Bounded-reorder packet scheduling in optical cut-through switch

Energy efficiency of optical packet switches (OPS) is the key to ensure the profitability of backbone network providers. However, due to lack of optical random access buffer, most optical packet switches rely on electronic buffer to resolve output contention, which requires power-hungry O/E/O conversion for all packets. The recently proposed optical cut-through (OpCut) switch holds a great potential in achieving high energy efficiency, as it allows optical packets to cut through the switch in optical domain whenever possible. The energy efficiency of OpCut switch hinges on the cut-through ratio, which is the percentage of packets that cut through the switch optically. On the other hand, it is generally desirable to maintain packet order in a switch. To achieve in-order transmission, an optical packet needs to be converted to electronic form and buffered when an earlier packet from the same flow is still in the buffer, which may lead to a low cut-through ratio. In the meanwhile, the Internet is designed to accommodate a certain degree of packet reorder, which is very common in practice due to path multiplicity. In this paper, we introduce a novel reorder metric, reorder degree, to accurately describe the extent of packet reordering, and propose a flow management scheme to bound the reorder degree of transmitted flows. We then design an efficient packet scheduling algorithm that significantly increases the cut-through ratio of the OpCut switch while allowing a small degree of out-of-order transmission. Our extensive simulation results show that the cut-through ratio can be drastically increased with only a very small reorder degree.