Figure 9.25 illustrates a model of a fiber optics link. Besides the supporting electrical
circuitry, it shows the three basic elements in an optical fiber transmission system: (1) the
optical source, (2) the fiber link, and (3) optical detector. Regarding the fiber-optic link
itself, there are two basic impairments that limit the length of such a link without resorting to repeaters or that can limit the distance between repeaters. These impairments are
loss (attenuation), usually expressed in decibels per kilometer, and dispersion, usually
expressed as bandwidth per unit length, such as megahertz per kilometer. A particular
fiber-optic link may be power-limited or dispersion-limited