MBONE: Multicasting Tomorrow's Internet

Network Bandwidth

A certain amount of network bandwidth is required to receive MBONE broadcasts. Although there is no definite minimum, I'll try to establish guidelines so that you easily can determine the approximate amount of bandwidth that you need to watch or participate in a conference.

First, whatever amount of bandwidth you have, you always should run a version of the multicast routing software that supports pruning. This requirement is especially true with low bandwidth links such as ISDN. When your multicast routing software supports pruning, it will only receive the traffic that is required. The required traffic is determined by which events are joined by the people who profit from the MBONE traffic you receive. You will only receive the traffic for those events. Without pruning, you will always receive the whole MBONE traffic. For pruning to work properly, not only must you support pruning, but your feed must support it as well. If only one of you supports pruning, you will again receive the whole MBONE traffic, whether the events are watched by your people or not. Another setup in which you would receive all this traffic is if you feed a site that doesn't support pruning. This site would ask for the whole MBONE traffic, causing your site to receive it all as well.

The people who developed the MBONE have estimated that the total bandwidth consumption of the MBONE should be around a maximum of 500 Kbps. Limiting the amount of data (video, audio, whiteboard, images, and so on) to the value of 128 Kbps for video is also a common practice. However, users tend to use less than this upper limit by limiting their frame rate or by choosing lower bandwidth audio/video encoding schemes. Audio streams almost invariably take up 64 Kbps. These two rules have remained pretty constant, even though the various tools have greatly evolved over time.

A video stream from an NV (an MBONE video tool) format produces between 25 Kbps and 120 Kbps at roughly 1-15 frames per second. VIC, another MBONE tool that can handle a very effective video compression scheme called H.261, is used more and more. This video compression algorithm allows VIC to take up less bandwidth than does NV. However, to be able to compress an H.261 stream, much more CPU is required, and unless you can afford a very powerful machine, you will produce a lower frame rate with VIC than with NV.

An uncompressed (u-law format) audio stream uses 64 Kbps, which becomes 78 Kbps on the network because of overhead packets. Software compression of the audio stream will lower the 78 Kbps. There are multiple audio compression schemes that can be used. Some schemes even permit sending audio data that uses as little as 5 Kbps of your bandwidth, the quality of the audio being proportional to the amount of bandwidth used.

If a site's network link is a T1 (1.5 Mbps), the site can fully participate in MBONE events. A T1 link is probably the minimum link to have if full participation is desired. Full participation is defined as the capability to create any number of simultaneous events and to join any number of existing events. The creation of events will, of course, have to remain within the MBONE constraints. These constraints are easy to understand; the MBONE has been designed with a total capacity of 500 Kbps. This amount of bandwidth must be shared between all the MBONE users in the world. This sharing of bandwidth suggests that a single user cannot send enormous amounts of data onto the MBONE without being noticed and flamed. A mailing list exists to announce future MBONE events. Announcing an event on this list eliminates any conflict that may occur for the scheduling of events.

If a site has a fractional T1 or a lesser link, the site is not automatically disqualified for MBONE enjoyment. One of the most frequent questions that comes up is related to the new popularity of ISDN links, which offer a good price/bandwidth ratio. Can a site take advantage of its ISDN link for participating in MBONE events? The answer is yes, under certain conditions.

The first condition, one we already discussed, is that you, your feed, and the people you feed need to support pruning. Because the MBONE is designed to operate at a maximum 500 Kbps, receiving all the MBONE traffic would prevent you from being able to participate in a single session.

Someone with an ISDN link could receive an MBONE event if the total bandwidth of the event is lower than that of the ISDN link. Given the numbers presented earlier, you can easily imagine that events that match this criteria are not too numerous.

One trick is to watch part of an event. Some events are created as many separate events, one for video, one for audio, and one for the whiteboard. You can select video only, sound only, or both video and sound. In many cases, however, the priority of the MBONE traffic gives you good sound but problematic video. The shortcoming is in the design of the multicast routing software. The various kinds of MBONE traffic are categorized and given different priorities. Audio data has the highest priority because it is the most important. It is very difficult to follow a speech if you receive only 60 percent (for example) of the data due to packet loss. The whiteboard has a medium priority and video a low priority.

If your site is linked to the outside world via an ISDN link, which is the link through which all your users access FTP, e-mail, news, and so on, than you should forget about watching the MBONE via that link. A big FTP transfer or a user downloading a Web page can easily disrupt the MBONE events you are watching.

However, sites with a low bandwidth link to the outside world are far from being prevented from taking advantage of these tools and the remote conferences. It is possible to create a session that uses lower-quality video and audio. The result still will be enjoyable, and you will be able to get the information.

It is also perfectly feasible to use the MBONE internally. Imagine a company that installs MBONE tools on all the machines so that employees can use them on the internal network. Users could remotely talk to colleagues four floors below without having to leave the office and miss an important phone call. The company could also pipe a satellite feed into the local MBONE so that everyone receives a special event without having to gather in a huge room equipped with a giant screen TV. Applications of the MBONE, local or global, are very easy to find.

A site with an ISDN link could free the link from time to time to receive a special conference from the larger MBONE. This solution may seem a bit impractical, but it is a small sacrifice to make when the executive staff wants to use the MBONE to inform employees of a new policy. You also can arrange for your network provider to send you a maximum amount of MBONE data so that regular traffic never falls victim to a flood from the MBONE and the amount of MBONE traffic you receive never exceeds the capacity of your link, which would create problems in all the MBONE media you receive.

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