han an LSR port, thus the hub design is convenient. Since we have done a very simple example, it is not SO relevant the fact that in practice, considering long-haul DWDM systems based on EDFA amplifiers; Z is slightly greater than one and V is small, perhaps between 0.5 and 0.2. More relevant is the observation that network optimization depends on cost balance and that frequently in the time interval between the planning and optimization of a new network and the end of its deployment, the cost framework is changed. Thus, in real design, it is important to capture qualitative trends that are constant with respect to market evolution to be able to keep the network design updated in all the circumstances. Convergent Network Management and Control Plane 8.4.1.2 Design for Survivability In the previous section, we have carried out a basic example of network design without taking into account any survivability strategy. In reality, survivability has to be taken into account. In the case of ASON/GMPLS networks, both protection and restoration can be implemented and each of them can be carried out both at layers 1 and 2. Depending on what solution is chosen for survivability, the impact on the network design changes and network availability has a different value. Since we are dealing with a simple example to underline the main points of network design, in considering survivability, we will assume to face only fiber cables cuts or DWDM line sites failure. We will assume that the probability of a cable cut in a given time interval At is constant (does not depends on the absolute time period but only on the interval duration) and for a sufficiently small At can be written as P(At)=pfAt (8.4) In our case, At is sufficiently small as far as the failure probability will remain much smaller than one and in this case it can be as long as a few months, since the probability that a single link will be broken in a transport network is generally low. Naturally, if the number of links in the network is very high, the overall probabil