[hylafax-devel] Re: Supporting Digital Modems in HylaFAX.

[Marcio Saito <marcio@cyclades.com>]

Steve!

> One reason that running modems via TCP/Ip is not currently supported by
> HylaFAX has been concerns of robustness.  HylaFAX prides itself on being robust,
> AND IT IS!!  Timing is very critical to successful faxing, and there have
> been concerns expressed that there is no way to GUARANTEE timing issues when
> using TCP/IP, because packets could get delayed on delivery, etc.

It is true. There is no way to guarantee timely delivery of data in a
TCP/IP based connection. I don't know how stringent are the
requirements for fax transmission.

Round-trip delay on a reasonably dimensioned 10BT Ethernet LAN is in
the order of tents of millisecond. When there are bursts of traffic,
that can get to a few milliseconds. Jitter is probably very high. On a
100BT Ethernet LAN, things get a little better, but jitter is still a
problem.

Isn't some level of buffering at the destination (at the RAS, right
before passing it to the modem) enough to overcome this problem?

That is certainly one of the potential problems to be
addressed/considered if we decide to do something in that direction.

> With the timing necessary for Class 1 fax modems, I cannot imaging that it
> would be possible at all over TCP/IP in a production environment, which is
> what you are talking about if it's a T1 environment.  I guess if the RAS box
> was on a switch direct to the host, it would probably be fine, but random
> network traffice ( busy bursts ) could easily cause the fax to fail.
> So, the first thing to ask would be... does your RAS box provide Class 2 or
> Class 2.0 fax support?  If not, then I would think you should not even
> consider implementing the fax side of things.

The modems in RAS supports Class 1 and 2 fax. From what you say, I
understand tha Class 2 faxes are more tolerant to timing variation.

> The opening of a socket and sending AT commands ( & receiveing responses )
> would actually be reasonably easy to implement in HylaFAX.  The problem is
> that HylaFAX ( and virtually every other comm package ) has been designed
> with SERIAL RS-232 communication to the modem.  Unfortunately, communication
> with the modem is not simply done through two file handles ( TX & RX ).
> There is other information sent via the other wires in the RS-232 spec.

I'm aware of that (I've written tty drivers). Most of the
ioctl-related functionality can be handled through a socket connection
(there are even standards for that), but the way to handle it is very
different than in tty ioctls.

I agree that supporting this probably means changing a lot of code in
the fax server and is a non-trivial issue.
 
> I am not aware of any "standards" to accomplish what you want, but really,
> that doesn't mean much, I don't exactly have my ear to the door on this
> particular subject.  As much as I hate to say it, do you plan on your
> product being supported under Windows ( NT/2000 )?  If so, then the place
> to start might be with standards for support faxing over TCP/IP in that
> environment, and see how realistic they would be to implement in the Unix
> environment.

Yes, I'll do some research on Fax over TCP/IP too. And I'll some more
reading on Fax technology.

With e-mail and the Internet, it would be natural to think that fax
becomes less and less important. But, besides the huge installed base,
I heard somewhere that the market for fax is still growing.
 
> You might consider trying to contact demon.co.uk ( think that is the domain )..
> They are providing fax coverage for most of England through the TPC.INT
> network ( www.tpc.int ) using HylaFAX.  I am not sure what they are running for
> hardware, but it must be pretty beefy stuff given the volume that they process.
> Someone on this list might know the contact, I can't seem to hunt it up..
> You could always try faxmaster@demon.co.uk & see what the turns up..

I will try to contact them.

Steve, thanks for your comments. The question of time sensitiveness or
fax transmissions had not crossed my mind.

Marcio.
_________________________________________________________

Marcio Saito
Director of Technology
voice (510) 770-9727 x214  mailto:marcio@cyclades.com
Cyclades Corporation http://www.cyclades.com
_________________________________________________________