MBONE: Multicasting Tomorrow's Internet

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 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.

Incidentally, 128Kbps is about nine times the speed of a 14.4Kbps modem. A dual-channel ISDN line can move data at 128Kbps, so if you are one of the lucky few who have ISDN, you have barely enough bandwidth to receive multicast video. (Sending video requires another 128Kbps, which makes using ISDN for two-way videoconferencing barely tolerable, if not impossible.) Most experts agree that in order to do multicasting effectively and get other work done, you need a T-1 or faster link to the Internet (although some users have managed to make the tools work with as little as 56Kbps). That 128K video stream uses nearly 10 percent of a T-1 line; several simultaneous high-bandwidth sessions can easily saturate network links.

High-speed connections to the Internet can cost thousands of dollars a month. Lower-speed connections cost much less. The faster you go, the more you pay.

In their paper, "MBONE Provides Audio and Video across the Internet," authors Michael Macedonia and Donald Brutzman write, "Only a few years ago, transmitting video across the Internet was considered impossible. Development of effective multicast protocols disproved that widespread opinion. In this respect, MBONE is like the proverbial talking dog: It's not so much what the dog has to say that is amazing, it's more that the dog can talk at all!" Macedonia and Brutzman's paper is reprinted in Chapter 8.

What is ISDN?

ISDN -- Integrated Services Digital Network -- is a type of telephone service that enables you to get high-speed data connections through your phone line. ISDN is basically the telephone network turned completely digital, using existing wiring.

ISDN is much cheaper than many other methods of moving data at high speeds, but it is still expensive relative to a normal phone line. (Normal phone lines -- the kind that work reliably with your 14.4Kbps modem, fax, answering machine, and the Sports Illustrated football phone, are known in some circles at POTS-- plain old telephone service.)

ISDN is new, but it is catching on. A major drawback to ISDN is that because it moves digital data instead of analog data, it doesn't work with your regular modem, answering machine, or football phone. You need special, expensive equipment to perform those functions at ISDN speeds.

On, the bright side, ISDN is faster than a standard modem. ISDN is available in various parts of the world, including Australia, Western Europe, Japan, Singapore, France, and portions of the U.S.

The World Wide Web site http://alumni.caltech.edu:80/~dank/isdn/ is one of the best Internet resources for finding out about ISDN. For more information, check out that site.

Audio multicasts, partly because of their lesser bandwidth requirements, are more common on the MBONE than are video multicasts. Multicast audio typically uses 56 to 64Kbps of bandwidth. (Thanks to heavy hacking and experimental compression tools, MBONE audio and interactive whiteboard traffic have been demonstrated by using as little as 9600 bps lines. These demonstrations are one indication that eventually those of us who access the Internet from home at 14.4Kbps will be able to have some access to the MBONE.)

"For the multicast broadcasting model that the MBONE establishes to succeed as a mainstream medium, current technologies simply have to advance," writes Internet guru Aaron Weiss. MBONE services simply eat more bandwidth than most of us can afford. Before multicasting becomes commonplace, either bandwidth needs to be available more cheaply, or our ability to compress bandwidth-hogging information into a limited bandwidth space needs to improve. "Network bandwidth has to fatten or audio-video compression schemes have to flatten," Weiss writes. "Presumably, both will occur, which also will require increased CPU power at the home computer level. Although it's probable that all three of these developments will take place, the time frame is not clear," he says.

Even if you could push 128 kilobits (or more) each second around the Internet affordably, it's a good bet that when enough of us could push that much data around that fast, the sheer load of all that data pulsing though the Internet would bring it to a standstill. One of the IETF's jobs is to plan for this eventuality.

"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 (million bits per second). At full tilt, the MBONE itself can handle no more than four simultaneous videoconferencing sessions or eight audio sessions.

"Although there is much to experience on the MBONE, there isn't much space for everyone. There is only about 500Kbps of bandwidth available to the entire MBONE community at any one time. With video streams typically running at about 128Kbps and audio streams at 64Kbps, there is a small and finite limit on the number of simultaneous transmissions the MBONE can handle," writes Weiss.

This limited resource environment presents MBONE users with what Weiss calls "the classic sandbox scenario": sharing and playing nice. Sharing means planning multicast events in advance and scheduling them with the rest of the MBONE community to eliminate conflicts. Internet e-mail lists have been set up for announcing scheduled events.

Special multicast programs are announced on the rem-conf mailing list. To subscribe, send e-mail to rem-conf-request@es.net with a message body of "subscribe".

Sometimes two planned events conflict (for example, a High Energy Physics conference conflicted with a planned IETF meeting, so the physics conference was broadcast at a later time). At times, oversight or naivete can wreak havoc. "In mid-1994, a host in Japan was found to have been sending 650Kbps video-streams over the entire MBONE, effectively trashing it," according to Weiss. The problem, as it turned out, was caused not by malice, but by a program bug that enabled the multicast packets to escape a local network. Such unintentional flooding happens periodically and is a testament to the experimental nature of the MBONE.

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