Exploring the queueing behaviour of ATM switches

In this paper we describe methods which we have used to verify queueing models of two ATM switches, namely the Fairisle ATM switch developed at Cambridge University Computer Laboratory and an AT&T Globeview 2000 switch installed in the experimental test-bed at Telia Research Laboratories. Our approach relies on methods from large deviation theory, which we believe provides an appropriate scale on which agreement may be expected between a simple model and an actual ATM switch. We found that for the purpose of estimating delays the single-server queue provides an adequate model, at the large deviation scale, of both switches under the conditions investigated. However it does not reproduce the output behaviour of the Globeview switch at high loads. The model was evaluated using both analytic techniques and simulation. We emphasize that the model is not exact, indicating that very complex models may be required in order to obtain results more detailed than those afforded by large deviation theory. This calls into question the common practice of using detailed models of network traffic in conjunction with simplified models of the network itself.