TION TO MULTIPLEXING 96 channels on a wire pair, depending on the characteristics of that wire pair. Coaxial cable can support many thousands of voice channels; line-of-sight microwave radio is capable of carrying from several hundred to several thousand voice channels. A single fiber-optic thread can support literally tens of thousand of channels. A communication satellite transponder can carry between 700 and 2000 such voice channels, depending on the transponder's bandwidth. There are essentially two generic methods of multiplexing information channels: 1. In the frequency domain; we call this frequency division multiplex (FDM). 2. In the time domain, which we call time division multiplex (TDM). The concepts of frequency division multiplexing are discussed in this chapter. Time divi- sion multiplexing (pulse code modulation) is covered in Chapter 6. 4.5.2 Frequency Division Multiplex 4.5.2.1 Introduction. With FDM the available channel bandwidth is divided into a number of nonoverlapping frequency slots. Each frequency slot or bandwidth segment carries a single information-bearing signal such as a voice channel. We can consider an FDM multiplexer as a frequency translator. At the opposite end of the circuit, a demultiplexer filters and translates the frequency slots back into the original information bearing channels. In the case of a telephone channel, a frequency slot is conveniently 4 kHz wide, sufficient to accommodate the standard 300- to 3400-kHz voice channel. Figure 4.17 illustrates the basic concept of frequency division multiplex. Output Mixer 995998 Frequency translator (multiplexer) Figure 4.17 The frequency division multiplex concept illustrated. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK In practice, the frequency translator (multiplexer) uses single sideband modulation of radio-frequency (RF) carriers. A different RF carrier is used for each channel to be mul- tiplexed. This technique is based on mixing or heterodyning the signal to be multiplexed, typically a voice channel, with an R