ctrum band are not considered. Telecommunication Networks 8-PoISK FIGURE 9.60 Residual penalty after pulse optical precom- Normalized ISI index (ATRs) pensation versus the ISI normalized index. Passing through all the states from the predistorted form and the final form, the pulse presents in correspondence to some link sections sharp peaks that creates a great amount of nonlinear effect. These are not simulated when the final shape of a normal transmitted pulse is inverted to find the correction function; thus the shape of the predistorted pulse is not perfect. This is also the cause of an expected marked difference between the behavior of phase modulation and polarization modulation. Polarization modulation is practically a form of double amplitude modulation and the impact of the nonlinear phase variations are less evident when the predistorted pulse propagates along the fiber link. Passing to the compensation capacity, it has been assessed by simulation of the deter- ministic part of the single channel fiber propagation through the link that we are consider- ing with a split step algorithm to determine the output pulse distortion. This has been precompensated by a suitable transfer function and the output SNo pen- alty has been derived and plotted in Figure 9.60. Since the technology errors are random variables, the penalty curves plotted in the figure are worst case curves, where all the errors are in the same direction and at the 30 point of their distribution. The compensation capability of the method is impressive: to arrive to have a penalty of 3 dB the pulse has to broaden in noncompensated propagation up to 13 times its initial width: no practical postcompensation would be capable of doing that. At this point, all the elements are available to evaluate the ISI penalty. The ISI index due to the interplay between chromatic dispersion and SPM is about 0.9 while, as far as the tight filtering is concerned, the relative bandwidth is 0.6, implying a relative ISI index of 0.3. Applying the ISI indexes summati