d of the span; this means that a DCM is placed at the preamplifier site to compensate the last span 3. Mixed pre-and post-compensation: In this case a DCM is preset both at the transmit- ter and at the receiver side Several studies have been done to compare post-compensation and pre-compensation in different situations. The final result depends on the specific system setup. In general, post-compensation performs better at the expenses of a more stringent toler- ance on the compensator parameters choice. An example of this behavior is reported in Figure 6.23, where the Q factor is reported versus the compensation ratio (ratio between residual dispersion and total link dispersion) for a system transmitting a single 10 Gbit/s signal with 13 spans of km of SSMF and a variable length of DCF, which is adjusted to define the compensation ratio. The fiber parameters are given in Table 4.1, Chapter 4 for the SSMF and in Table 6.3 for the DCF. A two stage EDFA with 5 dB noise figure per stage is used in each section to regenerate losses and to host the DCF. In Figure 6.23, experimental data reported in [31] are fitted with the performance evalua- tion model where the propagation of NRZ pulses through the fiber link is simulated with a program using the algorithm described in [8], that is based on the split step solution of the nonlinear propagation equation (see Section 4.39). Noise is taken into account, analytically, after the simulation of the deterministic part of the propagation. Maximum Q factor Post-compensation FIGURE 6.23 Q factor versus the post-compensation ratio for a single 10 Gbit/s channel in a system with 13 spans of 80km of Pre-compensation SSMF and a variable length of DCF, which is adjusted to define the compensation ratio. (After Peucheret, C. et al., IEEE Photon. Tech. Lett. 12(8), 992, 2000.) Compensation ratio (%) Transmission Systems Architectures and Performances Other situations exist where it is pre-compensation to be preferable, as in the examples reported in [32]. The situation changes complete