ible number of trails. Networks Fundamentals and Present Architectures In the case of multiring network, the first step is to optimally identify the rings starting from the given nodes and links. It is done only one time both in the case of dynamic and static traffic with the optimization algorithm reported in Ref. [67]. Once the rings are identified, the traffic routing is performed with the same algorithms used for the mesh, but on the ring topology. As far as survivability is concerned, in the static traffic case and in the preliminary dimensioning of the dynamic case, we suppose that restoration is present at SDH/SONET when using DXCs, at MPLS layers and at the DWDM layer only if OXCs are used. If a multiring SONET/SDH or OTN is considered, no restoration is present in the physi- cal layer [72]. The coexistence of multilayer restoration, for example, in the MPLS over mesh OTN model, is simply managed via hold-off times. In the case of a mesh network, fast protection is implemented at the physical layer for real-time traffic, while best effort traffic relies only on restoration. The protection is 1:1 MSP in case of SDH/SONET or 1:1 OCh in case of OTN. This is not the better solution especially for a mesh architecture; however, it is not the scope of this study to optimize protection resources, it is enough not to insert a bias favor- ing one or the other solution and we achieve this goal by using for all architectures the same protection scheme. To dimension the spare capacity needed in a layer for restoration we use a simple algo- rithm reported in Ref. [67]. 3.4.3 Test Networks and Traffic Model We consider as a test the network topology reported in Figure 3.2. In order to have sensitiv- ity to the impact of network topology on our results, we will repeat a few runs using also the topologies reported in Figures 3.55 and 3.56. The first is the core German network as reported in Ref. [70], the second, a proposal for the IP-based core Italian network. The architectures we are going to compare are as follows: Pa