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 7.2.2.4).
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 multiUniCastConference.
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 conference".
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 it.
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.
Regards,
Linh
----------------------------------------------------------- TRUONG Hong Linh email: hlt@zurich.ibm.com IBM Zurich Research Laboratory tel : +41 1 724 8434 CH-8803 Rueschlikon / Switzerland fax : +41 1 724 8955