sion [PMD]; in this case, the parameters would be D and DPMD). For a small impact of these phenomena, we can still approximate the BER with (6.1), where the ideal expression SN is substituted by an expression SN depending on A and B. Generally, the logarithmic units are used for the SN ratio; thus, passing from linear to dB and developing at the first order in the small parameters representing the impairing phenomena, we can write (6.2) where AA and AB are the deviations of the parameter A and B from the values guarantee- ing ideal system performances. If such deviations are sufficiently low, they can be written as (all in dB) (6.3) where ASN| is, expressed in logarithmic units, the variation of SN corresponding to a small variation of A. Thus, Equation 6.2 can be rewritten as SN(dB)=SN;(dB)+ASN A (dB) + ASN |B (dB) (6.4) This is the simple rule of penalties addition that says that, for small deviations from the ideal behavior, the SN ratio in dB can be obtained by adding to its ideal value the sum of the penalties relative to all the impairing phenomena, all expressed in dB. Equation 6.4 is the base for several design techniques, but it has always to be verified in its accuracy due to the approximation used to derive it. A frequent reason for impossibility to use Equation 6.4 is the fact that the distribution of the noise changes greatly due to the presence of some transmission impairment SO that the ideal expression of the error probability cannot be used. In these cases, Equation 6.4 can still be useful if an approximation of the relation between the BER and the SN is known or can be estimated via a suitable simulation. Transmission Systems Architectures and Performances Phenomena affecting system performances can be random or deterministic in nature. Deterministic phenomena often consist in unwanted distortions of the signal shape, like the effect of fiber nonlinearities or of the nonideal modulator transfer function. In this case, an intuitive way of quantifying the distortion is the so-called eye opening pe