ger L. Freeman CONCEPTS IN TRANSMISSION TRANSPORT in FDM configurations (see Section 4.5.2). Fiber-optic cable has replaced the greater portion of these coaxial cable systems. There is one exception. Coaxial cable is still widely employed in cable television configurations (see Chapter 17). Wire pair remains the workhorse in the subscriber plant. At the outset, we assume that these transmission links are digital and will be based on the PCM configurations covered in Chapter 6, namely, either of the DS1 (T1) or E1 families of formats. However, the more advanced, higher-capacity digital formats such as synchronous optical network (SONET) or synchronous digital hierarchy (SDH) (see Chapter 19) are now being widely deployed on many, if not most, new fiber-optic systems, and with lower-capacity configurations on certain radio systems. 9.2 RADIO SYSTEMS 9.2.1 Scope The sizes, capacities, ranges, and operational frequency bands for radio systems vary greatly. Our discussion will be limited to comparatively high-capacity systems. Only two system types meet the necessary broadband requirements of the long-distance net- work. These are line-of-sight (LOS) microwave and satellite communications. Satellite communications is really nothing more than an extension of LOS microwave. 9.2.2 Introduction to Radio Transmission Wire, cable, and fiber are well-behaved transmission media, and they display little vari- ability in performance. The radio medium, on the other hand, displays notable variability in performance. The radio-frequency spectrum is shared with others and requires licens- ing. Metallic and fiber media need not be shared and do not require licensing (but often require right-of-way). A major factor in the selection process is information bandwidth. Fiber optics seems to have nearly an infinite bandwidth. Radio systems have very limited information band- widths. It is for this reason that radio-frequency bands 2 GHz and above are used for PSTN and private network applications. In fact, the U.S. Federal Communications 