MultipointCapability's in H2250Capability

Hong Linh Truong hlt at ZURICH.IBM.COM
Wed Nov 19 11:14:08 EST 1997

Could anybody help me to shed more light into the meaning of the various
MultipointCapability's that have to be coded within the H2250Capability? I
went through its specification in H245 and the related sections in H323 and
came to the following conclusions, which I'd like to ask for comments:

Point 1: receiveMultipointCapability

This "indicates the receive capabilities of a terminal in a multipoint
conference" (H.245 section

If I understand H323 correctly, an endpoint should only support the
following four multipoint capabilities (H323 section 6.8):

     (i) centralized audio and centralized video,

     (ii) distributed audio and distributed video,

     (iii) centralized audio and distributed video, and

     (iv) distributed audio and centralized video.

Control and data are always centralized. The above capabilities are coded
within the mediaDistributionCapability of the receiveMultipointCapability.

"Centralized" means the endpoint cannot mix/switch the media stream; the
mixing/switching has to be done by means of an MP. "Distributed" means the
endpoint can do the mixing itself.

In case of "centralized" the endpoint can receive the mixed/switched media
stream either via an unicast or a multicast address; multi-unicast is not
relevant in this context. In case of "distributed" the endpoint can receive
the multiple media streams either via multi-unicast or via a multicast
address and do the mixing/switching itself.  The support of multicast
address or multi-unicast is indicated in multicastCapability and

Resumee: The H245 receiveMultipointcapability is appropriate to describe
the  receive capabilities of a terminal in a multipoint conference.

Point 2: transmitMultipointCapability

This "indicates the transmit capabilities of a terminal in a multipoint

>From a transmit point of view "centralized" or "distributed" are not
relevant. Relevant are only whether the endpoint support multicast and/or
multi-unicast. Therefore the mediaDistributionCapability (mandatory) within
the transmitMultipointCapability has no meaning and I'd propose to ignore

Point 3: receiveAndTransmitMultipointCapability

After having coded the  receiveMultipointCapability and
transmitMultipointCapability I don't know what kind of "new" information
should an endpoint put into this parameter. Since all three are mandatory
I'd propose to ignore that parameter!

Point 4: H323 Section 6.8.1 "Centralized multipoint capability", last
sentence: "Optionally, distributedAudio and distributedVideo may be used to
indicate multicast distribution of media streams".

I don't understand the meaning of that sentence, which is new added to H323
version 2. I thought, distributedAudio and distributedVideo indicate the
endpoint capability of mixing/switching the media streams itself. The
indication of whether multicast and/or multi-unicast is supported is done
by means of multicastcapability and multiUniCastConference, and not by
mediaDistributioncapability as stated in that sentence.

I'd appreciate if somebody could give me an explanation of that sentence.

I'd also appreciate any comments to the conclusions I put together above.



 TRUONG Hong Linh                    email: hlt at
 IBM Zurich Research Laboratory      tel  : +41 1 724 8434
 CH-8803 Rueschlikon / Switzerland   fax  : +41 1 724 8955

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