[Asterisk-Users] T.38 fax with SIP devices

[Mark Dutton]

I will give this a try Clay, but I don't think it will make a difference.
When the gateway detects fax tone, it switches to T.38 and the voice codec
becomes irrelevant. I am a relative novice to SIP transactions, but from
what I have seen from the debug output of my gateways, the gateway first
negotiates a voice codec, in this case g729. This is to be expected as this
stage happens as part of the call setup. As soon as either gateway detects
fax tone, the debug output of this gateway shows this detection and then it
appears to re-invite with the t39fax protocol. When the gateways are in peer
to peer mode, this re-invite is successful, but when all invites must be put
through the asterisk server, I think Asterisk denies the T39fax protocol. 

This leads me to the thought, is it hard to tell Asterisk about the protocol
so it will accept it? The Micronet gateways see t39fax as a codec, which
makes sense and they call it "t39fax". Even though t39fax is not strictly a
codec, it makes sense that gateways see it as this as for the purposes of a
SIP conversation between the gateways, it looks and acts like a codec.

More input on this subject is welcome. I can't be the only person who needs
fax through SIP devices and T.39 is as old as the hills.

I know I could turn off T.39 (actually I can't on the Micronet gateway) and
use spandsp, but this is a kludge compared to the rock solid T.39 protocol. 

Cheers

Mark


Date: Thu, 7 Apr 2005 09:07:33 -0400
From: "Clay Reiche" <creiche at accxx.com>
Subject: RE: [Asterisk-Users] T.38 fax with SIP devices
To: "Asterisk Users Mailing List - Non-Commercial Discussion"
	<asterisk-users at lists.digium.com>
Message-ID: <A842467212C1DC43A5D34E1C63C80C74016F8FEC at mail.accxx.com>
Content-Type: text/plain; charset="us-ascii"

Mark,
 
Try forcing ulaw on the devices, make a successful ulaw voice call, then try
the fax again.
 
Clay

________________________________

From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Mark Dutton
Sent: Thursday, April 07, 2005 7:01 AM
To: asterisk-users at lists.digium.com
Subject: [Asterisk-Users] T.38 fax with SIP devices


Hi there
 
I have a SIP ATA with a fax machine attached and a SIP FXO gateway to the
PSTN. When I try to send faxes in either direction, I get nothing but stony
silence. I have changed the gateway and the ATA to peer to peer mode to test
them and they happily do the T.38 thing and faxes flow.
 
It seems that they initially negotiate a G.729 codec, which is what I want
and then when the receiving end detects the fax machine, it wants to
re-negotiate and use the t38fax codec. This is the working the Micronet
devices use at least.
 
When I put the units into proxy mode and run them through Asterisk, they
fail
at the negotiation stage.
 
Now I have learned from my dealings with Asterisk and the newsgroup that
Asterisk does not do T.38. However, why should it not let devices do T.38?
My
debug messages from Asterisk don't show it saying no, but the gateways don't
wont' setup the T.38 on Asterisk.
 
I have chanded sip.conf to allow=all and there are no explicit rules in the
registrations for the gateways.
 
Does anyone have an idea here? 
 
For this venture to be truly usable, I have to be able to get FAX working at
this basic level.
 
Regards
 
Mark Dutton