 BER needed using the FEC an SNo of 15.8 dB is needed. This number of course does not take into account system margin and aging, and since the simulation is scalar, also polarization penalty is not accounted for. Thus, 5 dB have to be added a poste- riori, obtaining a simulation required SN of 20.8 dB that is not far from the 20 dB obtained in the last section, taking into account all the numerous approximations. 6.3.2.2 Long-Haul Transmission at 40 Gbit/s 6.3.2.2.1 Definition and Draft Design It is interesting to repeat the design in the case of 40 Gbit/s channels. Choosing a channel spacing of 200 GHz the overall bandwidth of the optical link, mainly determined by the cascade of the multiplexer and the demultiplexer, can be as tight as 60 GHz, attaining = 1.5 that gives a very good noise rejection. In this case, we can allocate 22 channels at 40 GHz in the extended C band (see Table 6.5). Reduced gain amplifiers have to be used at this speed due to the small number of chan- nels and to the greater efficiency of nonlinear effects. The choice of the average power along the link is a strategic choice, since the right value comes out from a trade-off between the need of limiting nonlinear effects that call for a low power and the need of a high SN ratio that would bring to select a high power. In our case, a fast look to the penalty plots relative to nonlinear effects reveals that at 40 Gbit/s a comb of 22 channels 200 GHz spaced suffers a penalty of 0.1 dB from FWM at maximum power per channel of 0 dBm, the penalty rises at 2 dB at a maximum power per channel of 5 dBm and to 4 dB at a power per channel of 6 dBm. Also XPM rises fast with the power per channel; it is smaller than 0.1 dB at a maximum power per channel of 5 dBm, but at 6 dBm is already slightly smaller than 2 dB. It results that a power per channel of 5 dBm is a sort of threshold for nonlinear effects allowing to maintain the overall nonlinear crosstalk penalty at 2 dB that is a practical limit for penalty coming from a single effect for the working o