When resto- ration is set up there is no preassignment of survivability resources to specific paths, but a set of resources is assigned to restoration and left unused up to the moment a failure occurs. When a failure occurs, the control plane eliminates the failed link from the topol- ogy and reroutes the failed GLSPs using restoration resources. In general, a complex algorithm is needed to reserve the minimum amount of restora- tion resources to face the desired set of failures (e.g., all the single link failures), but in the simple example we are dealing with, it is quite clear that if a spare wavelength is installed in links 1-2, 2-3, 1-4, and 3-4, whatever optical channel is involved in a single failure can Convergent Network Management and Control Plane be rerouted on the spare wavelengths. Thus restoration, in this specific case, to face all single-channel failure, needs 4 spare wavelengths (in each direction) and consequently 16 spare OXC ports. When a channel is restored using restoration resources, these are occupied; thus, in our example, no other channel can be restored. As in the case of shared protection, in complex situations it is quite difficult to evaluate the exact network survivability degree, but a lower bound can be evaluated. Such lower bound is obtained by assuming as failure probability the probability that two working paths will fail in the repair time plus the probability that a working path fails besides a single restoration reserved link. Thus we can write, as in the previous section, =1-P(fail,1)[1-P(fail,2)]-P(fail,1)[1-P(fail,2)]-P(fail,1)P(fail,2) (8.10) Where P( (fail,1) is given by Equation 8.11 and P(fail,2) (8.13) where NR is the number of links dedicated to restoration. In the example network with 100 links for working paths and 30 links reserved for res- toration only and Pf 2 X 10-4 the lower bound is D 2 0.99948. 8.4.1.2.5 Comparison of Various Designs for Survivability We have seen four possible single-layer survivability strategies starting from the more effective and als