writing it is not clear to what extent EDC will displace optical compensa- tion techniques in WDM systems at high channel datarates. We tentatively suggest that, for national and transoceanic applications, EDC combined with FEC and alternative modula- tion formats will reduce but not eliminate the dependence on NDF and other optical methods. However, the greatest scope for displacing all forms of optical dispersion manage- ment is in shorter-span systems, especially high-speed optical LANs, long-reach PONs, and metropolitan networks. Polarization Mode Dispersion Polarization mode dispersion is another form of dispersion that particularly affects high- speed signals in longer-range systems. This section describes the factors that give rise to polarization mode dispersion and its characteristics. Methods to minimize it and compen- sate for it in systems are also discussed. Nonideal Optical Fibers Light that is launched into an optical transmission system is preferably unpolarized, which means that it can be regarded as a combination of all possible states of polarization (Damask, 2005). The fiber through which it passes is normally designed for use in the fundamental HE11 waveguide mode, but the propagation is in reality the superposition of two orthogo- nally polarized guided waves. In an ideal fiber the polarized guided waves are "degener- ate," which means that they travel with the same group velocity and SO arrive at the detector at the same time. However, the real optical fibers and components that the signals encounter differ from the ideal in a number of minute but important respects, all of which influence the signal's State of Polarization (SoP) by creating two group velocities and SO destroying the degeneracy. Transmission and dispersion-compensating fibers and passive optical components exhibit Polarization Mode Dispersion (PMD) and Polarization-Dependent Loss (PDL), and the amplifiers commonly have at least a small Polarization-Dependent Gain (PDG). These effects may be of little concern over short tran