[Asterisk-Dev] SIP Jitter Buffer Testing

[steve at daviesfam.org]

On Fri, 16 Sep 2005, George Pajari wrote:

> It goes without saying that on most connections, getting faxes through 
> without the jitter buffer is almost impossible. The jitterbuffer makes a 
> phenomenal difference. But we are still experiencing more problems than 
> we should and have some questions on how to best proceed.


George:

The jitter buffer logic really isn't designed at all for the goal of
correctly passing modem signals.  I don't think you are experiencing more
problems than you should; even if the jitter buffer works perfectly
(actually, especially if it works perfectly) it won't pass modem signals
consistently.

Here's some reasons why:

1) If a frame doesn't arrive in time, it makes up a frame based on a 
simple model of what sounds OK to the human ear.  Certainly won't fool 
your fax machine.

2) If the code needs to expand or contract the jitter buffer, stunts are 
pulled with the audio that tries to hide the change.  Death to your fax 
machine's signal.

3) If the jitter buffer grows too large, latency on the connection will 
break certain time-critical handshakes in the protocol between the two fax 
machines.

Speaking with some confidence on behalf of Steve Kann: Passing modem
signals is not, and is very unlikely to become a design goal for the
jitter buffer.

Regards,
Steve