constructed by eliminating all the links traversed by the work- ing path. The path is routed again using the chosen criterion in this new topology to obtain the protection path. In the hypothesis of using the OXC switching capacity to double the signals and to feed the working and the protection paths, the cost of the network is given by Equation 8.1 wherein the counts of OXC ports and of links and wavelengths both protection and working paths have to be considered. In a very first approximation a service disruption occurs if a working path and its protec- tion path both fail during a repair time. Thus probability of failure of a path can be calcu- lated starting from the number of links composing the path. In particular, let us assume to have Np working paths (three in the example) and the same number of protection paths, each composed of H hops j = 1 2 Np), SO that in our example, always considering the first design method, [H] = (1, 2, 1). The path failure for the j-th path (that can be working if j < Npr p otherwise protection) is (8.8) SO that a failure that cannot be faced occurs when during a repair time both working path and its protection path fails. The survivability degree, in a first approximation that is valid if the path failure probability is much smaller than one for all the paths, is thus given by [1-P(s)] (8.9) In the example network with 100 links and Pf 2 X 10-4, we will assume a number of routed optical paths equal to 40 with an average length of 5 hops each. With this fea- tures D 2 0.99996. 8.4.1.2.3 Optical Channel: 1 + 1 Shared Protection From the routing of protection paths in the example network we are considering, it is evi- dent that some sharing of protection resources can be carried out, paying it with a lower efficiency of the protection mechanism because of losing capacity of facing simultaneous single failures on different paths. In particular, in our example, in the dedicated protection scheme, two protection wavelengths are routed along the link 2-3. If facing a simultaneous fa