uffi- cient gain to support WDM. These amplifiers operate only in the 1550-nm band. Allowing about 50 GHz of separation between optical carriers, an optical fiber can support some 100 DWDM channels where each optical carrier would carry on the order of 40 Gbps using the ITU frequency (wavelength) matrix. Thus, the aggregate bit rate capacity per fiber would be 40 X 100 Gbps or 4000 Gbps using only the 1550-nm band with current technology (2004, 2005). 9.4.9 Fiber-Optic Link Design The design of a fiber-optic communication link involves several steps. Certainly the first consideration is to determine the feasibility of such a transmission system for a desired application. There are two aspects of this decision: (1) economic and (2) technical. Can we get equal or better performance for less money using some other transmission medium such as wire-pair cable, coaxial cable, LOS microwave, and SO on? Fiber-optic communication links have wide application. Analog applications for cable television (CATV) trunks are showing particularly rapid growth. Fiber is also used for low-level signal transmission in radio systems, such as for long runs of IF and even for RF. However, in this text we stress digital applications, some of which are listed below: On-premises data bus LANs (e.g., fiber distributed data interface) High-level PCM or CVSD configurations² , SONET and SDH Radar data links Conventional data links where bit/symbol rates exceed 19.2 kbps Digital video including cable television It seems that the present trend of cost erosion will continue for fiber cable and components. Fiber-optic repeaters are considerably more expensive than their PCM metal- lic counterparts. The powering of the repeaters can be more involved, particularly if power is to be taken from the cable itself. This means that the cable must have a metallic element to supply power to downstream repeaters, thereby losing a fiber-optic advantage. Metal Guardbands are empty spaces to provide isolation between adjacent channels. This helps minimize interf