ly if some pulse pre-distortion is implemented, either at the electrical or at the optical level. Pre-distortion is intended to support in-line compensation in balancing the disper- sion of the line. It consists in transmitting chirped pulses with a positive chirp in the case of transmission in the third window, where there is anomalous dispersion. The chirp, that can be generated by the transmitter electronics or by the optical modula- tor, is compensated by the first part of the fiber and provides an intrinsic robustness to dispersion. In this case, in the great part of the systems the differences between pre- and post- dispersion compensation are much less pronounced. Naturally, even better performances are attained when both pre- and post-compensa- tion are used, if the overall link is suitably optimized. As further refinement, also a combined pre- and post-compensation with individual channel compensators at the transmitter and at the receiver has been proposed [29], SO attaining even better performances with respect to mixed pre- and post-compensation. The weakness of this solution is of course its cost, especially for systems with a high chan- nel count. From Figure 6.23, it also results that the higher value of the Q factor is realized with a cer- tain degree of under compensation that is compensating only 98% of the line dispersion. This result is reasonable, since a bit of residual dispersion allows SPM and XPM to be partially compensated, thus improving system performance. Extensive study of under-compensation technique has been done [30,33,34]. The optimi- zation technique is generally based on building a variable dispersion map, where the value of the DCF dispersion and dispersion slope are the same at each site and are the optimiza- tion parameters. Sometimes, when the requirements do not fix the amplifier sites a priori, also the span length is a third optimization parameter. The scalar signal propagation is simulated via a numerical model and the Q factor or the SNo are evaluated using the analyti