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Videoconferencing at UVa

H.239: Dual-Channel Data


The H.323 standard (Internet-based videoconferencing) is comprised of many sub-standards. One that has become more common recently is H.239, or dual-channel. The first channel is the accustomed video and audio of people. The second channel is a data channel. The data channel may show a shared desktop view, a specific application, or the output of a second camera. It is often referred to as the presentation channel.

H.239 was originally conceived and implemented by Polycom as their Content channel. Documentation for Polycom products may refer to the People + Content feature. The concept was developed into a standard and adopted into H.323. But since it is a newer feature, not all endpoints may have it.

Display Styles

Endpoints that are computer peripherals may easily display an H.239 signal in a second window. Viewing that second channel on an appliance may involve additional hardware.

Videoconference appliances usually are manufactured in a spectrum of models, often differentiated by the number of inputs/outputs built into the unit. In order for an appliance-based videoconference endpoint to be able to take full advantage of H.239, the hardware must provide a connection to an external projector or monitor so that both signals can be displayed simultaneously. Since more cables, more hardware, and (sometimes) more software configuration is involved, installing the unit will be more complicated. This does not make it necessarily a difficult task, but it does involve some time and testing that should take place before a scheduled videoconference begins.

Use Examples


The Polycom PVX desktop endpoint is able to originate and receive H.239 if it is connected to another endpoint with the same capability. If it is connected to a non-H.239-capable endpoint, attempts to start H.239 by sharing an application or by sharing the desktop simply fail silently. When the PVX receives H.239 the data is displayed in a second window on the desktop.

The user is provided with the option of sharing an individual application or whiteboard (similar to the old T.120 data sharing standard) or of sharing the entire desktop of the host computer. This does not share control of the desktop, which could be a security hazard, but does provide a second method of sharing a presentation that most other H.239 endpoints will accept.


In order to use H.239 in a multipoint videoconference, the MCU used to host the event must be H.239-enabled. UVa has a pair of Codian MCUs which are H.239-compliant.

When an MCU is involved, it centrally controls the signal being transmitted to each of the sites. For that reason, even older H.323 endpoints which do not have H.239 implementations may participate in a videoconference that uses H.239. The central MCU accepts the originating signal and sends it as part of the amalgamated view to a site which cannot accept H.239. Sites which can accept H.239 receive the differentiated signal so that it can be displayed however they choose.

How an H.239-capable MCU distributes video signals

Page Updated: 2012-02-16