[Asterisk-Users] T.38 fax summary

[Lee Howard]

On 2005.02.27 08:34 Martijn van Oosterhout wrote:

> Hi,
> 
> I read it and found it very enlightening. I do have one question
> regarding "Modems don't like relativity". It says modems need a
> constant delay; is there a limit to what it can handle. For example,
> would it be possible to configure a jitterbuffer right at the endpoint
> before the fax to put a constant delay of 1 second relative to the
> sender. This should be enough time to weed out any jitter. Basically,
> fix the jitterbuffer so the delay is constant. If a fax can handle a
> constant delay of up to a second you're home.

Fax cannot handle a one-second delay.  As Steve mentions in the 
article, per-spec fax has some timings (particularly silence in 
direction "switching") set at 75 ms +/- 20 ms.  So if the delay gets 
much larger than 75 ms, then there's likely to be trouble.  Now, some 
fax machines may tolerate larger delays, but that tolerance is beyond 
the spec, and thus should not be used as a gauge.

Lee.