on Networks The integral can be evaluated numerically via Fast Fourier transform (FFT) to have a cor- rect shape of the received signal for each pattern. However, a first idea of the broadening of the pulses composing the patterns can be attained much simply by the definition of D itself (see Equation 4.13). In a first approxima- tion the square pulse propagating along the fiber broadens of a factor At = DLA2. Naturally, during propagation the pulse shape does not remain squared (as resulting from Equation 6.23), but it is not a great error in regime of small distortion to imagine that the pulse broad- ens with triangular tails (a comparison between the real pulse shape and a triangular pulse is provided in Figure 6.4, in an extreme case, in which there is strong dispersion distortion). In this case, just to give an example, in a very first approximation the energy translated from the central "one" to one of the nearby "zero" in the pattern = 010 can be evaluated as Eg = 2PDLRX2/c where P is the relevant optical power [2]. If we want a rough estimate of the penalty due to pulse broadening in this case, we have to compare the signal power with the interference power which is the energy per unit time. Since Eg is evaluated in the bit interval, this ratio is Pen = =E/PT=2DLR2N2/d SO that, neglecting the noise, the penalty depends in a first approximation on the square of the bit rate and it is independent from the transmitted power. If SPM cannot be neglected, the propagation problem cannot be solved analytically and a numerical integration of the propagation equation is needed. In Figures 6.5 and 6.6 an example of the nonlinear evolution of an NRZ pattern is reported [8] comparing linear evolution (at a transmitted power of -10 dBm) and strongly nonlinear evolution (at a transmitted power of 13 dBm). From the figure, it is clear that the transfer of energy from the "one" to the nearby "zero" is much more pronounced in the linear case, even if the pulse shape is completely distorted in the case of nonlinear propagat