vert wavelength to fre- quency using Eq. (2.1). One theory is that fiber-optic transmission was developed by physicists who are more accustomed to wavelength than to frequency. SIGNALS CONVEY INTELLIGENCE Optical Fiber-optic Optical transmission source detector medium Output Driver circuit Electrical Electrical input output signal signal Figure 2.13 A simplified model of a fiber-optic link. A simplified model of a fiber-optic link is illustrated in Figure 2.13. In this figure, the driver conditions the electrical baseband signal prior to modulation of the light signal; the optical source is the transmitter where the light signal is generated and modulated; the fiber-optic transmission medium consists of a fiber strand, connectors, and splices; the optical detector is the receiver where the light signal is detected and demodulated; and the output circuit conditions the resulting electrical baseband signal for transmission to the electrical line (Ref. 3). A more detailed discussion of fiber-optic systems will be found in Chapter 9. 2.5.4 Radio Transmission Up to now we have discussed guided transmission. The signal is guided or conducted down some sort of a "pipe." The "pipes" we have covered included wire pair, coaxial cable, and fiber-optic cable. Radio transmission, on the other hand, is based on radi- ated emission. The essential elements of any radio system are (1) a transmitter for generating and modulating a "high-frequency"6 carrier wave with an information baseband, (2) a trans- mitting antenna that will radiate the maximum amount of signal energy of the modulated carrier in the desired direction, (3) a receiving antenna that will intercept the maximum amount of the radiated energy after its transmission through space, and (4) a receiver to select the desired carrier wave, amplify the signal, detect it, or separate the signal from the carrier. Although the basic principles are the same in all cases, there are many different designs of radio systems. These differences depend upon the types of signals to be 