n fluctuations) and their exact statistic is complex. In the realistic case, in which there are many WDM channels, several contributions are added on the same wavelength and in a first approximation the FWM can be considered a sort of nar- rowband colored Gaussian noise [2,8]. Once this approximation is made, the system performance in the presence of FWM can be evaluated by considering a central channel of the WDM comb (i.e., the channel in the worst situation regarding FWM products) and evaluating the number of products con- tained in the channel bandwidth and the overall FWM power as the sum of their indi- vidual powers [20]. As an example in Figure 6.17 [21], a system with nine channels, 30km long and without dispersion compensation is considered. The transmitted power is 0dBm per channel. In the figure, the power penalty due to FWM is shown versus the channel spacing. The first thing to notice is the oscillatory behavior of the curves. This is due to the oscil- latory behavior of the phase matching term and also the fact that only the main FWM terms are accounted for in the figure. Considering all the terms the oscillation results less deep in correspondence of the points in which the main terms nullify. Telecommunication Networks Sensitivity penalty (dB) FIGURE 6.17 FWM power penalty versus channel spacing for a system with nine channels 30km long without dispersion com- pensation. The other system parameters are reported in Table 6.1. Optical channel spacing (GHz) In designing a WDM system, it is not possible to exploit the fact that the FWM power is strongly reduced around some points for the aforementioned fluctuations of the phase matching conditions, and it is necessary to evaluate the FWM penalty using the envelope of the maximum values of the curve of Figure 6.17. The strong effect of fiber dispersion is also evident from Figure 6.17, where the FWM impairments are much more severe in the case of an NZ fiber. Last but not least, the impairment due to FWM has a step behavior typical of nonlinear phenome