and if there is none, it is a 0 (Conradi, 2002). Indeed, the word "bit" was derived from "binary digit." Nonbinary forms of digital communication are also possible; examples include "duo- binary" and "multilevel signaling." There can be, for example, three logic levels, such as 0, 1, and 2, supplied by no pulse, a low-power pulse, and a high-power pulse, respectively (Gallion, 2002). However, such formats are less common. Analogue optical modulation is also possible but its use is confined to specialist applications and SO is not considered in this chapter. Digital logic is preferred because it is more tolerant to noise and is compatible with the large numbers of computers and other electronic processors that are connected to tele- communications systems. An optical channel is formed by launching the modulated output from a laser into an optical fiber and then receiving the pulses at the far end of the system to recover the infor- mation in the form of electrical 1s and Os. The laser is an electrical-to-optical energy con- verter; the detector at the output is the opposite, converting back to electrical signals. The "datarate," or "bitrate," of the channel is simply the number of bits per second. 1 A glossary is provided at the end of this book. Single-Channel Transmission Light in an optical fiber can be characterized by its optical intensity, phase, wavelength, and polarization. The most common type of optical transmission is phase insensitive, which means that at the transmitter there is a simple electrical current-to-light conversion and the detector there is a reverse conversion (Ho, 2001). Similarly, most transmission systems use unpolarized light, which is, ideally at least, a superposition of all states of polarization. An expression commonly used for binary data sequences that do not make use of the light's phase or polarization properties is "On-Off Keying" (OOK), according to which the light is either on or off. Nonreturn-to-Zero and Return-to-Zero Signals The two main types of OOK transmission are kno