Inconsistent references to H.281

Dave Lindbergh lindbergh at ITU.CH
Sun Nov 23 18:21:42 EST 1997


The H.224 codepoint in DataApplicationCapability.application.h224 of
H.245 was put there for use with H.324; you can use H.224/H.281
to do far-end camera control in H.324.  (H.224 itself negotiates
the use of H.281; that's why you don't see H.281 in H.245.)

My feeling (and I think this has been the view of the group
as a whole) is that H.224/H.281 won't work for H.323.  The H.281
control model is timing based: You send a message to start moving
the camera (presumably at some slow, fixed rate), then another
to stop it.  Even with H.320 or H.324, this requires the user to
"lead" the camera to compensate for end-to-end round trip delay.

With H.323, any inconsistency in packet delivery times will make
it impossible for the user to predictably control the camera.  This
could be partially fixed by adding timestamps to the messages, but
only at the expense of buffering for the worst-case packet delivery

I think the RDC portion of T.130 is a much better approach: It is
not timing based, but based on absolute positioning of the camera
("turn left 24 degrees").

Since T.130 is stalled, I suggest we think about extracting the
RDC portion of T.130 (clearly needed for H.323) and approving it
as a separate Recommendation (not necessarily using T.120).  We
could follow much the same approach as with V.CHAT/T.CHAT: A simple
stand-alone protocol (the RDC ASN.1 structures from T.130) which
can be used point-to-point in a H.245 LC, and a multipoint version
that makes use of a T.120 "wrapper" (ala T.CHAT) around the same
simple protocol.

--Dave Lindbergh

At 03:01 PM 11/21/97 -0800, you wrote:
>We were poking around a bit for information on far-end camera control, and
>ended up searching for references to H.281 in most of the documents being
>decided in January. As a result, we uncovered some inconsistencies that may
>require editorial correction of the decided texts of H.245, H.246, and
>(1) H.245 section 2 refers to 1996 versions of H.281 and H.224. We can find
>only versions published in 1994. However, I note that although H.224 and
>H.281 are listed as references, they are not actually referenced in the text
>of H.245 anywhere, either by name or by the reference number.  Either these
>references should be deleted from section 2, or the dates changed to
>(2) H.246 refers to H.281 and H.224 in section A.9, but they are not
>included in the references in section 2. The solution here could be to
>simply remove section A.9, since it is "for further study"; if this is not
>done, H.281 and H.224 should be added to section 2.
>(3) H.323 version 2 refers to H.281 and H.224 in section 6.2.7, but neither
>of these are included in the references in section 2. H.281 and H.224 should
>be added to section 2. Note that H.323 version 1 had the same problem.
>The reference in H.323 and the presence of
>DataApplicationCapability.application.h224 in H.245 seem to imply that
>H.281/H.224 can be used in H.323 terminals. It would seem fairly
>straight-forward to open a camera control data logical channel associated
>with a video channel and send H.281 commands. Does anybody actually do that
>today? Has it even been prototyped? Have details been worked out, such as
>whether it would be a TCP or UDP connection? Or was this intentionally
>postponed while T.130 work proceeded? If that's the case, should it be
>resurrected considering the state of T.130? Is H.281 simply considered not
>suitable for use on IP/Internet (due to latencies)? If so, is there desire
>to work out H.245 extensions to do the same things?  I'm sure you've all
>discussed this before I was around; sorry.
>        -- Toby
>Toby Nixon, Program Manager - NetMeeting
>Microsoft Corporation, Applications and Internet Client Group, Redmond  WA
>+1 (425) 936-2792     Fax: +1 (425) 936-7329    mailto:tnixon at
Dave Lindbergh                       Email: lindbergh at
Manager, Technical Standards Group   Voice: +1 978 623 4351
PictureTel Corporation               Fax:   +1 978 749 2804
100 Minuteman Road - M/S 635         H.320: +1 978 437 0265 (twice)
Andover MA 01810  USA      
PGP fingerprint:  C8 3E 28 69 53 0A 4B 87  EF E0 11 CD AC 87 4D CD

More information about the sg16-avd mailing list