ies necessary for good BER. In contrast with other influences on BER, such as chromatic dispersion, PMD, amplifier noise, and finite transmitter extinction ratios, it is not possible to improve nonlinear crosstalk merely by increasing the mean received optical power. Such an action would make matters worse. For these reasons, system opti- mization for high-capacity WDM operation can impose very tight design margins. Fiber Effective Area One measure that we can use to lower nonlinear crosstalk relates to the effective area in the denominator of the nonlinear term in equation 6.35. Fiber designs with a large value of Aeff are therefore desirable. For the G.652 fiber, to which we referred in previous sections, Aeff is around 80 to 85 um2 in the C-band, which is rather higher than many other categories, such as DSF and NZ-DSF. System Design and Implementation Other specialized types of transmission fiber exist with effective areas in excess of 150 um - notably, the large effective area designs, which often have a pure silica core. However, they can be expensive and sometimes have relatively high bending loss, which tends to limit them to niche applications, such as repeaterless subsea communications (ITU-T G.654) For these reasons, operators commonly consider the G.652 fiber to be a good multi-application compromise for all terrestrial needs. As explained in the earlier section on chromatic dispersion mitigation, DCF for within modules often has a very small effective core area, with values in the 15 to 25 um2 range being typical. Consequently, it can contribute significantly to the system's accumu- lated nonlinear crosstalk. Manufacturers devote much effort to novel DCF waveguide designs to achieve the highest values of Aeff and |DDCF| simultaneously, with the aim of minimizing the required spans. Where it is possible to do so, it is best to minimize the use of DCF through the deployment of other compensating techniques, such as chirped FBGs and (especially) electronic methods. Nonlinear Crosstalk in Amplifier Chain