Performance of packet switches with input and output queueing

A single-stage nonblocking N × N packet switch is considered. In order to resolve a certain degree of output-port contention, the switching fabric allows limited queuing at the output ports. Overflow at the output queues is prevented by the use of a backpressure mechanism and additional queuing at the input ports. The impact of the backpressure effect on the switch performance is analyzed for arbitrary output buffer sizes. The investigation is based on two performance measures: the average delay and the maximum throughput of the switch. Closed-form expressions for these measures are derived for both an asynchronous and synchronous operation model. The results demonstrate that a modest amount of output queuing provides significant delay and throughput improvements over pure input queuing. With reasonable output-buffer sizes, the ideal performance of infinite output queuing can be closely approached. The maximum throughput is the same for the synchronous and the asynchronous mode of operation.