[Asterisk-Users] T.38 fax summary

[Lee Howard]

On 2005.02.27 09:30 Martijn van Oosterhout wrote:
> On Sun, Feb 27, 2005 at 09:10:48AM -0800, Lee Howard wrote:
> > 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.
> 
> Something's not right here.

Quite right.  I'm sorry to have misled.

What happens is this (as an example scenario):

The receiver will, for an example, receive the post-page message.  The 
sender expects a response to this.  The receiver, however, is required 
to wait between 55 and 95 ms before transmitting the response.  The 
sender will likely be looking for the post-page response immediately 
after transmitting the post-page message.  Per spec the sender will 
only wait about 3 seconds (per-spec between 2550 and 3450 ms) before 
giving up wating and retransmitting the post-page message (and then 
re-expecting the response).

So if there is a steady 1000 ms lag between the sender and the receiver 
(both ways, meaning we assume that both ends could have the 1-second 
jitter buffer), what will happen is this:

The sender will finish transmitting the post-page message.  One second 
later the receiver finishes getting it.  The receiver will introduce 
its own required pause, and add to that the overhead of any processing 
required, and then it will return the signal.  The sender will not get 
that signal for another 1000 ms.  That means that for the total 
processing of that to occur the 2550 ms danger-zone time is nearly 
reached.  Add to that buffer-time the latency time, and I'd say that 
you'd be looking at a signal failure quite certainly.

In real-world action, however, the 2550-3450 ms danger-zone time is 
practically never reached.  In normal use that time is often very close 
to 400 ms.

So yes, 75 ms latency is not accurate for a command-response 
interaction between two fax machines.  And, per-spec the response 
could, in theory, sustain a 1000 ms lag.  However, that would 
far-exceed normal behavior, and I'd be surprised if it would not prove 
fatal to most fax communications.

Lee.