, or bidirectionally as necessary to optimize overall performance. Such "midstage insertion" within the amplifier's gain medium is a compromise configuration that provides a reasonably low overall noise figure without unac- ceptably sacrificing pumping efficiency. Another approach, as explained in the earlier section Amplification in Optical Systems, is to use the DCF as a Raman gain medium and thus the DCM itself becomes a discrete fiber Raman amplifier (Namiki, Emori, & Oguri, 2005). The small effective core area of the DCF facilitates relatively high-efficiency pumping, and the pump lasers and their power supplies can be included in the DCM package. DCMs are normally located in equipment racks in node buildings. Whether Raman or erbium gain is used, it is necessary to provide electrical powering to the pump lasers. While they are not a problem in node buildings, electrical cables can be highly inconvenient and costly if geographical constraints impose placement of the DCM in the field. Therefore, network planners always try to avoid such situations. Subsea transmission requires carefully optimized dispersion compensation (Bickham Cain, 2002). Because the systems can be very long, loss minimization is a key issue. More- over, many of the system specifications are constrained by the accumulation of nonlinear crosstalk. Typically, the optimal dispersion map consists of about two or three parts PDF to one part NDF, in which the periodicity coincides with the 40- to 50-km spacing between repeaters. The term "Inverse-Dispersion Fiber" (IDF) or "Reverse-Dispersion Fiber" (RDF) is commonly used for the portions that have negative values of D. The PDF in subsea systems is normally a type of NZ-DSF with |DPDF| ps/(km.nm). Thus to satisfy equa- tion 6.31 we expect |DNDF| to be about 8-24 ps/(km.nm). In contrast to the DCF used in modules, it is possible to achieve the required refractive index profiles without either very small core sizes or very high GeO2 doping. This brings four benefits: The loss coefficient tends to 