BLE 8.11 Efficiency and Availability of Different Multilayer Survivability Strategies Only Optical Layer Resources 1 A (x10-6) 1 + 1 protection SP-R multilayer survivability R-R multilayer survivability Restoration type 2 We will use a three stage optimization, first optimizing the network for working traf- fic, then dimensioning the bottom layer survivability and at the end the upper layer survivability. The average availability is evaluated by simulation as in previous section. Two multilayer methods will be considered, assuming simple coordination through an hold-off time. The first method, which for simplicity we will call SP-R, is based on 1 + 1 optical GLSP at layer 1 and type 1 restoration at layer 2. This means that after a failure event, the MPLS LSR at layer 2 wait a certain time to let the optical layer to end its recov- ery attempt. After this time, all the LSR that has not been recovered by the optical layer has to be restored using already the available network resources to route them. The second method, which briefly we will call R-R, comprises wavelength level type 3 restoration at layer 1 and LSR type 3 restoration at level 2. The survivability method efficiency and the average GLSP availability are reported in Table 8.11 in the conditions detailed in the previous section. The two multilayer methods are both better with respect to single-layer methods, even if under different points of view. The R-R method attains almost the same availability of 1 + 1 optical protection, but at a much lower cost in terms of redundant equipment in the network. The SP-R method is quite expensive, but has the highest availability among the methods analyzed up to now. Not only double failures can be efficiently faced, but also a few of triple and quadruple fail- ure events are recovered and a good recovery capability also exists in front of node failures. This property is shown in Table 8.12, where resilience factors of different degree are evaluated. Besides the factor relative to multiple link failures, the resilie