 from subscribers to be supported. Chapter Seven In examining Figure 7.8, note that several amplifiers are shown as being unidirectional. While subscribers literally "at the end of the coax" cannot obtain a bidirectional transmission capability, this indicates how CATV operators can gradually "build out" their new infrastructure instead of having to replace everything at once. This also explains why subscribers located in different areas within a city or in different rural areas within close proximity to one another may not be able to use a cable modem at the same time. CAPACITY CONSIDERATIONS Another advantage associated with the use of a hybrid fiber-coax (HFC) system is the additional capacity made available. Under the HFC architecture, downstream frequencies beyond the typical 550-MHz limit of a fully coaxial system becomes available for use. If we remember our physics, high frequencies on an electrical system attenuate more rapidly than do low frequencies. This phenomenon would require cable operators to place attenuation equal- izers within their systems at closer spacings, which would drive up their network cost. Putting pen to paper, cable operators decided that it would be more advantageous to limit transmission to 550 MHz, result- ing in most systems initially installed limiting support to 42 channels of programming. Because the use of optical fiber provides a minimal loss of signal strength into a neighborhood, it becomes possible to support additional channels over coaxial cable that now has a shorter run length from the fiber node into homes and offices. Thus, HFC systems can support downstream frequencies of 750 MHz or higher, permitting support for 125 or more channels of programming. Now that we have a general idea of the direction where the CATV infrastructure is headed, we conclude this chapter by focusing on a lim- ited use of fiber. That limited use we will call fiber to the home, which at the beginning of this chapter we described as including fiber routed directly to residential houses as we