hannels in order to frustrate the necessary phase matching for the effect (Agrawal, 2001). With the aid of optimization algorithms based on the NLSE, it is possible to predict a set of channel frequen- cies that can minimize the problem. This is why, as explained in the earlier section on dense wavelength division multiplexing, the DWDM grid lines have been specified down to 12.5- GHz spacing. The lasers are selected to coincide with the grid lines that lie closest to the required frequencies. FEC: Method of Last Resort or Design Requirement? We have seen that many techniques can be used to improve the physical performance of optical networks and SO provide low measured BERs. However, some of the required means are costly, and operators' needs for greater spans and capacities or for wavelength routing can often be SO demanding that physical techniques are no longer viable. In particular, a previous subsection explained that PMD is inherently probabilistic and that improvements to fiber fabrication and component quality, or the use of compensation techniques, cannot guarantee to prevent outages. In that case, network designers have turned to FEC as a method of last resort. The use of good error-correcting codes can be our only hope for service continuity. It is as if algebra comes to the rescue of physics. Furthermore, it is increasingly recog- nized that FEC, which merely depends on electronic chip sets to implement mathematical algorithms, can be very cost effective. FEC is being implemented in network categories (such as long-reach PONs [Lin, 2006; Davey, 2006; Gilfedder, 2006]) for which it was not previously considered appropriate. To this end, its use is being incorporated into the initial design stage as a way to reduce the dependence on expensive optoelectronic hardware. Chapter 6 Optical Fiber Transmission Conclusion This chapter explained the main transmission techniques that are used in optical fiber systems. Its objective was to present engineering principles rather than to review recent results. The p