Optical Networks: A Practical Perspective - Part 52. 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. | 480 WDM Network Design Table Number of wavelengths required to perform offline wavelength assignment as a function of the load L with and without wavelength converters. The fixed conversion result for arbitrary topologies applies only to one- and two-hop lightpaths. Network Conversion Type None Fixed Full Limited Arbitrary min L 1 1 L L 2L - 1 Vm-L 2 Ring 2L - 1 L 1 L L Star L L L Tree L 2 L L Other topologies such as star networks and tree networks have also been considered in the literature. In star and tree networks wavelengths are sufficient to do WA-NC RU94 . In star networks L wavelengths are sufficient for WA-FC RS97 . The same result can be extended to arbitrary networks where lightpaths are at most two hops long. Table summarizes the results to date on this problem. It is still a topic of intense research. Multifiber Rings The wavelength assignment problem in multifiber rings is considered in LSOO . In a multifiber ring each pair of adjacent nodes is connected by k 1 fiber pairs k 1 fibers are used for each direction of transmission instead of 1 fiber. Recall that we are considering undirected edges and lightpaths and each edge represents a pair of fibers one for each direction of transmission. Thus such a multifiber ring is represented by k edges between pairs of adjacent nodes. There is no wavelength conversion but it is assumed that the same wavelength can be switched from an incoming fiber to any of the k outgoing fibers at each node. The following results on multifiber rings are proved in LSOO . Theorem LSOO Given a set of lightpath requests and a routing on a k-fiber-pair ring with load L on each multifiber link the number of wavelengths summed over all the fibers required to solve the wavelength assignment problem is no more than 1 . Maximum Load Dimensioning Models 481 Thus for a dual-fiber-pair ring k 2 the number of wavelengths required is no more than L - ij which is a significant improvement over the bound of 2L - 1 for a .
