ontrol. Since no solution in which the average power is higher than the threshold is acceptable, it is reasonable to optimize the overall link by maintaining constant the average power. If this is the optimization bond it is useful to introduce in Equation 6.51 a different expression of the amplifiers number N. From the equation G Na e-NaaLs where Ls is the span length, the following expression of N can be derived: (6.52) Substituting this expression of the number of amplifiers into the expression of the noise power generated by the line and using the noise factor to evaluate the overall ASE spec- trum SASE is obtained (6.53) Since the average power in fiber and the bit rate are fixed, the SNo and, thus, the error probability is optimized, if SASE is minimized. In order to minimize SASE! it is necessary to decrease Ls up to the limit of continuous amplification. In this limit, the amplifiers' gain is very small (tends to zero in the absence of DCF) and the noise figure is near to the quan- tum limit. In reality, a long amplifying fiber is not feasible, but this result is nevertheless very important. The penalty due to the use of concentrated amplifiers remains low if the span does not exceed 50-80 km and the use of lumped amplifiers is advantageous under many practical points of view. Once the link is optimized from the point of view of attenuation compensation, it has to be optimized also under the dispersion compensation. Chromatic dispersion is not the only component of the broadening of a pulse propagat- ing through an optical fiber. SPM and PMD introduce phase distortion and thus pulse broadening. On long-haul WDM systems the dispersion compensation is generally set SO to partially balance XPM and SPM with residual dispersion, since propagation happens generally in the third transmission window of the optical fibers, where the dispersion coefficient B2 is negative. This technique is called dispersion assisted transmission. Last but not least, since this class of systems is explicitly designed to reach long d