The ATM Paradox

Stuart Cheshire, July 1998.

ATM's big feature is guaranteed quality of service. When you set up a TCP/IP connection, the Internet does not reserve network bandwidth for you to guarantee that your data will not suffer network congestion or loss. ATM does offer guaranteed reserved bandwidth. This is its big advantage.

Or is it? If you reserve bandwidth for one user, then you have to refuse to let anyone else use that bandwidth. Everyone always talks about reservations in the context that you are the one who gets the bandwidth and it is everyone who is refused. What about when you are the one being refused? Reservations suddenly doesn't seem so wonderful any more, do they? The only way to make sure no one is refused service is to engineer your network so that you have enough bandwidth for everyone -- but if you have enough for everyone then why do they have to keep making reservations? That's the ATM paradox.

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