Dale,
Regarding the revision of TF21:
As you undoubtedly realize, but not mentioned in TF21, a major reason for specifying the fast switch capability is for calls that are between PSTN devices and gatewayed via an IP network. In this environment, a call can switch between voice and fax and back.
I'd like a little more clarification on the Fast-switch-capabity-from-Voice-coder-UDP-to-fax-coder-UDP mode that you define in TF21. Is the intent here that the change includes the encapsulation of the Fax data within RTP? This seems the most reasonable mechanism since it allows the maintenance of accurate timing and sequence when switching between modes. If the change between modes is in-band without any logical channel signalling, how would the RTP payload type of the fax packets be indicated, since it would be assigned as a dynamic payload type?
I have a similar question with regard to the Real-Time-Fax-over-UDP mode. Does it imply encapsulation within RTP as well? If you are using normal H.245 logical channel operations to change from voice to fax mode, particularly the replacementFor procedure, the mode change, while signalled in advance, is actually indicated when the RTP Payload type changes.
Since, in the PSTN<->IP<->PSTN gateway environmemt, it may not be possible to know whether or not the call will contain fax data until well after the end-to-end connection is established, it should be allowable for an endpoint to advertise its fax capabilities as well as voice capabilities and still complete the call even if the other end does not support a fax capability.
steve magnell Dialogic Corporation