ction a data rate of either 27 or 36 Mbits/s is supported. Bandwidth Limitations While a bandwidth of 27 or 36 Mbits/s may appear quite sufficient for most applications, a cable modem subscriber shares that bandwidth with other subscribers. Thus, if the cable modem operator is greedy and assigns a channel to 1000 active subscribers, then the average effective bandwidth per subscriber is not 36 Mbits/s but 36 Mbits/s/1000, or 36,000 bits/s, which could be less than that of a conventional modem. For this reason, some cable TV operators limit a serving neighborhood for cable modem service to 500 subscribers. Because the use of two bands of frequency on a CATV network must be limited to a neighborhood of 500 homes or offices before perfor- mance is significantly affected, the architecture of the CATV network must be revised. This revision requires much more than the simple replacement of unidirectional amplifiers with bidirectional amplifiers and explains why cable TV operators have dug deeply into their pockets to spend tens of billions of dollars upgrading their systems' Chapter Seven infrastructure One method used to upgrade a cable network involves the replacement of an all-coaxial-cable-based network with a hybrid fiber-coaxial system. Hybrid Fiber-Coaxial Systems The employment of hybrid fiber-coaxial (HFC) systems by CATV operators commenced during the mid-1990s in response to the need to provide not only support for more channels but also the ability to aggregate and route data originating from groups of subscribers to the headend and from the headend to a group of subscribers. In the downstream direction, routers must be placed at appropriate locations within the network infrastructure to send data to the applicable branch or subbranch, where the subscriber is located. Figure 7.8 illustrates the emerging hybrid fiber-coaxial (coax) net- work that many cable TV systems are migrating to. Under the HFC network architecture a star cabling infrastructure is employed, where fiber-optic cable is routed from a locat