s Today's advanced networks maintain mostly separate electronic connections for voice and data and achieve reliability by using rings based on the SDH/SONET communication standard. In this configuration, if one link is cut, traffic flows down the other half of the ring. The SDH/SONET multiplexer aggregates the traffic onto the ring, SO there's a stack, or hierarchy, of rings, depending on the scope of the network they're serving (refer to Figure 11.1). In the local realm, which might be a small city or metro area environment, tra- ditionally the rings are based on a capacity of OC-12, or 622Mbps. For slightly larger-scale rings, such as regional rings, OC-48, or 2.5Gbps, is often the capacity. Core backbone rings provide OC-192, or 10Gbps. Currently, as metro capacity requirements are increasing, we're migrating from OC-12 to OC-48. Similarly, some of the regional rings are migrating from OC-48 to OC-192, and we're looking at ways to expand the capacity of the backbone core, generally through the addition of Chapter 11 Optical Networking dense wavelength division multiplexers, which allow multiple 10Gbps streams to coexist on one fiber. Although some systems can now offer up to 40Gbps, they are in early stages of deployment. Rather than using rings, tomorrow's networks will channel all traffic over the same fiber connection and will provide redundancy by using the Internet's mesh approach of interlocking pathways. In that case, when a line breaks, traffic can flow down several alternating pathways. Optical switches will be the foundation for building these integrated networks. Optical switches all have point-to-point links between them (refer to Figure 11.2). If there is a failure in a given link or node, traf- fic can be quickly rerouted over any other alternative in the network configuration. There are several problems with the first generation of optical networks. Band- width efficiency is an issue at the higher capacities of SDH/SONET. The SDH/ SONET hierarchy is not optimized for large data transfers, mainly