Optical Networks: A Practical Perspective - Part 50. 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. | 460 WDM Network Design Figure An example to illustrate the difference between having and not having wavelength conversion. L lightpaths use this link L wavelengths will clearly be sufficient to accommodate this request. However without wavelength conversion the number of wavelengths required could be much larger. The important question is How much larger We will study this problem in detail in Section under various conditions but we consider one somewhat extreme example now. Example Consider the network shown in Figure . The set of lightpath requests is shown in the figure to be the following. Transmitter i - must be connected to receiver r -i i where N is the number of transmitters or receivers. Clearly there are many routes for each lightpath. Interestingly however regardless of how we route each lightpath any two lightpaths belonging to this set of requests must share a common link. Thus each lightpath must be assigned a different wavelength requiring a total of N wavelengths to satisfy this set of requests. If we are clever about how we route these lightpaths we can arrange matters so that at most two lightpaths use a given link as shown in the figure. This means that the load is 2. Thus two wavelengths are sufficient to satisfy this set of requests if full wavelength conversion is available at each node in the network. Does this mean that full wavelength conversion is absolutely needed Luckily for us the example shown here is a worst-case scenario. We will quantify the benefit due to wavelength conversion in Section . LTD and RWA Problems 461 Relationship to Graph Coloring It turns out that the WA problem described earlier is closely related to the problem of coloring the nodes in a graph. To understand this better consider a graph representation of the network G where the vertices of the graph represent nodes in the network with an undirected edge between two vertices corresponding to an optical fiber link between the .
