wavelength passing through an OXC only if a WC is present in that OXC and if not all the available WCs are already occupied by other wavelength paths. It is, in any case, intuitive that this ad hoc use of the WCs is not the best possible, and an optimized deployment of WCs will give better results. We will analyze this problem in detail using cyclic algorithms, that being much less complex from a computational point of view with respect to ILP, are better suited for such an investigation. 10.3.4.3 Problem of Transmission Impairments and the Use of Regenerators In the case of transparent network, as we anticipated in the introduction of this section, there is no certainty that the route individuated by a routing algorithm looking to mini- mize the network cost is also acceptable from a transmission point of view. This means that it is not possible to decouple the problem of the link dimensioning (comprising the placement of electronic regenerators where needed) from the routing of wavelength paths. The solution to this mixed problem is generally called Impairments Aware Routing and Wavelength Assignment (IA-RWA). The simpler case in which transmission characteristics have to be included into the design of the transparent network is the linear case, when nonlinear propagation effects can be neglected. This case in practice is quite common, even if the performances of very long DWDM systems are dominated by nonlinear effects. As a matter of fact, a great majority of the links of carriers in Europe or in Asia are shorter than 1000km, SO that the transmitted power can be maintained sufficiently low to neglect nonlinear effects. In this case, assuming a line rate of Gbit/s, the dominant transmission impairments are ASE noise, linear crosstalk, and chromatic dispersion. In order to insert in the model these effects, near the matrix [L] introduced in Chapter 8 to take into account the link lengths, other two matrixes can be defined to take into account dispersion and noise for every link and every OXC. If all the links a