Hi, To elaborate further on Franz's question, I trust that your vendor is partly right in this case. Though you may be able to negotiate any number of streams/ media types in the RE-INVITE SDP, it is essentially *only as a fallback mechanism to do fax over PCMA/ PCMU* in case the peer entity does not support T.38. However, I agree that it should not treat the multiple media types in SDP as incorrect. The correct solution is to negotiate any number of codecs in the RE-INVITE and based on the negotiation results, you may (as in a T.38 call) / may not (as in PCMA Fax call) require muting the audio channel.
This is because if you send/ receive fax using T.38, the mechanism is quiet different from audio transmission, notably, things like echo cancellation, silence suppression, hold have no meaning in a T.38 call and hence these should be disabled in a T.38 call. If you are to support T.38 without muting the audio, it will imply sending/ receiving 2 parallel streams (one carrying PCMA over RTP and the other carrying T.38 over UDPTL) - one requiring voice specific features like echo cancellation, silence suppression to be disabled while the other requiring these to be activated (to improve audio quality). Though it's highly implementation specific but when these features are implemented on dedicated signal processors (DSP) on hard phones & terminal adapters, it is not possible to achieve this in parallel i.e without muting the audio channel unless you have dedicated DSPs for each media type (highly unlikely). Of course, in this case, it's very well possible to do this while sending/ receiving fax over PCMA/ PCMU. I hope this helps. My 2 cents, Gaurav -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Romel Khan Sent: Monday, February 20, 2006 10:09 PM To: EDLER, Franz; [email protected] Subject: Re: [Sip-implementors]´Problem with a T.38 call Some implementations do this in the offer in the reINVITE. It would include the T38 image and G711 PCMU/PCMA just in case the other side doesn't do T38. The answerer side should choose T38 and mute G711 codecs, if it can handle T38. Otherwise, if it can't handle T38 but can handle a G711 codec type, it should mute T38 and accept PCMU or PCMA. This implementation is consistent with the offer/answer model RFC3264 with examples in RFC4317. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of EDLER, Franz Sent: Monday, February 20, 2006 9:08 AM To: [email protected] Subject: [Sip-implementors] ´Problem with a T.38 call Hello T.38 experts, I am faced with an issue, that a SIP Terminal-Adapter after detecting a T.38 preamble sends a Re-INVITE with two media streams in SDP (audio and image), but without muting the audio. The other side (a SIP/PSTN gateway) does not honor the image media in SDP and continues the audio media anly. The vendor of the gateway claims that there MUST be only one media stream (not both audio amd image). The question is: Who is right and who is wrong? Are there any clear arguments? regards Franz _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
