Maintenance Manual

■ Differing characteristics
The range of bit rates, resolutions and frame rates available to the MCU for sending the content channel via
H.239 to H.323 video conferencing endpoints is potentially as wide as that for the main video channel.
However, in general, the main video channel is used for motion video (i.e. high frame rate streams) and the
content channel for less dynamic video such as an accompanying presentation - typically high resolution, low
frame rate.
However, the MCU allows flexibility in terms of nominating which of the available streams forms the content
channel, as well as allowing control over which endpoints are permitted to start contributing content video.
■ Uni-directionality
For the main video channel, a video conferencing endpoint would normally be both contributing (sending) a
video stream to the MCU and receiving a video stream from it.
However the content channel works differently in that an endpoint can either be sending content video or
receiving content video, but not both. A given endpoint may switch between being the contributor and a
viewer during the conference, but it will never be both simultaneously.
Passthrough and Hybrid Content
In versions prior to 4.3, the MCU always decoded the incoming content stream and then re-encoded it using either
H.263+ or H.264 before sending it out. This original mode is called Transcoded content. As of version 4.3, the MCU
can be configured to pass-through a content stream without transcoding it to participants that can support the same
content codec as the source content stream. This can reduce latency and increase quality and does not require a
video port on the MCU. This is known as Passthrough content.
There are drawbacks with both of these content modes. In Transcoded mode, all participants will see the content
encoded with the specified outgoing transcoded codec and a single participant could reduce the quality for other
participants if it had limited codec support. In Passthrough mode, the MCU does no transcoding for content, which
means that fewer endpoints may be able to decode the passed through content.
Hybrid content mode helps to avoid these drawbacks. In Hybrid mode, the incoming content stream is passed through
to participants who are able to support the same codec as the original content stream (the Passthrough content
stream). The MCU also encodes a second content stream with the specified outgoing transcoded codec for
participants who cannot decode the original Passthrough content stream. Hybrid content mode uses up a video port.
The Outgoing transcoded codec for the content stream can be explicitly specified (either H.263+ or H.264) or set to
Automatic.
To edit these settings, go to the Content section of the conference's configuration page (go to Conferences then
click on the conference name).
H.323/SIP Endpoints' Content Channel Support
For H.323 endpoints, depending on the specific endpoint and how it is configured, the content video stream may be
displayed on a separate screen, or the endpoint may show the main video and the content video streams side by side
on the same screen.
Irrespective of its content receive capability, an endpoint may or may not be able to contribute the content channel -
typically, for this to be possible it will either need a second camera or some other video input such as a VCR or "video
in" connection.
Some H.323 endpoints may have no support for the H.239 protocol. However, it is still possible for such endpoints to
display the content channel - the MCU is able to show the content channel within a normal view pane in the same
way as it displays other conference participants. This ability is controlled by the unit-wide/blade-wide Display
content in normal video channel setting (see Configuring content settings).
Content Channel Sources
As described above, a conference's content channel as sent to the set of receiving endpoints has a single source.
There are several possible content channel sources:
■ H.239 video channel
This is the most conventional content channel behavior - a H.323 conference participant opens a H.239
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