rds Transmission impairments, which represent the influence exerted by amplifiers (especially noise), chromatic dispersion, polarization mode dispersion, and nonlinear optical effects Chapter 6 Optical Fiber Transmission The role of the receiver and how it influences BER-the main physical measure of a network's performance Overall system design, which includes a number of essential techniques to overcome transmission impairments: Mitigation and compensation of chromatic dispersion and polarization mode dispersion Avoidance of the worst effects of nonlinear crosstalk Use of optimized modulation formats and forward error correction (FEC) The design of optical systems and networks requires knowledge of many effects, and the design techniques used are constantly evolving. This chapter explains the underlying engineering principles. It does not aim to review the most recent results at the time of writing, which, if history is a reliable guide, will soon be superseded. Single-Channel Transmission When optical fibers were initially introduced, they were designed to carry a single stream of optical pulses by simple on-off keying of the signal light. As systems have developed, and fibers have been required to transport ever more information, the transmission tech- niques have become more refined. Basic properties of light impulses have been exploited and innovative methods have been developed to achieve higher efficiencies for data trans- mission. This section explains the operation of single-channel systems to lay a foundation for understanding the more recent advances. Optical Binary Digits: "Bits" Light guided by an optical fiber is normally a stream of optical pulses to convey a digital data sequence (Agrawal, 2002; Ramaswami & Sivarajan, 2002). The pulse stream is divided into equal, short time intervals known as "bitslots" or "bit intervals." The most common type of data sequence in optical communications is "binary," in which the data is in the form of logical 1s and Os. If there is a pulse in a bitslot, it is a 1 