ion in a neighborhood referred to as a fiber node (FN) back to the CATV headend. In examining Figure 7.8, note that in addition to an electrical to opti- cal converter, each fiber node that supports data transmission to one or more branches will have a router that will be used to aggregate trans- mission to and from the branch feeder cables. The router's transmission would be placed on a separate wavelength for transmission in the uplink direction toward the cable TV headend. In the downlink direc- tion a separate wavelength could be used to provide a data path to each router. However, programming was initially transmitted in the down- link direction via time division multiplexing (TDM). Thus, many cable operators continue to use TDM to each fiber node, where channels in each TDM slot are converted from the time domain into the frequency domain. The latter is required since subscribers tune their set-top boxes via frequency to locate a particular program of interest. RATIONALE The rationale for the use of fiber into a neighborhood is based on economics, capacity, and fiber-optic immunity to electro- magnetic interference. Because bandwidth capacity of fiber is several orders of magnitude above that of coaxial cable, the existing coaxial- cable drops can be used as is. Thus, the use of fiber provides the cable Fiber in the Neighborhood Figure 7.8 A hybrid fiber-coax distribution system permits the gradual upgrade of coax drop to support bidi- rectional transmission and also increases the number of chan- nels of programming that can be carried Cable TV by the CATV system. Headend Legend: subscriber Bidirectional amplifier Unidirectional coaxial amplifier Fiber node operator with an economical method for upgrading their distribution system. AMPLIFIER CONSIDERATIONS To ensure that the upgraded sys- tem is capable of supporting two-way communications, the unidirec- tional amplifiers within the remaining coaxial portion of the network must be replaced by bidirectional amplifiers. This then permits upstream transmission