs to send DTMF codes
to operate a remote device. While looking at linphonec, I noticed that a
pause of 1 second is inserted between tones (console/commands.c, function

This makes the sending of DTMF codes very slow, especially when the sequence
contains several digits. Since the tones are not actually being transmitted
as audio, I thought that the pause could be removed / shortened. However when
I tried this, the other end had problems detecting the DTMF codes. This
happened both with RFC2833 and with SIP INFO. I also verified that the same
problem occurs when both ends are running linphone.

Can someone explain why is this pause needed, and why does it need to be one
second? I could not find any reason for this, specially if SIP INFO is used.

The delay is to allow the SIP INFO message to be received and confirmed before sending the next message. 1 second is completely arbitrary and using a delay-based approach is not actually reliable. The correct implementation would be to have a queue of to-be-sent dtmf codes so the next one would be sent as soon as the in-flight one is ACKed.

Even then, it can take a long time to send long dtmf sequences. If your endpoints support it, you might want to use a single SIP INFO message to send the entire sequence by using application/dtmf instead of application/dtmf-info. However, that is even less standard.

Good luck!

John Wimer

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