Multi-hop scheduling for optical switches with large reconfiguration overhead

One of the limiting factors that prevents a widespread use of optical switching fabrics in high speed routers is their large reconfiguration overhead. Indeed, due to technological constraints, changing configuration in an optical switching fabric implies that the communication between input and output ports is established only after a given reconfiguration time. This reconfiguration time may be significantly larger than the packet transmission time, and cannot be considered negligible as in electronic switching fabrics. Recent works have addressed the problem of scheduling incoming traffic transmissions across optical fabrics by extending the traditional approaches conceived for electronic switches. However, for very large switch size (thousands of ports) and high speed links (tens of Gbps), performance in terms of average delays may be unacceptable. We propose a different approach, based on multi-hop scheduling, in which packets are relayed to the output port by a number of intermediate hops, i.e. successive transmissions through intermediate ports. Our approach achieves much smaller delays than those of traditional single-hop scheduling for low to medium loads, and comparable delays for high loads. We believe that multi-hop scheduling is a very promising scheme to achieve good performance when using optical switching fabrics in high-speed routers.