Dynamic queue length thresholds in a shared memory ATM switch

Abstract

Buffer management schemes are needed to fairly regulate the sharing of memory among different output port queues in a shared memory ATM switch. Of the conventional schemes, Static Threshold is simple but does not adapt to changing traffic conditions while Pushout is efficient and adaptive but difficult to implement. We propose a novel scheme called Dynamic Threshold which combines the simplicity of Static Threshold and the adaptability of Pushout. The key idea is that the maximum permissible length, for any individual queue at any instant of time, is proportional to the unused buffering in the switch. A queue whose length equals or exceeds the current threshold value may accept no more new cells. The Dynamic Threshold procedure presented here improves fairness and switch eflciency by guaranteeing access to the buffer space for all output queues. Computer simulation is used to compare the loss performance of the Dynamic Threshold technique with that of Static Threshold and Pushout. The Dynamic Threshold scheme is shown to, be a good compromise: while neady as simple as Static Threshold control, it offers most of the performance benefits of Pushoat. Like Pushout, the Dynamic Threshold method is adaptive, so it is more robust to uncertainties and changes in trafic conditions than Static Threshold control.