ity that one link breaks in the period can be not negligible. In particular, we can assume as a measure of the network reliability the probability that no service disruption occurs in a fixed time if the service was working at the start of the period. We will call this measure network survivability degree D. In Section 8.5, we will give a more precise definition of the standard parameter used for this kind of evaluations: the network availability. The survivability degree of a network with N L links without any protection and/or res- toration mechanism is (8.5) where P(failure, to) is the probability of a failure in the considered time interval when the network was working at the interval start (instant to) and it is assumed that the single link failure events are independent and their probabilities Pf equal and much smaller than 1. It is important to note, just for an example, that if the network has 100 links and the prob- ability Pf in the network repair time is X 10-4, the result of (8.5) is 0.98, that is clearly unacceptable in front of a normal request of 1 -D = 10-5. This clarifies the need for a survivability strategy. 8.4.1.2.1 Link 1 + 1 Dedicated Protection This is the most effective but also the most resource-hungry survivability strategy. Every single link is doubled with another identical link running physically on a differ- ent path. This is done by deploying, for example, two identical cables on different sides of a highway or of a railways SO that if one is broken, the other is sufficiently far not to be involved in the accident. Every node has a protection switch in front of the receiver and the transmitted signal is doubled and sent both on the working and on the protection path (see Figure 8.22). Telecommunication Networks Protection Transmission switch splitter Working Protection FIGURE 8.22 Block scheme of 1 + 1 link dedicated protection. If this protection strategy is chosen, the base cost of all the fiber infrastructure is more than doubled (since for each link we have twice the number of 