ilure of working connections 1 4 is not a requirement, a single protection wavelength can be Telecommunication Networks equipped on the link 2-3 SO that it is used both to protect connection 1-3 and to protect connection 2-4. The OXCs will suitably switch SO to create the correct routing of the protection path. Naturally, if dedicated protection needs only advertising through the control plane, but switches on without any protocol operation, this is not true for shared protection. As a matter of fact, the intermediate OXCs that have to switch to assign the shared protection resources to the right path have to be informed by a control plane protocol of the configu- ration to implement. In this case, the network cost is that explained in the previous section, without the wave- lengths and OXC ports that now are not needed due to protection resource sharing. A general expression of the survivability degree D is quite difficult to provide for the shared protection, as D depends on the effective sharing of protection resources and then on the topology. However, a lower bound for D is immediately obtained by assuming as failure prob- ability the probability that two working paths will fail in the repair time plus the prob- ability that a working path and its associated protection both fail during the same repair time. Thus the lower bound can be approximately expressed as D=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) and P(fail,2) are the failure probabilities for the two failures cases and are given by (8.11) (8.12) In the example network with 100 links and Pf 2: x10-4, a number of routed optical paths equal to 40 with an average length of 5 hops each. With these features D 22 0.9996. Shared protection can arrive in this example to lose one "9" in D, in front of a relevant sparing in protection resources when the sharing is relevant and thus D is near its lower bound. 8.4.1.2.4 GMPLS Restoration The extreme form of sparing in survivability resources is GMPLS restoration. 