H.245 vs. SDP

Mike Buckley mikebuckley at ATTMAIL.COM
Fri Apr 16 11:50:32 EDT 1999

Thanks for the clarification Tom.  This is exactly what I meant.

I agree that caps exchange will generally be a registration process. However I
would pose a question whether there might not be a need for a caps exchange
after registration to identify resources temporarily unavailable (traffic loading etc)
or unavailable in a specific set of circumstances.  Also is there a role for a caps
exchange to change parameters during a session? This might be particularly
important in multimedia situations.


____________________ Begin Original Message
Date: Fri Apr 16 09:24:57 -0400 1999
From: internet!NORTELNETWORKS.COM!taylor (Tom-PT Taylor)
Subject: H.245 vs. SDP
Content-Type: Generic-Text
Content-Length: 4210

I would like to clarify the issue of H.245 vs. SDP.  First off, what we are
talking about here is NOT capability negotiation.  The latter is handled by
signalling between MGCs.  A given MGC obtains the information needed to
support its end of the negotiation through use of the Audit command.
Megaco/H.GCP has the action to ensure that responses to Audit commands
convey the necessary information.

That said, the question becomes one of SDP vs. the H.245 OpenLogicalChannel
(OLC) and OLC Ack structures.  I think what Mike is saying is that there are
two issues in this discussion: content, and ordering of that content.

On content, I think we can get general agreement that if the H.245 OLC can
request something, the Media Gateway control protocol should be able to
propagate that request to the MG.  Thus there is no argument in general over
content, leaving aside housekeeping items like the H.245 logical channel
number which may be irrelevant to the MG.

The contentious point is therefore one of how the individual parameters
should be ordered.  What I suggest is that we see through example how they
are used, then decide on ordering.  We will probably find that there is a
dependency here on how we use contexts to represent multimedia.  A
single-medium context corresponds to one (bidirectional) or two
unidirectional H.245 logical channels.  There is an issue here: how does the
MGC correlate the second logical channel with the first one?  Do we end up
with a separate context for each H.245 logical channel?   A multimedia
context corresponds to a number of logical channels, but the correlation
rules are simpler: all logical channels associated with the same call (H.323
definition) are grouped together.

> -----Original Message-----
> From: Kumar, Vineet [SMTP:vineet.kumar at INTEL.COM]
> Sent: Thursday, April 15, 1999 4:37 PM
> Subject:      Re: H.GCP draft uploaded
> I agree with Mike in using H.245 between MGC and MG. SDP is essentially
> used
> for advertising loosely coupled sessions, so it does not have the
> functionality necessary to be considered in H.GCP. Also, H.GCP must re-use
> H.323 components whenever possible.
> vineet
> -----Original Message-----
> From: Mike Buckley [mailto:mikebuckley at ATTMAIL.COM]
> Sent: Wednesday, April 14, 1999 7:04 PM
> Subject: Re: H.GCP draft uploaded
> Bryan,
> I don't know whether this is the right place to comment on the Draft.  I
> have also
> copied some of these comments to the megaco list.
> I would like to see the use of SDP between the MGC and MG added to the
> issues list.  My understanding is that the symantics and syntax of SDP are
> different from H.245OLC.  Use of SDP in H.gcp will therefore involve a
> reordering
> of the information sent in H.245 between endpoints.  This will add
> transcoding
> complexity and latency.  In addition, all the features of H.245 I believe
> cannot be
> presently supported in SDP.  For maximum efficiency and flexibility
> therefore
> I think the mechanism used to convey capabilities should mirror the
> semantics
> and syntax of H.245OLC.  I don't believe there is any extra overhead in
> adopting this approach over SDP.  The coding used may be different from
> or
> BER.
> I think this also fits in with the list of H.gcp requirements where it is
> stated that
> all the features of H.245 should be supported.
> Mike
> ____________________ Begin Original Message
> ___________________________
> Date: Tue Apr 13 08:57:37 -0400 1999
> From: internet!VIDEOSERVER.COM!bhill (Bryan Hill)
> Subject: H.GCP draft uploaded
> Content-Type: Text
> Content-Length: 367
> Mr. Okubo,
> Please find a version of HGCP.doc and HGCP.zip that I have uploaded into
> the
> ptel incoming site.  These are drafts of the contribution I am preparing
> for
> Santiago.
> Best Regards,
> Bryan Hill
> _________________________________________________________
> Bryan Hill
> VideoServer Inc.
> (781) 505-2159
> bhill at videoserver.com <mailto:bhill at videoserver.com>

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