Re: [hylafax-users] 24 channel pri - iaxmodem - hylafax, or ??

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

Andres Paglayan wrote:
> Setup 1./
>
> PRI  <-> Asterisk <-> (24 ports of) Iax Modem <-> Hylafax (same box)
>
> from http://www.voip-info.org/wiki-Asterisk+fax I read:
>
> -"You might also consider Hylafax which runs outside of Asterisk but 
> can be bridged using IAXModem to Asterisk, this is a robust solution 
> when receving calls via PRI and forwarding to iaxmodem and hylafax on 
> localhost. "
>
> But I in the same page, it also says:
>
> -"Sadly the Hylafax/IAXmodem combination isn't a reliable solution. 
> After extensive testing (July/06) the short story is that Digium cards 
> don't do fax. If the remote fax machine supports ECM then IAXmodem 
> works well"

I don't know where that last comment is coming from, but its premise is 
wrong... as many hundreds of IAXmodem users could attest... and hundreds 
upon hundreds of thousands of faxes sent and received can substantiate.  
The person who wrote that probably was probably dealing with PCI latency 
issues surrounding the zaptel hardware.  The zaptel driver is quite 
demanding on CPU attention (in order to perform software echo 
cancellation), and it's rather easy for a hard drive or a network card 
to disrupt it.  On most systems there are ways to work things to give 
the zaptel driver/hardware the requisite attention.

> Setup 2./
> Callweaver with spandsp (no hylafax)
> PRI <-> Callweaver <-> Scripts
>
>
> So in case #1 I am puzzled by the two mutually exclusive comments,
> may IAX modem just work fine with the right T1 card? if so which one 
> is that card?

The second comment you provide for Setup #1 would also be applicable in 
Setup #2... unless Callweaver has rewritten the zaptel driver to not 
need so much CPU attention.  If the hardware driver is so troubled that 
it cannot be used to reliably deliver audio to iaxmodem... it won't 
reliably deliver audio to txfax/rxfax either.

> Case #2, stable not yet released and not enough mass testing yet.
>
>
> Did anybody dealt with a scenario like this?
> what was the experience?
>
> any recommendation? 

Well, you're probably asking the wrong group of people to get an 
unbiased answer.  So let me try to be as unbiased as possible by giving 
you a technical distinction between the two methods.

Both txfax/rxfax and iaxmodem use spandsp.  They both use the same core 
modems (V.21, V.27ter, V.29, V.17) within spandsp.  However, txfax/rxfax 
uses T.30 directly from spandsp's T.30 routines, and iaxmodem+HylaFAX 
uses T.30 from HylaFAX - interfacing with the modems via T.31 from 
spandsp's routines.

So chances are good that if there is a modem (DSP) incompatibility 
between iaxmodem and some remote endpoint that the same incompatibility 
will exist with txfax/rxfax.  (IAXmodem tunes the spandsp modems 
slightly different for itself than txfax/rxfax does, so that may not be 
entirely true.)  However, if the error is in T.30 (fax protocol) then 
the outcome could certainly be different with the other application.

HylaFAX+IAXmodem drive HylaFAX's T.30 through spandsp's T.31 to the 
spandsp modems.  Where txfax/rxfax drive spandsp's own T.30 "directly" 
to the spandsp modems.

Hopefully that makes sense to you.  I know it's confusing.

Now for the biased part...

I chose to write iaxmodem (and consequently help do a lot of the T.31 
development in spandsp and do debugging on the spandsp modems) because I 
wanted to continue to use HylaFAX.  I didn't want to have to redevelop 
T.30 in spandsp that I had already been developing for years in 
HylaFAX.  (In particular, realize that at the time I started iaxmodem 
that txfax/rxfax did not support ECM.)  Furthermore, I didn't want to 
have to abandon all the other HylaFAX niceties... or try to reproduce 
them in the soft-PBX application environment.  Furthermore, I wanted to 
be able to use soft-PBX modems along with more traditional fax modems 
together on the same fax software system (I was, at the time, creating a 
system of mixed modems ... Patton 2977 and IAXmodems and sometimes a 
Multitech ZBA).

So boiled down, I wanted to use HylaFAX, and I wasn't willing to abandon 
it.  I trusted it more, and I didn't want to redevelop trust in txfax/rxfax.

Now for the "in fairness" part...

In fairness it may be questionable to say that iaxmodem has had more 
"mass testing" than txfax/rxfax has.  I have known of some big-time 
txfax/rxfax deployments in the past.  It worked for them.  It did what 
they needed.  At the time they didn't care about having ECM or not.  So 
while I do believe that iaxmodem has had a huge amount of exposure (I 
send/receive tens of thousands of fax pages daily with it), I'm not 
entirely sure that it's had more exposure than txfax/rxfax has... 
however, HylaFAX certainly has been around for a lot longer and has had 
more than a decade of exposure and development.

Hopefully that helps somewhat.

Lee.


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