> From: Saurav Sarkar (sausarka) [[email protected]] > > I am aware that today T.38 (for Fax) is negotiated in mid-call, > i.e. after an audio call is established we can have a re-INVITE or an > UPDATE (in SIP) to negotiate Fax capabilities (T.38) depending on the > fax tone. > > What my query is that if there is any implementation that requires > T.38 in the initial call itself? (Say, for example, any Fax endpoint > that supports T.38 in the initial call?) > > I am asking this to know if a support of this sort is required on the > Session Border Controller (SBC). And if yes, then what is the > mechanism that is used (because there cannot be any fax tone to start > off with).
The general SIP concept is that a device should be willing to accept at any time any media type that it supports. So a device that can accept T.38 but requires it to be in the first offer is likely to not interoperate successfully. Over time, I expect more and more fax calls will be done via SIP from end to end, and those calls will omit the initial audio segment to transmit the fax tone. In regard to SBCs, generally they should be agnostic regarding media types, and simply pass the SDP media specifications through. If the SBC attempts to enforce particular patterns of media use, it will have to be updated frequently as new media usages become popular. Dale _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
