hysical measure that is of greatest concern to network operators is the bit error rate (BER), which is defined as the ratio of errored bits at the receiver to the total number of bits transmitted. We showed that a wide range of effects, such as chromatic and polariza- tion mode dispersion, nonlinear optical phenomena, amplifier noise and gain characteristics, and the nature of the transmitted pulses influence BER. System design normally requires compromises to satisfy many conflicting requirements. These become ever greater as the number of channels and the datarate per channel increase. Fortunately, there are numerous techniques to improve performance. They include optimi- zation of amplifier placement; use of distributed Raman amplification; management and compensation of chromatic dispersion, using both optical and electronic methods; applica- tion of optimal modulation formats; reduction and compensation of polarization mode dis- persion; appropriate choice of transmission fiber to minimize nonlinear effects; and use of advanced FEC codes. Clearly, the physical design of optical fiber systems and networks is a wide-ranging task that requires many skills. Acknowledgments The writing of this chapter was supported in part by the Spanish CICyT (Project TEC2004-05936-C02). I am grateful to Nuria Miguel Zamora and Oscar García López for Figures 6.8 and 6.9, respectively. Resources Afonso, A., et al. (2006) Dynamic Gain and Channel Equalizers or Wavelength Blocker Using Free Space: Common Elements and Differences, IEEE J. Lightw. Technol. 24(3):1534-1542. Agrawal, G. P. (2002) Fiber Optic Communication Systems, Third Edition, Wiley. Agrawal, G.P. (2001) Nonlinear Fiber Optics, Third Edition, Academic Press. Ait Sab, O. (2001) FEC Techniques in Submarine Transmission Systems, Proceedings Conference on Optical Fiber Communications (OFC), paper TuF.1. Aozasa, S. (2007) Highly Efficient S-Band Thulium-Doped Fiber Amplifier Employing High Thulium-Concentration Doping Technique, IEEE J. Lightw. Technol. 25(8):2108-2114. B