[asterisk-users] T.38 Gateway

[Kevin P. Fleming]

On 07/23/2012 06:30 AM, Paul Goldbaum wrote:
> Hi everyone,
>
> we're trying to setup a fax gateway from PRI to SIP. Our test setup
> consists of two asterisk 10 machines connected over PRI and a third one
> which is acting as a fax endpoint over SIP.

This 'third' system is the one responsible for initiating the switchover 
to T.38 mode for the call, since it is the last element in the path to 
the eventual called FAX terminal.

> On the machine which is receiving over PRI, we see the following message:
>
> [Jul 23 12:54:23] DEBUG[18288] res_fax.c: detected v21 preamble on
> SIP/xxx.xxx.xxx.xxx-00000001, but DAHDI/i1/-2 does not support T.38 for
> T.38 gateway session

This is expected behavior. If you run a gateway on a SIP channel, it 
will naturally assume that *that* channel is not capable of T.38 support 
(thus the need for a gateway) and it will attempt to request T.38 
support from the channel it is bridged to.

In your configuration, the Asterisk system acting as SIP FAX endpoint 
should *itself* request a switch to T.38 on the SIP channel before it 
ever emits any FAX tones at all.  The system in the middle will receive 
this T.38 negotiation request, and the gateway code (since it has been 
enabled) will realize that the DAHDI channel is not T.38 capable, and it 
will step in to provide T.38 gateway services.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
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