Towards an IP-centric Optimization: On the Topology Design of WDM-mesh Networks

We investigate the topology design in IP backbone networks while exploiting both the advantages of IP routing and optical networking. The emerging WDM and optical switching technologies enable the use of optically switched circuits to interconnect routers, which may not have direct fiber links with each other, in an IP-over-WDM network. All these “light circuits” form the underlying topology of the IP network. In this study, we optimize this WDM-layer topology to minimize the packet-loss rate under dynamic IP traffic. The dimensions of this optimization include finding the right connectivity pattern and allocate limited capacity to each light circuit. Giving the fact that the total interface capacity of a router is bounded by its internal packet-switching speed, we found the problem manifests itself as a trade off between the interface count and the interface speed. Dense connectivity leads to low average interface speed and utilization, vice versa for sparse connectivity. Interestingly, low interface utilization alone do not guarantee low packet-loss rate since higher speed pipes carry traffic more efficiently. Using both analytical and experimental methods, we verified that there are two critical loads for any network. When the network load is higher than the high critical load, the best topology is always the clique. When the network load is lower than the low critical load, the best topology is always the ring. When the network load is between these two extremes, which we believe to be the normal operating point of a backbone network, the best topology has a moderate connectivity density.