Understanding Data Sharing

Introduction

Sharing data in a videoconference is frequently crucial to the successful outcome of the conference. To understand the subject, we must first define the term "data sharing". There are perhaps three common meanings to the term.

1. The ability to send static computer windows (e.g. PowerPoint) from the presenter's computer to a display at the remote site(s) that are a part of the videoconference.
2. The ability to send computer-based video (perhaps with audio) to the remote site(s).
3. The ability to send static windows in such a manner that a user at a remote site(s) can interact with the presentation (modify or change the content of the window or application).

There are three basic ways to share data:

1. The data is sent "in band" via the audio/video stream from the videoconferencing unit in place of the normal video from the units camera.
2. The data is sent "in band" via the videoconferencing unit's data stream- some videoconferencing units have dual streams: one for the normal audio/video stream and one for data. This is normally referred to as H.239 data sharing.
3. The data is sent between computers "out of band" of the video stream- that is the data stream will share the same Internet as the videoconferencing audio/video streams but not be sent by the videoconferencing unit. This will require a secondary connection between computers at each site using VNC.

In Band Data Sharing

On some videoconferencing units, you can connect your computer directly to the videoconferencing unit. As an example with the LifeSize videoconferencing unit and some Polycom videoconferencing units, you can make the connection from your computer using a VGA cable. The videoconference unit can select to send the computer data either via the videoconferencing unit's audio/video channel or to the unit's data channel.

The data channel supports a protocol called H.239. It will send data at very high resolution, but it will not be as smooth as the video since there aren't as many frames per second being sent. Data sent via the unit's audio/video channel will be sent at a low resolution compared to the data channel, but it can send the computer data at 30 frames per second that is good for computer-based video clips. Data sent through the videoconferencing unit whether by the data channel or via the audio/video channel is referred to as "in band" data sharing. You will have to refer to your videoconferencing unit's manual to understand how to select the various options.

When data is sent via the audio/video channel, the data will replace the image of the camera connected to the videoconference unit. When the data sharing has ended, the camera video will return allowing each site to see the people again. This is true whether the videoconference is point-to-point or whether it uses the MCU.

Data sent via the videoconferencing unit's data stream, however, can be a little more complicated. In all cases it can be seen on the remote unit's display as long as H.239 support is enabled on the videoconferencing unit. Any newer Polycom, Tandberg or LifeSize units will support H.239 and each type can be configured to display the video and data stream on one screen or the data on a second screen.

Out of Band Data Sharing

For out of band data sharing, the data stream is sent over the Internet, and is not sent via the videoconferencing unit's video or data streams. There are two common ways to accomplish this both using an open source software product called VNC:

• Interconnect a computer at each site over the Internet using VNC
• Connect the presenters computer to the Codian MCU using VNC- the remote sites see the data as if the presenter had sent it as a H.239 stream (see discussion of H.239 above).

When computers are directly interconnected over the Internet using VNC, the presenter's computer is run in a "VNC server mode". All other computers run VNC in a "VNC client mode". The client computers connect to the server computer by dialing its IP address. It is then up to each site to connect their computer to a local display source to view the data. Each site will see the data change as it is changed on the presenter's computer. You should note that the process only supports a few frames per second and is thus not good for viewing video clips.

We recommend obtaining and installing VNC software from: http://sourceforge.net/projects/ultravnc.

You can also install VNC software on the presenter's computer and have that computer connect to the VNC capability on the Codian MCU. The Codian MCU will treat the data as H.239 data and send it to all remote endpoints in the videoconference. Those that support H.239 will get high resolution data. Those that do not will see the data as a lower resolution video stream.

In order to use a VNC connection to our Codian MCU, you will need to reserve that service through our videoconference scheduling system or contact the AT Videoconferencing Team at mailto:video@listserv.it.northwestern.edu.