Distance Education Using Linux and the MBone
The primary components of the distance education software used at NC State are the MBone tools. These are vic, rat, wbd, and sdr are available for download in binary and source form at University College London's (UCL) web site at http://www-mice.cs.ucl.ac.uk/multimedia/software/. vic is an MBone video-conferencing tool. It was originally developed by Steve McCanne and Van Jacobson at the Lawrence Berkeley National Laboratory (LBNL) Network Research Group. The version being used at NC State is currently under development at UCL. This version provides video-capture support using Video4Linux, so many existing video-capture cards are compatible with it. It incorporates a number of codecs including H.261 and H.263. It provides controls to adjust frame rate, bandwidth, and video quality, as well as many other options. Users can switch between thumbnail and full-screen video windows, and switch between a number of video formats. vic runs in multicast- conference mode or point-to-point-unicast mode.
The Robust Audio Tool (rat) is an MBone audio-conferencing tool. rat was developed by UCL's Networked Multimedia Research Group. There are two versions of RAT: a stable, toll-quality (i.e., telephone-quality) version 3, and an improved, though experimental, high-quality version 4. rat supports both ALSA (Advanced Linux Sound Architecture) and OSS (Open Sound System), so it is compatible with a large number of sound cards. rat provides a number of audio codecs, as well as packet-loss concealment schemes. Other features include automatic gain control, silence suppression and encryption. rat also provides a graphical interface showing conference participants and audio levels. Like vic, rat can operate in point-to-point-unicast mode or in multicast-conference mode.
wbd is an MBone shared whiteboard. It allows a number of participants in a conference to share a single whiteboard workspace. It was originally written by Julian Highfield at Loughborough University. The most recent development work on it has been done by Kristian Hasler at UCL. wbd is compatible with the original LBNL whiteboard, wb, developed by Steve McCanne. Because wb is available only in binary form for UNIX platforms, Julian Highfield wrote wbd primarily to fill the need for a Windows version of wb. Since the source is freely available, we have chosen to use wbd over the binary-only wb on Linux. wbd has a standard set of whiteboard features, such as font, color and line-width options, text input capability, drawing tools and various page orientations. wbd can import both PostScript and text files. wbd was designed to work in both point-to-point and shared multicast modes, though currently only the multicast mode functions properly in Linux.
Instead of each user in a conference connecting to every other user, MBone users join a multicast group. Anything sent to the group is received by all current members of the group. None of the MBone tools discussed so far provide any means of locating or advertising these groups. This is accomplished through the session directory tool, sdr. In a sense, the session directory is like a television guide which shows all the currently available “shows” on the MBone. sdr was originally written at UCL and was modeled after another LBNL tool called sd. When the user loads up sdr, a listing of all public and private MBone sessions appears. To get more information about a specific session, the user clicks the session name. To join a session the user clicks the join button for that session and sdr then loads up the various tools needed to participate in the session. To create a new session, the user clicks the New button and enters various information about the session. sdr then generates the multicast addresses and advertises the session for other users to see.
The bandwidth requirements for the MBone tools are relatively low by current standards. Each video source requires only about 128KBps at a rate of ten frames per second. The audio requires about 64KBps at telephone quality. Higher frame-rate video is possible, but we've found that high-quality whiteboard data in combination with good-quality audio more than compensate for the slow video. The video mainly orients the participant and provides visual cues, while the actual content is provided via the audio and whiteboard data. For some classes, it is more important to provide full-motion video, and when adjusted appropriately, vic can provide this.
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