Data Communications Networking Devices - 4th Ed.pdf

4.9 BROADBAND MODEMS ________________________________________________________ 493technique involves the use of a second cable to provide a return path, while thesecond technique uses a different portion of the coaxial cable bandwidth to obtain areturn path.Both techniques involve a considerable change to the existing CATV cablinginfrastructure. For example, using a different portion of the frequency spectrumfor the return path requires the installation of bidirectional ampli®ers, while the useof a second cable requires both a new set of ampli®ers and cabling.Recognizing that the use of the CATV cabling infrastructure could notaccommodate the requirements of a large subscriber base for video-on-demand,high-speed data transmission, and even digitized telephone service, cable operatorsbegan to install hybrid ®ber/coax HFC) systems during the mid-1990s. UnderHFC a star cabling infrastructure is used, with ®ber cable routed from a cable TVswitch to an optical distribution node commonly located in a subdivision orbuilding complex. Then, coax is routed from the distribution node to individualsubscribers. Under the HFC architecture downstream frequencies of 50 MHz to750 MHz can be subdivided to provide a variety of analog and video channels toinclude data services and telephony. Upstream operations are limited to the 5 MHzto 40 MHz frequency band for telephony, data services, and control channels usedby interactive video set-top boxes.Figure 4.73b) provides a general schematic of the evolving CATV HFC cablinginfrastructure. Note that the ampli®ers shown in Figure 4.73b) must bidirectionalto enable upstream transmission from subscribers to be supported. Table 4.33compares the current and evolving operating characteristics of CATV andtelephone compares transmission facilities. The evolving CATV transmissionmethod in Table 4.33 requires a degree of elaboration. Currently one-way CATVsystems broadcast frequency-multiplexed signals. While the evolving HFCinfrastructure will still use a broadcast transmission method for delivery of basiccable video services, it will also employ switching technology to provide routingand delivery of voice, data, and video-on-demand in a manner similar to that byTable 4.33Cable TV CATV) versus telephone operating characteristicsOperating characteristic CATV TelephoneBandwidth 6 MHz/TV channel 4 kHz/lineUse:Current Video Voice and dataEvolving Video, data, voice Voice, data, VideoDirectionality:Current One-way Two-wayEvolving Two-way Two-wayTransmission method:Current Broadcast Switchedvolving Broadcast/switched SwitchedMedia:Current Coaxial cable Twisted-pairEvolving Fiber + coaxial cable Fiber + twisted-pair