As the chair of the Q1 SVC Adhoc Group, I have meet informally with several folks, including Vidyo and Tandberg.
Since the IETF SVC draft is now moving into the RFC editor's queue, it is an appropriate time to start working on ITU-T signaling.
There is general consensus that H.32x systems should support the SST mode of SVC that is specified in the current IETF draft. Although Radvision suggested last year that the MST mode also would be required, there seems to be little support for that requirement. The MST mode is intended for multicast, and is considerably more complex than SST.
There is language in H.323 (Annex B) on the use of layered codecs. This
text seems more applicable to MST than SST, since SST is sent as a
single RTP session. Some normative statements (for instance the way
timestamps of the various layers are coordinated) appears to be
incompatible with the IETF SVC draft. We also note that there is an analogous RFC in the IETF which also defines the
use of layered codecs. This signaling was not employed in the SVC
draft.
Athough the Annex B text should be
reviewed more carefully, there is general consensus that simple
interworking through H.32x<->SIP gateways and in multi-protocol
conference bridges is more important than continuing the use of the
existing Annex B procedures.
There is also general consensus that attempting to precisely define the various combinations of layers that a receiver is capable of receiving is very complex, and should not be attempted in the signaling.Polycom at least sees the value in being able to actively negotiate
which layers are being sent, in a way that is compatible with the
current IETF draft. As chair, I believe
that most others also support this view. Certainly no one has objected
to including this capability. One way this could be accomplished in H.241, is to combine a simple SVC profile capability with a layer negotiation procedure that would be analogous to the set submode procedures.
Please do respond (either to this list or to me personally) on this topic, especially if you have a different view on the topics above, or if you see additional requirements that are not included. I am not planning to hold an audio conference call at this time, but if people see a need for this I will schedule one.
Stephen Botzko
Polycom