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Multimedia in the Teaching Space
4.4 THE MBONE and MULTICASTING
This section contains extracts from two major articles and books on the Mbone and multicasting. This information is contained on the Web and an investigation of the web sites will provide much more detailed and comprehensive information. The book MBone: Multicasting Tomorrow's INTERNET by Kevin Savetz, Neil Randall, and Yves Lepage (ISBN: 1-56884-723-8) and published by IDG Books Worldwide, Inc., gives comprehensive information. An extract from this book is shown in section 4.4.1
An extracts from a second book, - MBONE, the Multicast BackbONE, written by Mike Macedonia and Don Brutzman of the Naval Postgraduate School is described in section 4.4.2.
4.4.1. Multicasting - Tomorrow's INTERNET Today, (by Kevin Savetz, Neil Randall, and Yves Lepage).
The MBONE is a critical piece of the technology that's needed to make multiple-person data, voice, and video conferencing on the INTERNET -- in fact, sharing any digital information -- cheap and convenient.
What Is Multicasting?
Multicasting is a technical term that means that you can send a piece of data (a packet) to multiple sites at the same time. (How big a packet is depends on the protocols involved-it may range from a few bytes to a few thousand). The usual way of moving information around the INTERNET is by using unicast protocols -- tools that send packets to one site at a time.
You can think of multicasting as the INTERNET's version of broadcasting. A site that multicasts information is similar in many ways to a television station that broadcasts its signal. The signal originates from one source, but it can reach everyone in the station's signal area. The signal takes up some of the finite available bandwidth, and anyone who has the right equipment can tune in. The information passes on by those who don't want to catch the signal or don't have the right equipment.
On a multicast network, you can send a single packet of information from one computer for distribution to several other computers, instead of having to send that packet once for every destination. Because 5, 10, or 100 machines can receive the same packet, bandwidth is conserved. Also, when you use multicasting to send a packet, you don't need to know the address of everyone who wants to receive the multicast; instead, you simply "broadcast" it for anyone who is interested. (In addition, you can find out who is receiving the multicast -- something television executives undoubtedly wish they had the capability to do.)
How is the MBONE different from Multicasting?
Unfortunately, the majority of the routers on the INTERNET today don't know how to handle multicasting. Most routers are set up to move traditional INTERNET Protocol (IP) unicast packets -- information that has a single, specific destination. Although the number of routers that know how to deal with multicast are growing, those products are still in the minority.
Router manufacturers have been reluctant to create equipment that can do multicasting until there is a proven need for such equipment. But, as you might expect, it's difficult for users to try out a technology until they have a way to use it. Without the right routers, there's no multicasting. Without multicasting, there won't be the right routers. Catch-22.
What is a router? A router is a device that connects a local area network -- such as an inter-office LAN -- to a wide area network -- such as the INTERNET. The router's job is to move information between the two networks.
Many routers today are unicast routers: They are designed to move information from a specific place to another specific place. However, routers that include multicasting capabilities are becoming more common. In 1992, some bright fellows on the INTERNET Engineering Task Force (IETF) decided that what no one would do in hardware, they could do in software. So they created a "virtual network" -- a network that runs on top of the INTERNET -- and wrote software that allows multicast packets to traverse the Net. Armed with the custom software, these folks could send data to not just one INTERNET node, but to 2, 10, or 100 nodes. Thus, the MBONE was born.
The MBONE is called a virtual network because it shares the same physical media -- wires, routers and other equipment -- as the INTERNET.
The MBONE allows multicast packets to travel through routers that are set up to handle only unicast traffic. Software that utilises the MBONE hides the multicast packets in traditional unicast packets so that unicast routers can handle the information. The scheme of moving multicast packets by putting them in regular unicast packets is called tunnelling. In the future, most commercial routers will support multicasting, eliminating the headaches of tunnelling information through unicast routers.
When the multicast packets that are hidden in unicast packets reach a router that understands multicast packets, or a workstation that's running the right software, the packets are recognised and processed as the multicast packets they really are. Machines (workstations or routers) that are equipped to support multicast IP are called mrouters (multicast routers). Mrouters are either commercial routers that can handle multicasting or (more commonly) dedicated workstations running special software that works in conjunction with standard routers.
So, what's the difference between multicasting and the MBONE? Multicasting is a network routing facility -- a method of sending packets to more than one site at a time. The MBONE is a loose confederation of sites that currently implement IP multicasting.
The MBONE -- or multicast backbone -- is a fancy kludge, a hack. It is at best a temporary utility that will eventually become obsolete when multicasting is a standard feature in INTERNET routers. By then there will be an established base of MBONE users (which should make the router manufacturers happy). The utilities and programs that work on today's MBONE will undoubtedly work on the multicast backbone of tomorrow.
Perhaps the most sought-after function that the MBONE provides is videoconferencing. The MBONE originated from the INTERNET Engineering Task Force's attempts to multicast its meetings as INTERNET videoconferences. MBONE video is nowhere close to television quality, but at a few frames a second, video quality is good enough for many purposes.
The MBONE's capability to carry remote audio and video makes it a wonderful tool for seminars, lectures, and other forms of "distance education." Imagine sitting in on a lecture that's being given live thousands of miles away and even asking questions or contributing to a panel discussion. Indeed, much of what happens today on the MBONE is of a technical nature, information that most of us would find dull. However, the nerds don't get to keep the MBONE to themselves. Besides esoteric engineering events, the MBONE is home to more exciting fare, such as multicasts of concerts, a do-it-yourself-radio station, and even poetry readings.
Can Your Computer Handle the MBONE?
Although anyone who has the right equipment can use the MBONE, the hardware and connectivity requirements for using the MBONE are much greater than what's available on the equipment that most INTERNET users have in their homes. A PC or Macintosh system coupled with a standard modem doesn't have enough computing power or bandwidth to send or receive MBONE transmissions. You need a good deal of power to handle multicast IP. Today, multicasting software -- the behind-the-scenes tools for moving, encoding, decompressing, and manipulating multicast packets -- is available only for high-end UNIX workstations, but the situation is changing, it's not too much to imagine that MBONE tools will soon be available for home computers -- PCs that are running Microsoft Windows and Macintosh computers. It will probably take the most powerful home computers (with Pentium and PowerPC chips), but it seems to be a likely eventuality. The software tools are being built: PC/TCP Version 2.3 from FTP Software Inc. supports multicasting for PCs, as does Windows 95, and it is rumoured that the next version of MacTCP will support multicasting.
How Much Bandwidth Is Necessary?
Even if users had the hardware to do multicasting today, another huge hurdle would prevent the MBONE from taking over the INTERNET: Most users don't have enough bandwidth. A multicast video stream of 1 to 4 frames per second eats about 128Kbps of bandwidth (ISDN) and gives you slow, grainy, bandwidth-hogging video. (By comparison, television-quality video scans at about 24 frames per second). Remember though, that a video stream uses the same bandwidth whether it is received by 1 workstation or 100.
In their paper, "MBONE Provides Audio and Video across the INTERNET," authors Michael Macedonia and Donald Brutzman (see below section 4.4.2) write, "Only a few years ago, transmitting video across the INTERNET was considered impossible. Development of effective multicast protocols disproved that widespread opinion.
"Until recently, experts believed that the MBONE could not be used for transmission of simultaneous video, audio, and data because of limited bandwidth," notes Professor Don Brutzman. "This effort to push the envelope of computing technology has provided valuable data to computer scientists and has shown that methods can be employed to work around the bandwidth problem."
There's a ceiling to the amount of information that can move around on the MBONE as a whole: 500Mbps. At full tilt, the MBONE itself can handle no more than four simultaneous videoconferencing sessions or eight audio sessions.
4.4.2. MBONE, the Multicast BackbONE, (by Mike Macedonia and Don Brutzman).
MBONE stands for Multicast Backbone, a virtual network that has been in existence for about three years. The network originated from an effort to multicast audio and video from the INTERNET Engineering Task Force (IETF) meetings. MBONE today is used by several hundred researchers for developing protocols and applications for group communication. Multicast is used because it provides one-to-many and many-to-many network delivery services for applications such as videoconferencing and audio that need to communicate with several other hosts simultaneously.
Multicasting has existed for several years on local area networks such at Ethernet and FDDI. However, with INTERNET Protocol (IP) multicast addressing at the network layer the service group communication can be established across the INTERNET. The reason that MBONE is a virtual network is that it shares the same physical media as the INTERNET, though it must use a parallel system of routers that can support multicast (e.g. dedicated workstations running with modified kernels and multiple interfaces) augmented with "tunnels". Tunnelling is a scheme to forward multicast packets among the islands of MBONE subnets through INTERNET IP routers which typically do not support IP multicast. This is done by encapsulating the multicast packets inside regular IP packets.
The key to understanding the constraints of MBONE is thinking about bandwidth. The reason why a multicast stream is bandwidth-efficient is that one packet can touch all workstations on a network. Thus a 125Kbps video stream (1 frame/second) uses the same bandwidth whether it is received by one workstation or twenty. That is good. However that same multicast packet is prevented from crossing network boundaries such as routers or bridges. The reasons for this restriction are religious and obvious: if a multicast stream which can touch every workstation could jump from local network to local network, then the entire INTERNET would quickly become saturated by such streams. That is very bad! Thus the MBONE scheme encapsulates multicast streams into unicast packets which can be passed as regular INTERNET protocol packets along a virtual network of dedicated multicast routers (mrouters) until they reach the various destination local area networks. The use of dedicated mrouters segregates MBONE packet delivery, protecting standard network communications such as mail and TelNet from MBONE experiments and failures. Once properly established, an mrouter needs little or no attention. Given this robust distribution scheme, responsible daily use of the MBONE network consists only of making sure you don't overload your local or regional bandwidth capacity.
When a host on an MBONE-equipped subnet establishes or joins a group it announces that event via the INTERNET Group Management Protocol (IGMP). The multicast router on the subnet forwards it the other routers in the network. MBONE sessions use a tool developed by Van Jacobson of Lawrence Berkeley Laboratories called sd (session directory) to display the announcements by multicast groups. sd is also used for launching multicast applications and for automatically selecting an unused address for any new groups.
Groups are disestablished when everyone leaves, freeing-up the IP multicast address for reuse. The routers occasionally poll hosts on the subnets to determine if any are still group members. If there is no reply by a host, the router stops advertising that hosts group membership to the other multicast routers.
The magic of MBONE is that teleconferencing can be done in the hostile world of the INTERNET where variable packet delivery delays and limited bandwidth play havoc with applications that require some real-time guarantees. It is worth noting that only a few years ago putting audio and video across the INTERNET was considered impossible. Development of effective multicast protocols disproved that widespread opinion.
In addition to the multicast protocols, MBONE applications are using the Real Time Protocol (RTP) on top of User Datagram Protocol (UDP) and IP. RTP is being developed by the Audio-Video Transport Working Group within the IETF. RTP provides timing and sequencing services; permitting the application to adapt and smooth out network-induced latencies and errors. The end result is that even with a time-critical application like an audio tool, participants normally perceive conversations as if they are in real-time, even though there is actually a small buffering delay to synchronise and sequence the arriving voice packets. Protocol development continues. Although operation is usually robust, many aspects of MBONE are still considered experimental.
Another aspect of this research is the need to compress a variety of media and to provide privacy through encryption. Several techniques to reduce bandwidth include Joint Photographic Experts Group (JPEG) compression and the ISO standard H.261 for video. Visually this translates to velocity compression: rapidly changing screen blocks are updated much more frequently than slowly changing blocks. Encoding for audio include Pulse Coded Modulation (PCM) and GSM. Outside of the concerns for real-time delivery, audio is a difficult media for the MBONE and teleconferencing in general because of the need to balance signal levels for all the parties who may have different audio processing hardware (e.g. microphones and amplifiers). Audio also generates lots of relatively small packets, which are the bane of network routers.
Besides basic networking technology, MBONE researchers are developing new applications that typify many of the goals associated with an "information superhighway." Video, audio, and a shared drawing whiteboard are the principal applications, provided by software packages called nv (net video), vat (visual audio tool) and wb (whiteboard).
Groupwork on groupware.
The MBONE community is active and open. Work on tools, protocols, standards, applications and events is very much a co-operative and international effort. Feedback and suggestions are often relayed to the entire MBONE mailing list (as an example, this article was proofed by that group). Co-operation is essential due to the limited bandwidth of many networks, in particular transoceanic links. So far no hierarchical scheme has been necessary for resolving potentially contentious issues such as topology changes or event scheduling. Distributed problem solving and decision making has worked on a human level just as successfully as on the network protocol level. Hopefully this decentralised approach will continue to be successful even in the face of rapid addition of new users.
It is not every day that someone says to you "Here is a multimedia television station that you can use to broadcast from your desktop to the world." These are powerful concepts and powerful tools that tremendously extend our ability to communicate and collaborate. These tools are already changing the way people work and interact on the net. See you later!
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