 Optical Fiber Communications, Second Edition, Prentice Hall. Stolen, R. H. (1980) Nonlinerity in Fibre Transmission, Proceedings IEEE 68:1232-1236. Wong, W. S., et al. (2005) Breaking the Limit of Maximum Effective Area for Robust Single-Mode Propagation in Optical Fibers, Opt. Lett. 30:2855-2857. 6 Optical Fiber Transmission Paul Urquhart Universidad Pública de Navarra Introduction Optical fiber communication provides data capacities that are unavailable from any other transmission means, and SO it exerts a substantial influence on the information society (Hecht, 1999). Optical cables have been installed in many environments-within buildings, under streets, suspended from overhead pylons, and across the world's deepest oceans (Urquhart, 2004; Keiser, 2006; Lin, 2006; Halabi, 2003; Chesnoy, 2002). The networks they serve carry data for many end users, including domestic consumers, manufacturing, finan- cial services, hospitals, educational establishments, and government departments. The history of the subject is one of record-breaking achievements, especially as measured by distance, total capacity, and number of customers served. This chapter explains the main physical aspects of fiber transmission systems, without specific reference to network topology. The treatment is system oriented but, when appro- priate, points to implications for the main categories of fiber networks. Many physical phe- nomena influence system design and implementation (Agrawal, 2002). Moreover, the broad spectrum of component technologies that are required, ranging from the transmitter to the receiver, all affect system performance (Liu, 2005). Our main aim is to describe the various operating impairments, explain how they influence the system's bit error rate (BER), and discuss ways to compensate for them. The topics considered in this chapter can be categorized as several interlocking themes: Single- and multiple-channel transmission, which includes Synchronous Digital Hierarchy (SDH) and Wavelength Division Multiplexing (WDM) standa