[Users] Support for T.38

Max Gregorian gregorian442 at googlemail.com
Tue Jan 23 01:15:55 CET 2007

Yes, i'm beginning to think the gateway isn't really doing what it's
supposed to be doing.

I read about the uncompressed codec thing somewhere, but what threw me was
some of the signalling I was getting back which made me wonder whether I
needed any special configuration on Openser.

I guess a "488 Media Not Supported" error back after the intial INVITE-100
Trying, when using PCMU or PCMA (as suggested by the device manufacturer)
probably means that T.38 (auto detection) has not been fully
implemented/supported yet on that Gateway.

When I added other codecs in Fax pass-through mode, it seemed to default
(negotiate) to G.729 but the destination Fax machine rings, although it does
not connect because the Gateway was not responding to the reinvites - which
confirms what you just said.

On Openser I saw the re-INVITE come in with T.38 in the SDP then other
INVITEs but with G.729 so it seemed to be trying both audio and Fax. Quite
clearly G.729 will not work and is not meant to work anyway.

I will re-test with other gateways that support T.38 (and other codecs) and
see if I will have any success.

Thanks for replies.

(I guess you learn something new everyday.)

On 21/01/07, Stephen Varga <varga at zbzoom.net> wrote:
> Max,
> OpenSER does not get involved with the media stream, it just handles
> call setup and therefore does not generate FAX tones. It is up to the
> UAs to negotiate CODEC used for the call. In the example you provided of
> the call path, both the IP gateway and the PSTN gateway would have to
> support the T.38 specification. If one side does not support T.38 they
> will most likely use a normal voice CODEC to handle the FAX tones and
> FAX tones do not do well when compressed with voice CODECs.
> Steve
> Max Gregorian wrote:
> > Hi,
> >
> > Does anyone know if OpenSER supports T.38 for Fax over IP?
> > Or has anyone managed to get it to work?
> >
> > I did some quick tests and what I got was the destination Fax machine
> > rang,
> > but the fax message did not arrive. I don't think the destination fax
> > picked
> > up.
> >
> > Setup was:
> >
> > Fax (PC modem) -> IP Gateway -> OpenSER -> PSTN -> Fax machine
> >
> > Is it possible the Fax tones are not being properly generated by
> OpenSER,
> > and so not being recognised by the Fax machine? (For reference, the
> > Gateway
> > used is advertised as supporting T.38)
> >
> > ------------------------------------------------------------------------
> >
> > _______________________________________________
> > Users mailing list
> > Users at openser.org
> > http://openser.org/cgi-bin/mailman/listinfo/users
> >
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