Ultimately the design of a digital communication system depends on the properties of the channel. The channel is typically a part of the digital communication system that we cannot change. Some channels are simply a physical medium, such as a wire pair or optical fiber. On the other hand, the radio channel is part of the electromagnetic spectrum, which is divided by government regulatory bodies into bandlimited radio channels that occupy disjoint frequency bands. In this book we do not consider the design of the transducers, such as antennas, lasers, and photodetectors, and hence we consider them part of the channel. Some channels, notably the telephone channel, are actually composites of multiple transmission subsystems. Such composite channels derive their characteristics from the properties of the underlying subsystems.Section 18.1 discusses composite channels. Sections 18.2 through 18.4 review the characteristics of the most common channels used for digital communication, including the transmission line (wire pair or coaxial cable), optical fiber, and microwave radio (satellite, point-to-point and mobile terrestrial radio). Section 18.5 discusses the composite voiceband telephone channel, which is often used for voiceband data transmission. Finally, Section 18.6discusses magnetic recording of digital data, as used in tape and disk drives, which has characteristics similar in many ways to the other channels discussed.The most prevalent media for new installations in the future will be optical fiber and microwave radio, and possibly lossless transmission lines based on superconducting materials. However, there is a continuing strong interest in lossy transmission lines and voiceband channels because of their prevalence in existing installations. Thus all the media discussed in this chapter are important in new applications of digital communication.
COMPOSITE CHANNELSIt is common for many users to share a common communication medium, for example by time-division and frequency-division multiplexing (Chapter 16).Example 18-1. Voice signals are roughly bandlimited to frequencies lower than 4 kHz. A suitable baseband channel therefore needs to pass only frequencies up to 4 kHz. Such a channel is often derived from a much higher bandwidth physical medium that is shared with other users. A voice frequency (VF) channel derived from a coaxial cable (Section 18.2) using single-sideband modulation is shown in Fig. 18-1. The SSB modulator translates the VF channel to the neighborhood of a frequency ω c for transmission on the coaxial cable. A VF channel can be used for digital communication, as long as the modulation technique conforms to the limitations of the channel.The channel in Fig. 18-1 is an example of a composite channel, because it consists of multiple subsystems. If the VF channel was designed for voice transmission, it has certain characteristics which are beyond the control of the designer of the digital communication system. The VF channel characteristics in this case depend not only on the properties of th...