Davide Cherubini - PhD Thesis - UniCA Eprints

7.3 The IBCN European networkSurvivability optimizationIntroducing the survivability constraints, the obtained results are significantlydifferent, and a detailed discussion is needed. Table 7.5 shows the followingstatements:• in case of single link failure, “default” metrics produce the forecasted congestion;• the IS-IS metaheuristic metrics optimization reduces the objective functionvalue by 26.45% carrying it to the 74.06%. Even if this could seems a goodresult, as the network is always under the congestion level, it is still abovethe empiric threshold of the 70% link occupation.• The MPLS LP model reduces the maximum link occupation by 36.38% withrespect to the “default” metrics and by 13.50% with respect to the IS-ISoptimized metrics. The resulting maximum occupation, in case of failure ofany one link, assumes a value of 64.06%.Phase u max % Gain (Def) % Gain (IS-IS) # LSPDefault u max = 100.7% – – 0IS-IS opt. u max = 74.06% 26.45% – 0MPLS opt. u max = 64.06% 36.38% 13.50% 543Table 7.5: Survivability optimization results for the IBCNThe following figures show in the y-axis the objective function value in caseof failure of the edge indicated in the x-axis.In Figure 7.6, it can be seen that, when edge 15 fails the network suffers acongestion event with a consequent loss of performance. Moreover, when edge 34fails the maximum occupation is over 90%.Figure 7.7 shows the same histogram obtained when the IS-IS metrics have beenoptimized using the metaheuristic procedure, while in Figure 7.8 the results ofthe LP model are depicted. In this latter case, it is important to underline thesmoothness (variance close to zero) of the objective function in case of failure ofone of any of the edge of the network.72