ies only at one particular wavelength; for all others it is merely an approximation. Figure 6.23 shows the outcome in Chapter 6 Optical Fiber Transmission Length Wavelength G.652 Fig. 6.23 (a) Variation of the dispersion coefficient of the PDF over M channels. (b) Consequences for the channels with the most extreme wavelengths. (Effects are exaggerated for illustration.) WDM operation if some channels are not well compensated because equation 6.31 does not apply sufficiently closely. Therefore, we must take the dispersion slopes of the two fibers into account and an additional equation, equation 6.32, must be satisfied (Wandel et al., 2001, 2002): (6.32) in which S = dD dr. Normally the values of DPDF and SPDF are fixed by the type of transmis- sion fiber in use and, if it is G.652 compliant, the values can be deduced from Figure 6.18. However, the negative-dispersion fiber has to be designed to ensure the closest possible agreement with equations 6.31 and 6.32 over the range of operating wavelengths. There are two strategies by which alternating PDF and NDF can provide dispersion management: Dispersion-Compensating Modules (DCMs) and incorporating the NDF within the cables. On the whole, operators prefer to use DCMs in terrestrial systems and cabled NDF in subsea operation. When network usage changes (for example, by incorporat- ing additional optical add-drop multiplexers or WDM channels), the dispersion map of the whole system may have to be revised. The use of DCMs to compensate for dispersion enables simple upgrading by module replacement. In contrast, transmission loss in subsea communications is a critical concern and is minimized by including the negative-dispersion fiber within the overall system span. Moreover, there is seldom, if ever, sufficient space within submerged repeaters for bobbins of negative-dispersion fibers. The simplest version of a DCM is a packaged bobbin of fiber having a negative disper- sion, known as "Dispersion-Compensating Fiber" (DCF). Its dispersion coefficient, DDCF, should hav