 Mitigation The following subsections contain an overview of the main techniques for mitigating chro- matic dispersion. Techniques Figure 6.20 categorizes the techniques we will consider. The methods are not mutually exclusive, and high-performance systems normally use more than one. "Dispersion accommodation" refers to a collection of techniques to make chromatic dispersion less detrimental. "Dispersion compensation" is a collection of techniques that counteract the pulse broadening and that can be categorized as optical or electronic. At the time of writing, most installed systems use optical means, but operators and system vendors are increasingly devoting their efforts to Electronic Dispersion Compensation (EDC). Because electronic methods offer potential cost savings and sometimes overall performance improve- ments, there is a migration away from optical techniques. However, even if EDC eventually dominates, legacy optical compensators will remain in use for many years. Dispersion Accommodation Chromatic dispersion can be mitigated by setting the terms in equation 6.30 to values that reduce it. We designate such methods as examples of "dispersion accommodation." Unfor- tunately, despite the 22 dependence, there is limited scope to lessen the difficulties merely by appropriate selection of wavelength. With the exception of short-span and low-datarate applications, such as unamplified PONs (Gilfedder, 2006), where chromatic dispersion is seldom a constraint, fiber loss dictates that we favor the C- and L-bands. Nevertheless, with Chapter 6 Optical Fiber Transmission reference to equation 6.30, we can obtain useful improvement in two other ways: through alternative modulation schemes and with fibers that are designed for reduced dispersion coefficients. Much effort has been devoted to developing low-bandwidth pulse-coding formats (as described in the later section Modulation Formats) and in this way we can reduce the value of b. Concerning the transmission fibers, their refractive index profiles can be designed