Distance Education Using Linux and the MBone
One of the most promising applications of the Internet is distance education. The most significant advantage of Internet connectivity over previous communication paradigms for distance education is that a high degree of interaction among teachers and learners is possible. Education is a highly interactive process. If it weren't, schools wouldn't be needed—textbooks alone would be sufficient. The fact that the Internet protocols support two-way communication and virtually unlimited possibilities for integrated multimedia (given sufficient bandwidth and quality of service) presents unprecedented opportunities for extending the reach of education beyond the traditional classroom.
Most applications of Internet technology to distance education have been web-based courses. In a web-based course, the course content is developed as modules in HTML, and interaction takes place asynchronously through discussion boards or plain old e-mail. Some web-based courses also support synchronous interaction through chat sessions. Some advantages of web-based courses are that they work well at modem bandwidths and require only a web browser on the student's computer, thus maximizing the possible audience and minimizing the technical support requirements. However, a quality web-based course requires a great deal of effort to produce; and in disciplines such as engineering where the content must be updated frequently, maintenance of the course can also be significant. Another important limitation is that interactions are essentially limited to textual media. In answering a question for a student in an engineering class, the teacher often needs to construct or mark up a diagram while explaining the concept. This is best accomplished in an environment which supports real-time audiovisual interaction, which is why the traditional face-to-face classroom has survived the last few hundred years so well.
With this in mind, the “MBone virtual classroom” was developed at North Carolina State University to allow students to attend live engineering classes from a remote location by “tuning in” to the class from a workstation. The concept was to replicate as nearly as possible the face-to-face environment with real-time interaction including audio, video and graphics. To solve the problem of having many remote students attending the classroom virtually, IP-multicast and the MBone tools were employed. The virtual classroom has been in use since the fall of 1996 to provide access to engineering classes to students at several locations in North Carolina. This article describes how IP-multicast and the MBone tools were used to create the virtual classroom environment, and the development of DETA, the Distance Education Teaching Assistant, a Tcl/Tk-based wrapper which provides a simple, unified interface to the many hardware and software components of the virtual classroom.
IP-Multicast, the class-D addressing scheme in IP which makes the MBone network possible, was developed by Steve Deering in his PhD thesis at Stanford, and later developed and implemented at Xerox Palo Alto Research Center (PARC). The first multicast tunnel was established between BBN and Stanford University in the summer of 1988. The Internet Multicast Backbone (MBone) was subsequently established as a virtual network of multicast tunnels over the existing Internet. In 1992, the same year that the Internet grew to one million hosts and Mosaic was created at NCSA, the MBone carried its first real-time audio and video traffic.
IP multicast is useful in that it provides an efficient mechanism for “broadcasting” data over the Internet. It is best understood in comparison to IP unicast. When sending the same data to multiple clients using unicast, multiple separate connections to those clients must be opened. When the number of clients grows significantly, the load on the sender increases dramatically. With IP multicast, the data needs to be sent only once. The multicast-enabled network sends copies of the data to all of the clients who wish to receive the data. In this way, the sender transmits the data only once, regardless of the number of clients wishing to receive the data. It is very similar to a television broadcast, where a single transmitter sends out a single video transmission, and anybody within range of the signal can receive the transmission. Because it is more efficient at sending the same data to multiple recipients, multicast is ideal for multimedia network applications such as video-conferencing or live netcasts.
The ability to send and receive IP multicast is primarily dependent on your network. The network's routers must know how to deal with multicast packets. A few years ago, there existed very few routers capable of handling multicast packets. At that time, a method was needed to send multicast packets over networks designed to handle only unicast packets. This method became the Virtual Multicast Backbone, or MBone. Software that uses the MBone essentially packages multicast packets inside of unicast packets, which non-multicast-enabled routers know how to handle. Multicast-enabled routers are able to identify and process the multicast packets, as well as computers running MBone software.
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