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

[Steve Underwood]

On 07/18/2012 09:43 PM, Matthew Jordan wrote:
>
> ----- 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
>
One thing that page doesn't mention is only spandsp supports T.38 
gateway right now. The Digium FAX module does not.

Regards,
Steve