On the effect of bandwidth fragmentation on blocking probability in elastic optical networks

In elastic optical networks (EONs), bandwidth fragmentation refers to the existence of non-aligned, isolated and small-sized blocks of contiguous subcarrier slots in the optical spectrum. As they are neither contiguous in the spectrum domain nor aligned along the routing paths, the network operator will have difficulty to use these slots for future connections. In this work, we analyze the effect of bandwidth fragmentation on the blocking probability in EONs. Our theoretical analysis indicates that two factors related to bandwidth fragmentation have effects on the blocking probability: 1) the extent that the available slot-blocks (i.e., blocks of contiguous slots) on different links are aligned on spectrum locations, and 2) the sizes of the available slot-blocks in links' spectra for future requests. When an EON's spectrum becomes more fragmented, the first factor actually reduces the blocking probability, while the second one increases the blocking probability. Their mixed effect determines the overall trend of how the blocking probability will change with bandwidth fragmentation. Our theoretical model can forecast this trend and reveal the relation among the blocking probability, bandwidth fragmentation, request bandwidth distribution, and spectrum utilization. We have also conducted numerical simulations to verify the theoretical analysis, and the simulation results exhibit similar trends as predicted by the theoretical model.