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Unformatted text preview: 596 WDM Network Design In the full, limited, and fixed conversion cases, the WA problem must be suitably modified. In the case of full conversion, the constraint on a lightpath being assigned the same wavelength on every link it traverses can be dispensed with entirely. In the case of limited wavelength conversion, the wavelength assigned to a lightpath can be changed but only to a limited set of other wavelengths. In the case of fixed- wavelength conversion, the wavelength assigned to a lightpath must be changed at each node. Given a set of lightpath requests and a routing, let l i denote the number of lightpaths on link i . Then we define the load of a request to be L = max i l i . From the first constraint, we need at least L wavelengths to accommodate this set of lightpath requests. If we have full wavelength conversion in the network, the problem of wavelength assignment becomes trivial because it no longer matters what wavelength we assign to a lightpath on a given link. As long as no more than 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 10.5, under various conditions, but we consider one (somewhat extreme) example now. Example 10.5 Consider the network shown in Figure 10.13. The set of light- path requests is shown in the figure to be the following. Transmitter t i must be connected to receiver r N i + 1 , where N is the number of transmitters or receivers....
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- Spring '09