[Asterisk-Users] Using Hylafax and Digium T100P

[Rich Adamson]

> > Any analog modem (fax or pc) is going to be limited to 9600 baud or 
> > slower,
> > and will only achieve that speed if g711 is used through the entire path
> > (including asterisk). If a modem call comes in one T1 (or PRI) and goes
> > out another, asterisk is still handling the pcm packets. The packets don't
> > magically jump across T1 cards (or T1 ports on the same card).
> 
> Rich,
> 
> I'm not quite sure what you're concluding here, but we routinely fax 
> hylafax->asterisk->hylafax and hylafax->asterisk->PSTN in a variety of 
> analog and digital configurations, with and without channel banks at speeds 
> up to 33,600.
> 
> There's no reason Lee's outline:
> 
> T1 --> TE405P(1) --> Asterisk --> TE405P(2) --> Patton 2977 --> HylaFAX
> 
> won't work perfectly well. Granted the Patton 2997 is limited to 14,400 
> maximum, but with other T1 cards such as Eicon Diva Server and Brooktrout 
> TR1034 there's no problem negotiating and sustaining V.34 speeds 
> (14,400 ->33,600).
> 
> Forgive me if I musinderstood your post, and wield your clue bat gently! ;-)

In the post that I was responding to, the writer hinted his understanding
was that T1 to T1 channel connections didn't involve any asterisk code.
His impression seemed to suggest that codec selection, etc, wasn't a
factor since the analog fax modem signals were coming in one T1 channel
(or PRI channel) and going out another without passing across the * pci 
bus. (Purhaps I've have read too much into his post though.)

If the analog modem signals are transitioning the * pci bus, there 
is a high likelihood the modem signals will not be accurately handled 
by * and thus limit the speed at which the fax modem will function.
Don't read that as "it will", but rather "it might". (See many past
posts relative to analog modem usage through asterisk, and many other
posts where readers didn't understand the significance of g711 usage
and analog modems via asterisk.)

As a sort of side note, modems that operate at rates higher then about
9600 bits/second actually use encoding techniques (such as trellis
encoding) on top of a 9600 "baud" signal (as an example only), thus 
achieving 28800 or whatever speed. There is a difference in the terms 
baud and bits/second. The more sophisticated the encoding technique,
the more difficult it is to accurately reproduce analog signals. I'm 
not a fax modem expert by any stretch, but I'm under the impression 
that fax modem standards are very old and limited to rather slow 
speeds (on the wire). I would very quickly defer to Steve Underwood
for a more accurate description of that entire topic however.

Rich