The new information services provided worldwide through the Internet are fostering the upgrade of existing access and transmission plants, and the de- ployment of new ones. The bandwidth bottlenecks of existing electronic plants are being gradually removed by the massive use of optics at all levels. The latest technological developments in optical system components have finally made the huge bandwidth of optical fibers available both for increas- ing the amount of transmitted information and for reducing the transmission cost per information bit. Wavelength Division Multiplexing (WDM) is now...