 meaning that it's a shared- access architecture. (Chapter 6, "Local Area Networking," describes bus and other LAN topologies.) Figure 12.3 shows the topology of a cable TV operator's HFC net- work. On the left side are the headends, from which the information is being broadcast. Those headends are tied together, generally with fiber, referred to as fiber in the backbone. By moving to a fiber-based backbone, the cable TV operators have also made improvements to the performance of their networks as well as the costs associated with operating and maintaining them. The backbones, then, feed into the neighborhood nodes, or the optical nodes (i.e., the access points), and Cable TV Networks Headend Optical Amplifiers and Transmitters Optical Primary Amplifiers Fiber Fiber Secondary Neighborhood Node 500- to 2,000-Home Area Figure 12.3 The topology of an HFC network from there coax goes out to the homes. You can see in Figure 12.3 that HFC is a shared infrastructure; this is one of the drawbacks of the HFC architecture. The HFC architecture uses Frequency Division Multiplexing (FDM) to derive individual channels, some of which are dedicated to the support of telephony ser- vices, others of which are reserved for analog TV, and still others of which are reserved for DTV and future interactive broadband services. This multiple-access coax system represents a bit of a hostile environment: The points where the coax connects into set-top boxes or cable-ready TV sets tend to collect noise, SO the cable picks up extraneous noise from vacuum cleaners or hair dryers. If every household on a network is running a hair dryer at 6:30 AM, the upstream paths are subjected to this noise, and hence there will be some performance degradations. Extra signal processing must therefore be added to overcome the impairment in the return channel. The major concerns with HFC include security, privacy, reliability, and return- path issues, particularly in support of telephony. With twisted-pair, we're used to having a private line to the local ex