Re: [hylafax-users] faxing over sip

[A J Stiles <hylafax_resp@xxxxxxxxxxxxxxx>]

On Thursday 22 Nov 2007, Gerard Webb wrote:
> Hey,
>
> I have a sip server for phone calls.
>
> i would like to use hylafax to send and receive faxes this way. The modem
> page does not mention this aspect at all.
> http://www.hylafax.org/site1/modems.php i know my SIp termination provider
> can handle faxes, so its just a matter of me being able to send and receive
> then from my end correctly.

A fax is just a collection of zeros and ones.  These are conventionally sent 
over a circuit-switched analogue connection using a modem.  There's no real 
reason why the raw zeros and ones that make up a fax image shouldn't be sent 
over a packet-switched digital connection.  In practice, that doesn't happen; 
and they end up being transformed into a different stream of zeros and ones, 
representing the sounds that would have been made by a modem had it been fed 
the original zeros and ones.

There are plenty of good reasons why this shouldn't work.  A fax machine is 
expecting to be connected to a circuit-switched network, where certain kinds 
of error are impossible:  the delay from end to end is always constant, and 
there are no silent gaps where a mis-received packet could not be resent in 
time to reinsert it into the sequence.

There will be naysayers on this mailing list, but I have seen with my own eyes 
a fax machine connected to an Asterisk PABX via a Grandstream Handytone 286 
ATA  (analogue telephone attachment adaptor; essentially a VoIP phone without 
a phone in it, just an RJ-11 socket where a standard phone can be plugged 
in).  And, amazingly, it worked.  If that horrendous bodge could work, 
there's no reason why using Hylafax in conjunction with iaxmodem shouldn't 
work -- provided that packets are getting through to the Asterisk server fast 
enough.  

It *won't* work -- or will work very badly -- if you have variable latency, 
because this introduces exactly those sorts of "impossible" errors which fax 
machines simply were not built to handle.  And you will have to make sure to 
use a non-lossy codec, such as G.711 A-law.

If your network is set up properly, with decent quality switches  (they must 
have enough RAM to store a comprehensive routing table; those weedy little 
8-port jobbies really don't cut it, and cascading them together is just 
asking for trouble.  They keep forgetting what was attached to which port, 
and then have to wait for an opportune moment to broadcast to all ports and 
see what responded)  and a gigabit connection from the switch to the Asterisk 
PABX, it has a better chance of working.

-- 
AJS
(insert figure one before at sign if replying off-list)


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