Data Communications Networking Devices - 4th Ed.pdf

4.9 BROADBAND MODEMS ________________________________________________________ 495LANcity introduced the world's fastest and least expensive personal cable TVmodem, named LCP LANcity Personal Cable TV Modem). This modem can beset to operate in any 6 MHz TV channel in the transmit range from 5 to 42 MHzand the receive frequency range of 54 to 750 MHz.This modem uses Quadrature Phase Shift Keying QPSK) modulation to obtaina symmetrical 10 Mbps operating rate, requiring approximately 5 MHz of RFbandwidth in a 6 MHz channel. Under QPSK the phase of the carrier signal isvaried based upon the composition of the digital data to be transmitted. Forexample, a digital `1' could be transmitted by generating a 180 phase shift in thecarrier, while a `0' might be represented by a 0 phase shift. The `quadrature' aspectof the modulation scheme results in the carrier being capable of being shifted to oneof four possible phases 0 ,90 , 180 , 270 ) based upon the dibit value of the data tobe transmitted. Figure 4.74 illustrates the four-point QPSK constellation pattern.Figure 4.74Four-point QPSK constellationAccess protocolBased on the method by which CATV channels are routed on the main trunk andfeeder cables through drop cables to the subscriber, bandwidth is shared amongsubscribers. In comparison, a local loop twisted-pair connection from thetelephone company central of®ce to a subscriber represents dedicated bandwidth.This means that access to the 6 MHz channel the cable modem uses must be sharedamong many subscribers, requiring an access protocol to govern the orderly ¯ow ofdata onto the cable. LANcity and most cable modem vendors support the EthernetCarrier Sense Multiple Access/Collision Detection CSMA/CD) protocol toprovide a mechanism for cable subscribers to gain orderly access to the 6 MHz TVchannel. To accomplish this the LANcity LCP cable modem is cabled to anEthernet 10 Mbps adapter card installed in a personal computer. The PC usessoftware to establish a TCP/IP protocol stack which results in the computer