interact with molecules that make up the fiber core. This interaction results in a loss of signal energy by the photons, whose end effect is the absorption of light. A second effect resulting from the flow of light in a fiber is scatter- ing. As noted earlier in this chapter, the effect of scattering is to redirect a portion of light out of the core into the cladding, causing other rays to bounce from core to cladding to core as they propagate along the fiber. Because the end result is a stretched pulse, the effect of scattering or modal dispersion is to attenuate the pulse. If we compare and contrast the three types of material used for optical fibers, we can categorize them with respect to their attenua- tion, cost, and utilization. Table 3.1 summarizes these properties. We will now describe and discuss these relationships by covering each type of fiber. Chapter Three TABLE 3.1 Fiber material Attenuation Utilization Comparison of Fiber Compositions Glass core Lowest Highest High bandwidth, long distance Plastic core Highest Lowest Low bandwidth, short distance Glass core with Medium Medium Medium bandwidth, plastic cladding short to medium distance (plastic-clad silica) Glass Fiber A glass-based fiber-optic cable has a glass core and glass cladding, with impurities added to obtain the desired indices of refraction necessary to guide light rays through the fiber. This type of cable has the lowest value of attenuation; however, it is also the most expensive of the three types of materials used to create an optical fiber. Concerning utilization, a glass-based fiber-optic cable is by far the most commonly used type of optical cable. Thus, it also represents the type of cable that installers are most familiar working with, which means that functions such as cable splicing and pulling techniques are well known. Because of its low attenuation, a glass fiber is always used for single- mode optical cable. Because single-mode fiber requires a very thin case, its composition eliminates the potential use of plastic, which 