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

[John Wimer]

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
> linphonec_parse_command_line).
>
> 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