e the topology graph has nodes representing the physical network links. An example of corresponding links and node topologies are given in Figure 8.24. FIGURE 8.24 Example of (a) node topology and (b) corre- sponding link topology. Telecommunication Networks We will image that all the fiber cables are owned by the carrier that is designing the network SO that the bare use of a free fiber does not entail a cost and a fiber is always available on each route where a cable is present. However, the model can be generalized without changing the base definitions if the fibers are leased and using a fiber is a cost for itself. Also a possible limitation of fiber availability can be easily introduced without substantially changing the model. The topology will be meshed at every network layer; design of multiring net- works is an important subject, based on different algorithms, and we encour- age the interested reader to start the study on multiring networks design from Refs. [10-12]. The layering is also fixed: physical layer is composed by a WDM transport net- work based on OXCs. We will assume that the OXCs are opaque and that every link between adjacent OXCs has been correctly designed under a transmission point of view SO that we need not deal with transmission systems design when designing the network. The client layer is assumed to be an MPLS layer that interfaces the optical layer via the GMPLS overlay model interface. This means that a node comprising an LSR only cannot exist in the network since in the overlay model, the LRS cannot interface DWDM systems and OXCs directly in other nodes. The design of optically transparent networks is an important subject that we will con- sider in Chapter 10. The opaque hypothesis also implies that wavelength continuity is not required in crossing an OXC, since the opaque switching machine also performs wave- length conversion. In principle, not always an LSR is superimposed on an OXC, and only when it is true, it is possible to communicate between layer 2 and layer 1. Since in