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Last Fit

The last fit allocation policy [6, 100] invariably tries to select the highest indexed available slot and use it for lightpath provisioning. When the lightpath is released, the slot is added to the list of free slots. The last fit spectrum allocation policy matches the computational complexity of the first fit spectrum allocation policy. This policy is not suitable for transmission systems that do not adopt dispersion compensation including digital signal processing. If we do not adopt dispersion compensation, the overall dispersion effect of a lightpath using last fit is higher than that of using first fit. This is because the dispersion effect increases with increase in the wavelength range. As an example, the dispersions of the wavelengths of 1.52 m and 1.53 цm are 18 ps/nm/km and 19.5 ps/nm/km, respectively.

To overcome the problem of last fit policy, the work in [109] suggests that longer lightpath requests need to be assigned from the smallest indexed spectrum slot, where the dispersion effect is less, and shorter lightpath requests need to be assigned from the largest indexed spectrum slot, where the dispersion effect is more. As a result, a less robust modulation technique is used to maintain the QoT threshold level for the longer lightpath requests, and hence a lower number of spectrum slots are required for lightpath establishment.

First-Last Fit

The first-last fit spectrum allocation policy [161, 162] is a combination of first fit and last fit spectrum allocation policies. Lightpath requests in the network are divided into two groups; one group uses the first fit allocation policy and the other group uses the last fit spectrum allocation policy. The group formulation can be performed in several ways. In f 161], lightpath requests are grouped based on disjoint and non-disjoint paths. Lightpaths with disjoint paths are allocated using the first fit allocation policy, whereas lightpaths with non-disjoint paths are allocated using the last fit allocation policy. The intention of this policy is to avoid spectrum fragmentation in the network. The performance of this policy depends mainly on how lightpath groups are formed; if the lightpath groups are not properly formed, its performance is degraded [100].

 
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