ption of error-correcting codes in this section is introductory. The principles have been known for many years because of their applica- tion in nonoptical transmission technologies. The original innovations were often for radio and microwave communications because they suffer numerous adverse influences on signal quality. Whatever physical transport medium is used, the objective is for it to transmit each channel at a higher bitrate than needed for the data that it carries. This is because an additional number of error control bits are added as an overhead to the data, according to a predetermined mathematical algorithm (Kumar et al., 2002). After the receiver we have an electronic bitstream consisting of 1s and Os, some of which have been badly corrupted during transmission and therefore erroneously assigned at the decision circuit. Dedicated electronic processors (in the form of ASICs) are located after the receiver cir- cuitry. They perform mathematical operations on the data and the overhead bits and SO correct for many of these errors. There are numerous types of FEC that are different various respects, one of which is how many errors they are able to correct. In a WDM system, FEC is implemented on each channel individually. The sections on chromatic dispersion mitigation and polarization mode dispersion described how electronic compensation for chromatic or polarization mode dispersion can be installed at the receiver end of the system. If this is done, the FEC processors are placed after the EDC and the decision circuit, as shown in Figure 6.30. Chapter 6 Optical Fiber Transmission Decision Coder Circuit Decoder Detector Coder Decoder Laser Transmission Fiber and Amplifiers Coder Decoder Fig. 6.30 Placement of FEC circuits in a WDM system. EDC = electronic dispersion compensation (if used at the receiver end); mod = electro-optic modulator; MUX and DE-MUX = wavelength multiplexer and de-multiplexer, respectively. Useful Definitions Some definitions are useful to know in order to understand the literature 