Scheduling multicast cells in an input-queued switch

We consider policies for scheduling cells in an input-queued multicast (ATM) switch. It is assumed that each input maintains a single queue for arriving multicast cells and that only the cell at the head of line (HOL) can be observed and scheduled at one time. The policies are assumed to be work-conserving, which means that cells may be copied to the outputs that they request over several cell times. When a scheduling policy decides which cells to schedule, contention may require that it leave a residue of cells to be scheduled in the next cell time. The selection of where to place the residue uniquely defines the scheduling policy. We prove that for a 2/spl times/N switch, a policy that always concentrates the residue, subject to a natural fairness constraint, always outperforms all other policies. Simulation results indicate that this policy also performs well for more general M/spl times/N switches. We present a heuristic round-robin policy called mRRM that is simple to implement in hardware, fair and performs almost as well as the concentrating policy.