ntrol Plane The description of multilayer networks in terms of virtual topologies The balance between grooming and transparent pass-through in multilayer nodes The possibility to express the network cost as a linear function of the design variables The necessity of deciding a survivability strategy A way in which network survivability can be quantified The different impact on costs of different survivability strategies Last but not least an idea of how the design can be carried out in conditions of uncertain traffic 8.4.2 Design Based on Optimization Algorithms The most direct way to generalize the design methods we have used in our example is to set up an optimization algorithm working on a completely general multilayer topology and aimed at optimizing the cost of the network evaluated with a more accurate and gen- eral model. Leveraging on the observation that in our example, neglecting the integer part appear- ing in the platform cost, the cost function was always linear in the design variables we will target a problem formulation compatible with integer linear programming (ILP), which is a well-known method to model a class of NP-hard problems whose optimization variables are integer [6-9]. 8.4.2.1 Optimized Design Hypotheses The first step to do to develop a general design procedure that can be applied to every topology and traffic request is to define the design problem data. We will assume a few very general hypotheses to restrict the problem and make it solvable. The infrastructure is completely given; that means that geographical position of Points of Presence of the carrier and of fiber cables are given. Equipment can be installed only in the points of presence, but for in-line amplifier sites of DWDM systems. We will assume that no OADM branching point exists in DWDM systems. We will work in terms of nodes topology: the topology in which the physical net- work nodes are represented by the nodes of the topology graph. A completely equivalent model can be carried out in the SO called links topology, wher