):55-67. Resources Michelson, A. M. and Levesque, A. H. (1985) Error-Control Techniques for Digital Communication, Wiley-Interscience. Peterson, W. W., and Weldon, E. J. (1972) Error-Correcting Codes, Second Edition, MIT Press. This page intentionally left blank Part 2 Transmission Media This part discusses the transmission media that are used in core telecommunica- tions networks. Before 1980 the predominant transmission medium was copper cables (either copper pairs or coaxial cables). A smaller proportion of systems were carried over radio channels. However, since the advent of commercial optical fiber transmission systems in 1980, copper cables for transmission have been displaced either to the edges of the network or to connections within or between equipment. They are therefore not covered in this volume. Microwave systems, on the other hand, are still widely used, although the proportion of traffic they carry in core networks is small compared to fiber systems. The basic principles of radio propagation are therefore covered here. However, radio remains a key technology for mobility and for local-area network applica- tions, such as in hotspots or home and office networks. These applications will be discussed in more depth in Volume 3 of The Cable and Telecommunications Profes- sionals' Reference. Optical fiber systems, however, have become firmly established as the mainstay of telecommunications transmission. From modest beginnings in the first commer- cial systems in 1980, with a single channel per fiber, bit rates of 2 Mbit/s and system ranges around 30 km, they progressively increased in bit rate to 2.5 Gbit/s per channel by 1990. At this stage wavelength division multiplexing was not considered economic; each time a higher-capacity system was needed, digital electronic tech- nology was able to increase its speed to meet the requirement. (A commonly applied rule of thumb was that each time speed increased by a factor of 4, cost increased by a factor of 21/2). Further increase of speed from 1990 onward pr