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transmitted down the fiber as digital pulses, usually from a semiconductorlaser.Light then propagates in a step-index optical fiber through its internalreflections. That is, as the light pulses travel along the fiber’s inner core, theyhit the surface of the cladding glass and bounce back, in a series of glancingcollisions. Using laser diodes, the propagated burst of light is to indicatebinary digit 1 and no light indicates binary digit 0. The light input is thenmodulated by a scheme known as optical on–off keying (OOK). At the otherend, a light-sensitive laser diode, for instance, detects the pulses and turnsthem back to electrical signals for sending to another terminal; seeFig. 3.23(b).The different angles of rays entering an optical fiber are calledpropagation modes. Different propagation modes have different speeds ofpropagation along the fiber. Only a finite set of modes can propagate along anoptical fiber. If the radius of the optical fiber is small enough, then only onemode can propagate (i.e., single-mode fibers). Propagation in optical fibers issubject to multimode pulse spreading, attenuation, and dispersion. Theseeffects limit the transmission rate and usable length of a fiber. This is whyoptical fiber links sometimes require repeaters every 30 to 50 km. Encouragingresearch has shown how inline optical signal amplification is possible, therebymaking the receivers more sensitive, since such signals are weak and quicklyovercome by noise, particularly thermal noise.With wavelength multiplexing—a term used to describe the combinedtransmission of multiple optical signals of different wavelengths over acommon optical fiber—optical fiber links have shown a larger capacity tocarry traffic. An example is the dense wavelength division multiplexing(DWDM) system, which uses light of different colors travelling down anoptical fiber cable.3.5.2 MicrowaveIn a microwave radio system, telecommunications traffic is transmitted in theform of direct beams of microwave energy. Microwaves, like light, travel in astraight line between antenna dishes that concentrate the beam. In some cases,the microwave dishes are used as part of the transmission network; they act asrepeater stations, which receive, amplify, and retransmit the signal theycollect. These repeater stations are located at regular intervals in such a waythat the overall transmission route is slightly zig-zagged to account for theearth’s curvature and, importantly, to prevent signals from passing between twoCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.