l, are provided in terms of a maximum attainable BER. The draft evaluation, essentially, consists in calculating all the relevant penalties in a situation in which the considered phenomenon is the only impairment affecting the ideal system and adding the obtained penalties with the system margin to arrive at a final requirement in terms of SN ratio. Generally, it is easier to work at the optical level, specifying the SN, but sometimes the presence of electronic nonidealities at the receiver renders it even easier to evaluate the target SN The target SN is compared, at the end, with the SN coming out from the design to verify the system is working correctly. Naturally, it is well possible that the system does not meet the requirement in terms of SN after the first design, in this case, the main causes of distortion and noise are identified and the design is reworked again to mitigate the corresponding degradation. We have seen in the previous sections that a long-haul system with 90 channels at 50 GHz spacing satisfies the draft requirements. A realistic penalty allocation is reported in Table 6.4. It is to be noted that, besides impair- ing effects, also engineering margins are added, 3 dB of system margin that takes into account fluctuations of components specifications and imperfect design implementation and 1 dB of aging margin. As far as the aging penalty is concerned, there are two possible approaches in con- sidering components degradation with time. Here we have decided to perform all the calculations using end of life parameters, that have to be evaluated by components sup- pliers during the components qualification stage. With this choice, a further aging pen- alty is only a sort of second margin on a wrong evaluation of the degradation of some component. Another approach, followed, for example, in [2], is to perform design calculations using the new components parameters. In this case, all the aging degradation has to be added at this stage. Generally, in this case, an aging penalty of 3-4dB is consider