plitting a fiber into 128 colors. In addition, other vendors announced the development of prototype DWDM systems that have derived 1024 channels through a common fiber using a tunable laser. With just one optical frequency on a fiber providing several magnitudes of the capacity of a copper pair, it is quite easy to visualize the superiori- ty of DWDM to the use of metallic pair wiring. Specifically, one strand of optical fiber is now capable of transporting the equivalent of a half- million T1 lines, with each of the latter capable of carrying 24 voice con- versations. Thus, in terms of data transmission rate and capacity, a DWDM system can transport approximately 12 million voice calls! While the capacity improvement afforded WDM and DWDM are con- siderable, it should also be mentioned that this improvement often occurs over an existing optical fiber infrastructure Because the cost associated with the installation of optical fiber commonly accounts for most of the total cost of installing an optical transmission system, the ability to expand capacity by changing the transmitter and receiver represents a Introduction tremendous economic benefit. This also explains why communications carriers have literally been gobbling up WDM and DWDM components as rapidly as the manufacturers of such components can produce them. Electromagnetic Immunity In a copper cable environment the flow of electrons can be altered by electromagnetic interference (EMI). In an optical transmission system, light in the form of photons traverses down the fiber. Photons are not affected by electromagnetic interference, and there is no photonic equivalent. For instance, optical systems in a building are immune (i.e., impervious) to a noisy electrical environment resulting from EMI produced by machinery including devices such as an electrical light fluorescent ballast and an elec- tronic pencil sharpener. In a WAN environment, protection is extended to obtain immunity to sunspots and other electrical disturbances. The immunity from EMI permits op