Packet-Mode Emulation of Output-Queued Switches

Abstract

Most common network protocols transmit variable size packets, whereas contemporary switches still operate with fixed- size cells, which are easier to transmit and buffer. This necessitates packet segmentation and reassembly modules, resulting in significant computation and communication overhead that might be too costly as switches become faster and bigger. It is, therefore, imperative to investigate an alternative mode of scheduling in which packets are scheduled contiguously over the switch fabric. This paper investigates the cost of packet-mode scheduling for the combined input-output-queued (CIOQ) switch architecture. We devise frame-based schedulers that allow a packet-mode CIOQ switch with small speedup to mimic an ideal output-queued switch, with bounded relative queuing delay. The schedulers are pipelined and based on matrix decomposition. Our schedulers demonstrate a trade-off between the switch speedup and the relative queuing delay incurred while mimicking an output-queued switch. When the switch is allowed to incur high relative queuing delay, a speedup arbitrarily close to two suffices to mimic an ideal output-queued switch. This implies that packet-mode scheduling does not require higher speedup than a cell-based scheduler. The relative queuing delay can be significantly reduced with just a doubling of the speedup. We further show that it is impossible to achieve zero relative queuing delay (that is, a perfect emulation), regardless of the switch speedup. In addition, simpler algorithms can mimic an output-queued switch with a bounded buffer size, using speedup arbitrarily close to one. Simulations confirm that packet-mode emulation with reasonable relative queuing delay can be achieved with moderate speedup. Furthermore, a simple and practical heuristic is shown by simulations to also provide effective packet-mode emulation.