[asterisk-users] Using Asterisk 10.6 as a T38 Fax gateway

[Matthew Jordan]

----- Original Message -----
> From: "Alejandro Recarey" <alex at recarey.org>
> To: "Asterisk Users Mailing List" <asterisk-users at lists.digium.com>
> Sent: Wednesday, July 18, 2012 6:30:26 AM
> Subject: [asterisk-users] Using Asterisk 10.6 as a T38 Fax gateway
> 
> Hi all, and thanks for taking the time to read this.
> 
> I am trying to configure Asterisk 10.6 as a T38 Fax gateway. I am
> receiving calls through the PSTN and want to send them to my VoIP
> carriers as T38. This is my dialplan:
> 
> [fax]
> exten => _X.,1,Set(FAXOPT(t38gateway)=yes,20)
> exten => _X.,n,Dial(SIP/${EXTEN}@x.x.x.x)

The correct setting is not FAXOPT(t38gateway) - that is not a valid parameter
to pass to the FAXOPT function.  As you mention below, the correct setting
is Set(FAXOPT(gateway)=yes).  The optional timeout is fine.

https://wiki.asterisk.org/wiki/display/AST/Asterisk+10+Function_FAXOPT

> I have tried with both FAXOPT(t38gateway) and FAXOPT(gateway). I have
> also tried setting t38pt_udptl = yes,redundancy in sip.conf. None of
> these things work. When we send a fax:
> 
> 1. Asterisk does NOT send a REINVITE with the t38 offered. Reading
> the
> documentation, it should detect the fax tone with the audiohook and
> then send a REINVITE with t38 capability.

Have you confirmed that Asterisk does not send the re-INVITE using either
a packet sniffer or by monitoring the log with 'sip set debug on'?  Without
seeing the SIP message traffic and a DEBUG log, its hard to say what
might be the cause of your issues.  

Typically, I would expect to see something like the following in a DEBUG log:

[Jul 18 08:29:18] DEBUG[20234] res_fax.c: detected v21 preamble from SIP/ast1-g711-00000001
[Jul 18 08:29:18] DEBUG[20234] res_fax.c: requesting T.38 for gateway session for SIP/ast1-t38-00000000
 
Note that this also answers your question in a subsequent e-mail: you
should be using res_fax, with either res_fax_spandsp or Fax for Asterisk.

> 2. Asterisk does not offer t38 in the SDP of the initial INVITE. This
> is not a problem if it correctly detects and REINVITES for faxes, but
> our destination carriers tell us that they cannot do the REINVITE
> themselves because we do not offer t38 in our SDP, so they believe we
> do not have that capability.
> 
> Obviously I would prefer to just detect the fax myself and have
> asterisk do the REINVITE.
> 
> I have read all of the documentation on the asterisk wiki (which is
> rather short) and anything else I could find online. Unfortunately
> most of it is out of date and refers to asterisk versions 1.4 to 1.8,
> which do not have T38 Gateway capability.

There typically isn't a lot of configuration that is needed for T.38
gateway support.  The necessary dialplan configuration is documented
here:

https://wiki.asterisk.org/wiki/display/AST/T.38+Fax+Gateway

> Does anybody have any experience in making this work?
> 
> Thank you!
> 
> Alex
> 


--
Matthew Jordan
Digium, Inc. | Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at: http://digium.com & http://asterisk.org