Worst - Case Cell Delay in an Atm Switch

STUCKEY, JEFFREY J. Worst-Case Cell Delay in an ATM Switch using EDF Scheduling. (Under the direction of Professor George N. Rouskas and Professor Ioannis Viniotis.) Asynchronous Transfer Mode (ATM) is well-suited for carrying real-time tra c. Because real-time tra c must arrive at its destination prior to its playback instant, end-toend delay is one of its most critical QoS parameters. We focus our attention on analyzing delays under worst-case conditions. In particular, our interests are in characterizing the tail behavior of cell delay functions in an ATM switch using the EDF scheduling algorithm. We use simulation techniques to observe cell delays and the behavior of EDF scheduling. In this study we examine four di erent source models and their e ect on the tails of cell delay functions. We observe that cell delays are a ected by many factors such as the source model behavior, the degree of multiplexing, the leaky bucket behavior, and the synchronization of cell transmissions. Our results suggest that cell delays approach their theoretical maximum delay bound during a switch's warm-up period. Further, we conclude that cell delays increase as the degree of multiplexing decreases. We examine two di erent methods of worst-case leaky bucket behavior in terms of admitting individual cells and bursts of cells. We nd that very large cell delays result in both of these worst-case leaky bucket behaviors. Finally, we show that worst-case cell delays occur when sources are aligned to begin transmitting cells simultaneously and that these delays can be greatly reduced if sources begin transmitting cells at random points in time. WORST-CASE CELL DELAY IN AN ATM SWITCH USING EDF SCHEDULING