o more expensive in terms of network resources and arriving at the most effective in terms of resources allocation but also less effective in terms of survivability degree. In any case naturally the resulting availability is at least two "9" better than the value of D achieved without any spare resource in the network. Naturally multilayer survivability strategies can also be implemented, but it is not the scope of this simple example to go SO farther and we will return to these more complex solution in a more general analysis later. Now we want to carry out on our simple network an economic comparison between the different strategies taking as a reference the mini- mum node cost design method. The survivability degree for the network in Figure 8.20 if the different survivability strategies are used is reported in Table 8.6 if Pf=10-4. From a network cost point of view, we can take as cost reference the cost of the network designed with the same criterion but without any survivability strategy. The relative cost TABLE 8.6 Survivability Degree for the Network in Figure 8.20 if the Different Survivability Strategies Are Used Survivability Strategy Survivability Degree Link dedicated 1 + 1 protection 0.99999996 (seven "9"s) Optical channel dedicated 1 + 1 protection 0.9999998 (six "9"s) Optical channel shared protection 0.9999997 (six "9"s) Optical channel restoration 0.9999996 (six "9"s) Telecommunication Networks Cost relative to the design without survivability 1+1 link pro. 1+1 Och DP OchSP FIGURE 8.23 Restoration Dimensioning results: cost of survivable networks normalized to the cost of the first method of design without survivability for different values of the cost parameters. z=X01XL of the various survivability strategies are reported in Figure 8.23 versus the normalized cost parameters V and Z. It is evident that the difference in the cost of survivability is great: the most expensive case is 1 + 1 link protection and the dedicated 1 + 1 channel protection. The relationship between the cost of shared prot