层次网络的体系结构接入技术.ppt

* The cable TV industry uses a portion of the RF electromagnetic spectrum. Within the cable, different frequencies carry TV channels and data. At the subscriber end, equipment such as TVs, VCRs, and High Definition TV set-top boxes tune to certain frequencies that allow the user to view the TV channel or, using a cable modem, to receive high-speed Internet access. A cable network is capable of transmitting signals on the cable in either direction at the same time. The following frequency scope is used: Downstream: Transmitting the signals from the cable operator to the subscriber, the outgoing frequencies are in the range of 50 to 860 MHz. Upstream: Transmitting the signals in the reverse path from the subscriber to the cable operator, the incoming frequencies are in the range of 5 to 42 MHz. The downstream frequency range is divided into channels (6 MHz channels in North America and 7 to 8 MHz for Europe). The range of frequencies between the upstream and downstream frequency ranges is called a guard band. The guard band is required because of the cutoff characteristics of the high-pass and low-pass filtering. The filtering ensures that the signal does not spill into the adjacent spectrum. The cable industry defines the cable TV spectrum for the downstream path as follows: VHF low band (TV channels 2 to 6) VHF midband (TV channels 98, 99, and 14 to 22) VHF high band (TV channels 7 to 13) VHF superband (TV channels 23 to 36) VHF hyperband (TV channels 37 and higher) There is no frequency plan for the upstream path. The cable operator can monitor the frequency band of the upstream and place the upstream data signals into clean areas where there is no interference from noise and other signals. The area between 5 and 15 MHz is usually noisy and unusable. * Modern cable operators use an Hybrid Fiber-Coaxial (HFC) Cable network that deploys fiber in the trunks. Fiber has several benefits over regular coaxial cable: Reduces the number and cost of amplifiers Thin and ligh