In optical transmission systems, there are three key elements: the transmitter (laser and modulator), the photodetector, and the optical transmission medium (the fiber). Typically, the detector is characterized by a level of sensitivity to impinging optical power., PIN diode or avalanche photodiode). Demodulation circuitry to extract the transmitted data. The optical fiber cable itself makes up. This chapter describes the key optical components used in a contemporary optical communication system; basic signal and noise parameters; major channel impairments, including chromatic dispersion, polarization mode dispersion (PMD), and fiber nonlinearities; and the system design process. Fault Detectability in DWDM provides a treatise on fault mechanisms are detected.