hannels and it is given by (6.47) where a is the attenuation in m-Superscript(1) Sj,k is the so-called walk-off factor, that is the inverse of the difference between the group velocities of the considered channels is related to the fiber dispersion by the equation = DX2/4, a being the central wave- length of the WDM comb and C the light speed in glass L is the length of the link Equation 6.46 has to be integrated numerically, but it is much easier than a simulation. We will use several times this approach for its simplicity. 6.2.6.3 Jitter, Unperfected Modulation, Laser Linewidth, and Other Impairments Besides the phenomena we have dealt in the previous sections, there are several other potential performance impairments that have to be considered when evaluating the per- formance of an optical transmission system. These effects are not SO important as dispersion or FWM, but if not managed with a cor- rect system design they can heavily affect system performances. Timing jitter: This phenomenon consists of the fluctuations of the receiver deci- sion circuit sampling instant. It is intuitive that, if sampling does not occur at the center of the bit interval, a penalty is generated. Timing jitter generally depends on the imperfect working of the receiver PLL, causing the reconstructed clock to be affected by phase fluctuations. A correct design of the digital receiver PLL can make this phenomenon negligible [24]. Telecommunication Networks Pulse jitter: This phenomenon, affecting mainly RZ transmission, consists in the fact that the transmitted pulses are not located exactly at the center of the bit inter- val. The cause can be phase noise in the clock at the transmitter, a random com- ponent in the switch-on or switch-off time of the transmitting modulator, or even pulse attraction due to nonlinear propagation for quasi-soliton RZ pulses when very long, amplified systems are considered. Transmitted pulses' position fluctua- tions cause ISI and have to be carefully controlled during system design [25]. Limited trans