On the number of input queues to efficiently support multicast traffic in input queued switches

The paper studies the design of IQ cell-based switch architectures with a reduced number of queues at input ports to support multicast traffic delivery. The design involves the definition of a queueing policy (to select how and where to enqueue packets) and of a scheduling policy (to select when and from which queue to transfer packets, satisfying the transmission constraints given by the switching fabric). The possible tradeoffs between performance figures, queueing policies and scheduling algorithms, are discussed. Clever design can lead to non-negligible performance gains; thus, several queuing policies and scheduling algorithms are examined by simulation. The main contributions of the paper are the following. First, it is shown that a small number of queues is sufficient to obtain good switch performance under typical traffic patterns. Second, multicast traffic patterns difficult to schedule are obtained by concentrating the load over few input ports, while keeping the output port load high. Third, the scheduling algorithm is shown to have a deeper impact on the switch performance than the queuing policy. Fourth, queueing policies that balance the load among all the available queues are shown to perform better than queuing policies based on other criteria. Finally, a novel greedy scheduling policy that improves switch performance is presented.