In the past decade, there were two offers rates up to Mbps modem rate of new technology 11 CableModem and ADSL (Asymmetric Digital Subscriber Line) Modem. CableModem in the hybrid fiber / coaxial network ( HFC ) To achieve up to 10Mbh / s data transmission, while the ADSLModem is the twisted-pair cable up to 6Mbit / s data transfer, the two technologies in the Internet access network Video on Demand (VOD), video conferencing, distance learning and other areas has a broad market.
1 Comparison of basic skills Typical CableModem the transmission lines of the spectrum is divided into 2, that is 5? 50MHz and 50MHz uplink channel above (other than the analog TV channel) and downlink channels. CableModems analog frequency division multiplexing is based on the application of integrated analog and digital transmission technology, optical fiber Coaxial cable Technology and RF technology, realized in the 5? 750MHz analog TV signals simultaneously transmitted and digital signals, while the use of commercial efficiency of 64QAM (quadrature amplitude modulation) technology, downlink channel rate up to 30Mbit / d. CableModem the transmission rate and transmission distance has nothing to do with the fiber itself, the noise and the number of users simultaneously access the.
ADSL transmission feed line to the spectrum is divided into 3, the low band transmit ordinary telephone service (POTS); the middle band digital channels for the uplink, the maximum transmission rate of 384kbit / s for control information; the other digital channels for the downlink transmission 6Mbit / s of digital information. ADSL technology uses two modulation schemes: CAP (no carrier amplitude modulation) and DMT (discrete multi-tone modulation). CAP approach is a two-dimensional manner similar to sending program QAM, DMT modulation is a form of multicarrier modulation. DMT is the available bandwidth is divided into N-sub-channel, send data according to sub-channel capacity, the data dynamically allocated to each subchannel, so that bandwidth utilization is increased, to minimize error and noise, thus enhancing the transmission capacity. In addition, ADSL using adaptive filtering of new technologies, grid codes and interleaving FEC methods to overcome the Gaussian noise interference, increased channel capacity, for the first time using asymmetric echo cancellation (echocancellation) to eliminate back wave. However, the transmission rate by ADSL transmission limit, long-distance transmission rate of no more than 1.5Mbit / s.
2 basic network structure 2.1CableModems system structure CableModem in the HFC network, while the HFC network is part of the use of fiber optic transmission lines, wiring, and access line some full use of existing CATV coaxial cable network to provide Broadband Access. Each network will provide online access to up to 100 users, each line must be set in two-way Amplifiers To support the uplink channel, in each client to install a CableModem, in Front-end equipment To install an IP router in, it is noteworthy that the coaxial cable network can be used as a data concentrator, to avoid over-concentration of the use of routers, in some cases, front-end devices and can be placed on high-speed buffer Proxy Data server to ease the congestion. Use of HFC broadband access network enables telecommunications network, CATV network and data network Triple play .
2.2ADSL network structure
(L) SKIP (starterkitInternetplatform) network structure shown in Figure 2, SKIP relatively simple network structure, in each central office to set up a router and a network switching device and ATU-C (ADSL transceiver), network switching devices will focus on access lines and enter the router unified operation.