Optical Networks: A Practical Perspective - Part 48. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 440 WDM Network Design 10 links a b Figure a The lightpath topology of the three-node network corresponding to Figure a that is seen by the routers. Routers A-B and B-C are connected by 10 parallel links b The lightpath topology of the three-node network corresponding to Figure b that is seen by the routers. All pairs of routers A-B B-C and C-A are connected by 5 parallel links. In our example the fiber topology is a linear one with three nodes and the traffic requirement is 50 Gb s between every pair of these nodes. The task is to design a lightpath topology that interconnects the IP routers and to realize this topology within the optical layer. In our example two lightpath topologies that meet the traffic requirements are shown in Figure . We call the first problem the lightpath topology design LTD problem. We call the problem of realizing the lightpath topology within the optical layer the routing and wavelength assignment RWA problem for reasons that will become clear shortly. The RWA problem is simple to solve in this example because there is only one route in the fiber topology between every pair of nodes. In a general topology the RWA problem can be quite difficult. The realization of the two lightpath topologies of Figure are shown in Figures b and c . Another problem we face in the design of wavelength-routing networks is that of grooming the higher-layer traffic. The term grooming is commonly used to refer to the packing of low-speed SONET SDH circuits for example STS-1 into higher-speed circuits for example STS-48 or STS-192 . This is the function provided by digital crossconnects. While the term is usually not applied to IP routers conceptually IP routers can be considered to provide the grooming function at the packet level. In order to reap the benefits of optical passthrough the higher-layer traffic must be groomed appropriately. For example in Figure c all the traffic destined for node B must be groomed onto a few .
