> From: Andrea Rizzi [andrea_rizzi_m...@yahoo.it] > > From the logical point of view, the T.38 stream will replace the audio > stream, so the best option (as recommended by RFC3264) is to reuse the same > m-line, so that there's no ambiguity about what the offerer is asking for. > This way would require a new INVITE in case the answerer doesn't support > T.38, to fallback to G.711.
What I've seen in practice is that the initial INVITE offers G.711. Once the fax handshake tone is recognized, the gateway device re-INVITEs, changing the one m= line to type "image" with the various "a=T38..." attributes. If the receiving device supports T.38, it responds 200 to the re-INVITE. If the receiving device does not support T.38, it responds 488 Not Acceptable Here, conveniently leaving the media configuration unchanged, with G.711 available as a codec. I suppose that if the initial offer/answer did not allow G.711, there would need to be an additional re-INVITE cycle to activate it. Dale _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
