Re: [hylafax-users] faxing over sip

[Robert Moskowitz <rgm@xxxxxxxxxxxxxxx>]

Gerard Webb wrote:
> 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.
>  
> Any hints about if hylafax can do this woudl be good.
There is a lot of work-in-progress on this.

Steve Underwood gives a lot of the information on why fax (T.30) over IP 
(really over UDP/RTP) fails more than it works.  The short of it is T.30 
relies on a half-duplex protocol (HDLC) and some critical timings.  This 
works just fine on a dedicated path (V.34 deals with working out latency 
on international calls; I remember my early modem days....) provided 
within the world's PSTN.  But just does not map onto the asynchronous 
nature of IP and the unpredictable latencies of TCP.

Interestingly, IAXModem is enough like V.34 (It does have Steve's 
spandsp code built-in) to work over most Internet connections (I am told 
it does breakdown when the Net gets congested and RED kicks in on the 
routers).

There is lots of information about running IAXModem out there.

But the real way forward is T.37 and T.38.  T.37 is fax over SMTP (ie 
MIME email).  Problem with T.37 is there is no feature negotiation, so 
it tends to be the lowest common denominator.

T.38 is designed address the realities of T.30 and make communications 
work over the Internet. Unfortunately, the people at the ITU that did 
this worked independent of the IETF (not the case with T.37), and did 
their own UDP protocol.  It works, but if there are firewalls along the 
way, you have to provision for it.  There is also T.38 over UDP/RTP 
(real VoIP, what a SIP trunk uses) and over TCP.

All the printers that I know of that have implemented T.38, use the 
ITU's UDP protocol; they don't work with SIP gateways.  Most of the ATAs 
(like Grandstream and Sipura) work with RTP and SIP, to leverage what 
they do for voice.  I seem to recall that Cisco and Avaya gateways do it 
all.

So where am I going with all this.

Asterisk 1.4 has implemented T.38 'gateway'' meaning that it can connect 
2 T.38 SIP/RTP connections.  It cannot terminate the fax connection.  
This is where spandsp should come in.

SpanDSP has for the longest time provided rxfax and txfax apps, that if 
your T.30 over RTP does not fall apart, works.  The current version of 
SpanDSP supports T.38, we are all looking forward to getting this all 
together.  Some people over on Digium's Asterisk forum have done this.  
A number of us over on Trixbox are waiting for next week's 2.3 beta 
update.  Also once IAXmodem incorporates this version of spanDSP, it 
should work as well with T.38.

Another piece of the puzzle is T38Modem for Hylafax.  A number of people 
use that.  But us Trixbox people cannot as it requires kernel patches to 
Centos :(

Now my personal observation:

FAX is holding back large-scale movement to VoIP over classic switched 
circuits.  Classic switch circuits are going away and except for 
'lifeline' will start costing more.  More people are going with GSM for 
their only phone, and GSM CANNOT support T.30; no how/no way.

T.38 will be the key piece that pulls fax over to the Internet along 
with everything else.

BTW, just like some others here pointed out their livelyhood is based 
off of PSTN equipment, I have been in TCP/IP standards for about 15 
years, and you will find me on a number of RFCs (and IEEE 802 standards 
as well).



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