rcent. Another development is the application in optical transmission of a class of FEC known as "turbo codes," which perform very efficiently in the wireless domain and are more recently being explored in optical fiber systems (Berrou, Glavieux, & Thitimajshima, 1993; Heegard & Wicker, 1998). System Design and Implementation This section explains the many concepts that system engineers must take into account in whole-system design. It uses all of the techniques presented up to this point. Design Methodologies The design of a whole system is multifaceted. It demands knowledge of all transmission impairments (loss, amplifier noise, chromatic and polarization mode dispersion, nonlinear crosstalk, etc.), subsystem and component technologies (transmitters, amplifiers, optical cross-connects, add-drop multiplexers, filters, isolators, passive splitters, detectors, etc.), modulation formats, and FEC coding. We now have sophisticated software with advanced visualization tools to carry out the modeling and allow developers to understand how design modifications influence BER. System and network suppliers normally perform as much simulation as possible with this software to constrain their need for expensive laboratory testing, especially when develop- ing very-high-capacity systems. In all stages the work has to be undertaken as a major coordinated effort. It is outside the scope of the chapter to describe the planning, modeling, testing, and implementation processes. Instead, this section provides insight into some of the design compromises that must be made. All of the nonlinear effects that were discussed in the section Nonlinear Optical Trans- mission cause crosstalk and thereby increase BER. We have seen that the problem becomes greater with high-launched powers. When operating at low datarates we can use relatively low-power transmitters and therefore often avoid the difficulties altogether. However, as we increase the datarate we need ever greater incident powers on the detector to retain the spectral power densit