| 单词 | DOCSIS (Data Over Cable Service Interface Specification) |
| 释义 | DOCSIS (Data Over Cable Service Interface Specification) A set of specifications for high speed, full duplex (FDX) data communications over CATV networks, DOCSIS was developed by the Multimedia Cable Network Systems Partners Ltd. (MCNS), and currently is administered by Cable Television Laboratories (CableLabs) an industry research and development organization.The original specifications were for set-top boxes and cable modems (CMs). Matching cable modems positioned at the service provider’s headend and the customer’s premises communicate over a hybrid fiber/coax (HFC) network comprising optical fiber, at least in the trunk portion, and embedded coaxial cable from the neighborhood node to the premises. DOCSIS specifications are for always-on Internet access, which avoids the dial-up delays characteristic of circuit-switched modem access via the public switched telephone network (PSTN). DOCSIS also specifies asymmetric data transmission, which is typical of high speed Internet access services. Downstream transmission takes place over one or more 6-MHz channels in the range between 50 MHz and 750-850 MHz. DOCSIS 2.0 specifies several variations of Quadrature Amplitude Modulation (QAM). At the lowest level, 64-QAM yields six bits per symbol and a signaling rate of 36 Mbps per 6-MHz channel. Alternatively, 128-QAM yields seven bits per symbol and increases the signaling rate to 42 Mbps, although it is more sensitive to noise.The standards also provide for the use of 256-QAM, which increases the raw signaling rate to 48 Mbps.These variations on QAM are compatible with Moving Picture Experts Group-2 (MPEG-2), the compression technique specified for digital video transmission in CATV networks. Downstream data is encapsulated into MPEG-2 packets of 188 bytes. Upstream transmission, in early DOCSIS versions, is supported over 6-MHz channels in the range between 5 MHz and 42 MHz for United States systems, and 8-MHz channels in the range between 5 MHz and 65 MHz for European systems.The channels each support a signaling rate of 12 Mbps through 74570c04.qxd 9/11/07 12:19 PM Page 142 |
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