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First-Last Fit

In the first-last fit spectrum allocation policy [99], all spectrum slots of each link can be divided into a number of partitions. The first-last fit spectrum allocation policy always attempts to choose the lowest indexed slots in the odd number partition from the list of available slots. For the even number partitions, it attempts to choose the highest indexed slots from the list of available spectrum slots. Use of first fit and random fit spectrum allocation approaches always attempt to choose the lowest indexed slots for each partition and randomly selects slots, respectively, from the list of available spectrum slots. This may lead to a situation where spectrum slots may be available, but connection requests cannot be established due to unavailability of contiguous aligned slots. The first-last fit allocation policy is expected to give more contiguous aligned available slots than the random fit and first fit allocation policies.

Least Used

The least used spectrum allocation policy [6,7] allocates a spectrum to a lightpath from a list of available spectrum slots that have been used by the fewest fiber links in the network. If several available spectrum slots share the same minimum usage, the first fit spectrum allocation policy is used to select the best spectrum slot. Selecting spectrum in this manner is an attempt to spread the load evenly across all spectrum slots.

Most Used

The most used spectrum allocation policy [6,7] assigns spectrum to a lightpath from a list of available spectrum slots, which have been used by the most fiber links in the network. Similar to the least used spectrum allocation policy, if several available spectrum slots share the same maximum usage, first fit spectrum allocation policy is used to break the tie. Selecting spectrum slots in this way is an attempt to realize maximum spectrum reuse in the network.

4.3.3.7 Exact fit

Starting from the beginning of the frequency channel, the exact fit allocation policy [98] searches for the exact available block in terms of the number of slots requested for the connection. If there is a block that matches the exact size of requested resources, this policy allocates that spectrum. Otherwise, the spectrum is allocated according to the first fit spectrum allocation policy. By selecting spectrum slots in this way, we can reduce the fragmentation problem in optical networks.

To illustrate the functionality of the above mentioned spectrum allocation policies, we use the example shown in Fig. 4.8. If two connection requests arrive that use link 2 and link 3 with one slot demand for establishing lightpaths, the strategies proceed as follows. First fit spectrum allocation policy selects spectrum slot 2 for the first connection request, and slot 3 for the second connection request. First-last fit spectrum allocation policy selects spectrum slot 2 for the first connection request, and slot 12 for the second connection request. Slot 6 and slot 4 have been used three times and two times, respectively, in this example. Therefore, slot 6 and slot 4 are used by most used spectrum allocation policies. As slot 2 and slot 9 have not been used so far, the least used spectrum allocation policy selects these two slots for the two connections requests. Random fit allocation policy selects any two of slot 2, slot 3, slot 4 and slot 12 with equal probability. Exact fit spectrum allocation policy selects spectrum slot 6 for the first connection request, and slot 2 for the second connection request.

Spectrum slot usage pattern for a network segment

Figure 4.8: Spectrum slot usage pattern for a network segment.

 
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