rgin on the upper inequality is only 0.5 dB. This means that fluctuations in the values of the parameters risk to generate receiver saturation. Probably, since the sensitivity is not a problem, a receiver with a higher saturation power has to be adopted, even with lower sensitivity. Regarding the transmission line power budget, that is, Equation 6.75, spans as long as 136 km can be achieved. Increasing the span length generally the cost per Gbit/s per km of the system decreases, due to the decreasing of the number of amplifiers per unit length, but also decreases the maximum system reach. The relationship between the system length, the span length, and the amplifier number is shown in Figure 6.22. The choice of the span length is thus a key choice to position a DWDM product on the market, being one of the main parameters governing the balance between cost per Gbit/s per km and system performances. Transmission Systems Architectures and Performances System relative length (%) FIGURE 6.22 ASE limited system length versus the amplifier spac- ing. The system length is normalized to the maxi- mum value attainable with distributed amplification. Span length (km) System parameters are reported in Table 6.1. 6.3.2.1.2 Erbium-Doped Optical Fiber Amplifier Amplified Systems Design: Dispersion Management From the results shown in Section 6.2.3 it is clear that, in order to fully exploit the reach potential of DWDM amplified systems, chromatic dispersion has to be compensated. Due to the close interaction between chromatic dispersion, SPM, XPM, and FWM, the way in which chromatic dispersion is compensated strongly influences also the way in which nonlinearity impacts transmission. Different solutions are available to implement dispersion compensation: Use of in-line dispersion compensators (like DCF or fiber Bragg gratings [FBG]) Pre-chirp of the transmitted pulses Pre- and post-electronic compensation and quite often all solutions are used together in order to find the best mix to compensate dispersion and control nonlinear