revised version of TF21 on gcl incoming area

[Steven Magnell]

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