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Real Time Traffic

The Internet has been used for more than 4 years now to carry audio and video traffic around the world. The problem with this traffic is that it requires guarantees: It has a minimum bandwidth below which audio becomes incompressible, and even compressed video is just not usable. For human interaction, there is also a maximum delay above which conversation becomes intolerable.

In the experimental parts of the Internet, we have re-programmed the routers which provide the interconnection to recognise these kinds of traffic and to give it regular service.

There are two aspects to this: First we must meet the minimum traffic guarantee, and this is done by looking at the queues of traffic in the net more frequently for traffic that needs more capacity. This then also means that the delay seen by this traffic is only affected by the amount of other traffic on the network, and the basic transmission time (speed of light, or thereabouts, although around the world, this is still a significant factor. However, it is one we are not at liberty to alter!). As we increase the other traffic, our video or audio experiences increasing delays and variation of delay. So long as this stays within tolerable limits, the receiver can adapt continuously (e.g. in silences in audio or between video frames) and all is well. Meanwhile, any spare capacity carries the old best effort traffic as before.

However, when the total amount of traffic is higher than capacity, things start to break.

At this point there are three views on how to proceed.

  1. Engineer the network so that there is enough capacity. This is feasible only while most peoples access speed is limited by the "subscriber loop", or tail circuits that go to their homes/offices. When we all have fiber to the home/desk-top, the potential for drowning the net is alarming. Note though, that the phone network is currently over engineered, so for audio capacity, we could certainly switch everyone over to the Internet, and switch all our phones over to Internet based terminals, and have a flat fee model.

  2. Police the traffic, by asking people who have real time requirements to make a "call setup" like they do with the telephone networks. When the net is full, calls are refused, unless someone is prepared to pay a premium, and incur the wrath of other users by causing them to be cut off!

  3. Simply bill people more as the net gets busier. This model is proposed by economists at Harvard, and is similar to models of charging for Road Traffic proposed by the Transport Studies group at UCL. We believe it is optimal. Since we have already re-programmed the routers to recognise real-time traffic, we have the ability to charge on the basis of logging of this traffic.

    Note that we can charge differentially, as well. And until we could make the guarantees, we would have a hard time placing a contract for this. Now it is feasible.

    But we have maintained all the original advantages of the Internet (no call setup, easy to rendezvous and so on).

next up previous contents
Next: Receiver Driven Resource Up: Problems with the Previous: Best Effort and

Jon Crowcroft
Wed May 10 11:46:29 BST 1995