Davide Cherubini - PhD Thesis - UniCA Eprints

2.4 Restoration SchemesHowever, failures occur almost everyday [49] and an understanding of their characteristics,properties and effect is extremely valuable.Accordingly with [59], failures are classified in two main groups: those resultingfrom scheduled maintenance activities and those generated by unplannedfailures. While the events belonging to the former set can be easily tackled, thoseincluded in the latter set must be identified as the shared link failures (furtherdistinguished among those that share IP routers and those that share opticaldevices) and the single link failures.The distribution in time and space of the faults affecting a backbone networkshows that:• 20% of all failures can be attributed to scheduled network maintenanceactivities• 80% of all failures can be attributed to unplanned failures, where:– 30% are identified as shared link failures. Half of them are derivingfrom router problems, which are hardware dependent faults, while theremaining half are directly connected to the optical infrastructure;– 70% are classified as single link failuresThe present dissertation is mainly focused on finding the optimal configurationof the network parameters, which guarantees the delivery of all traffic flows evenin case of single link failure (100% survivability level). As will be shown inChapter 6, the proposed network configuration is a combination of IS-IS routingand MPLS LSP.2.4 Restoration SchemesRestoration schemes are classified as link restoration and path restoration asshown in Figures 2.3, where the blue line represents the primary path followedby flow 1 → 4 while the red line is the backup one.As shown in figure 2.3(a), in path restoration, the origin/destination nodes ofthe traffic traversing the failed link, initiate the rerouting process.12