[asterisk-users] Asterisk 1.6 T.38 generation towards a Cisco voice router

[David Backeberg]

On Wed, May 13, 2009 at 3:30 AM, Jon Schøpzinsky <jos at firstcom.dk> wrote:
> We are having some problems using t.38 together with a Cisco voice router at
> one of our providers end.
>
> We are using the new digium asterisk fax module to generate the fax, and
> when we use together with our internal Audiocodes Mediant 2000 gateways, we
> have no issues what so ever, and the faxes go right through.
> I can see that asterisk discards all RTP T.38 packets sent from the
> provider, which the error message also indicates.
>
> Is there a known problem, connecting to cisco hardware using t.38 in
> Asterisk 1.6? or does anybody know of a patch that fixes this problem?

I doubt that there is a known problem, as I'm using Cisco with
asterisk and T.38 and having success. Do you have full control over
the Cisco gear?

Please post the dialpeer info from the Cisco gear and I'll take a look
at it. You can also go back through the archives for similar posts
because we've discussed this a few times in the last few months. Among
other things, I saw that your fax tried to transmit at 2400bps. The
gear should be able to support 9600. So that's already fishy.

What happens if you try to send a 'normal' audio fax over voip through
that gear?

 Some things you should know:
* do not compress voip faxes. Faxes are already compressed. If you try
to use a compression codec you'll wreck the fax.
* on the cisco dialpeer be darn sure that you've turned off vad
* for sip on asterisk, you need to enable reinvite, and you also need
to configure a  t38pt_udptl = yes entry in your sip.conf, but you
probably already to that right if you were T.38-ing to the other gear.
Are you sure you weren't just sending a normal audio fax to the other
gear?