[asterisk-users] Asterisk T38 and Dialogic DMG 2000

[Raj Jain]

JR,

On Mon, Sep 8, 2008 at 3:08 PM, JR Richardson <jmr.richardson at gmail.com> wrote:
> The DMG invite sends to asterisk:
> m=audio 49016 RTP/AVP 0 101          [notice the m=audio]
> a=rtpmap:0 PCMU/8000/1
> a=rtpmap:101 telephone-event/8000
> a=fmtp:101 0-15
> m=image 0 udptl t38
> a=T38FaxRateManagement:transferredTCF
> a=T38FaxUdpEC:t38UDPRedundancy
>
> And Asterisk responds with the 200 OK:
> m=image 29475 udptl t38                   [notice the m=image]
> a=T38FaxVersion:0
> a=T38FaxFillBitRemoval:0
> a=T38FaxTranscodingMMR:0
> a=T38FaxTranscodingJBIG:0
> a=T38FaxRateManagement:transferredTCF
> a=T38FaxMaxBuffer:400
> a=T38FaxMaxDatagram:400
> a=T38FaxUdpEC:t38UDPRedundancy
>
> I've been corresponding with Dialogic engineering on the messaging and
> they report that the gateway receiving m=image is not compatible or is
> telling the gw to immediately setup the call at T38 with is not
> compatible.  The gateway wants to setup the call as audio first, hear
> the CNG tones and then re-invite to t38.

That's how T.38 gateways typically work. This particular gateway seems
to have some advanced knowledge that the call may be converted into
T.38 later. They are offering m=image with port number set to 0 in the
INVITE. By doing this, they seem to be offering a sort "heads up" that
the call may be converted to T.38 later but no T.38 now because the
port number is zero. This hint is of no use to Asterisk. If you can
get the gateway to not send the m=image line in the first INVITE, they
you may be in luck.

> So my question:  Is there a way for configuring Asterisk to respond
> with m=audio instead of m=image?  If I disable udptl in Asterisk, call
> setup fine with audio.

This seems like a bug in Asterisk. Asterisk is encoding the SDP in 200
OK incorrectly on two fronts. It's dropping the m=audio line
completely and it's activating T.38 stream when the remote end hasn't
asked it to do so.

--
Raj Jain