Docsis The Cable Modem Standard

A bi-directional cable modem technology must take into account existing CATV networks and CATV network quality and requirements for supporting IP networks. Although several proprietary cable modem network technologies exist, the internationally accepted (and by far the most popular) standard for cable IP networking is DOCSIS. Created by the MCNS to drive superior cable IP technology and to encourage competition between equipment vendors, DOCSIS and the analogous European standard Euro-DOCSIS detail the communication between a cable modem and the device to which it connects, the CMTS. Other aspects of a cable IP network are also addressed by the standard such as provisioning, security specifications, and network management. The fundamental value of DOCSIS allows the cable operator to reliably bill for service and use equipment from more than one vendor in an essentially interchangeable fashion.

The DOCSIS 1.0 specification describes the mechanisms by which cable devices such as cable modems can connect to the cable IP network, become provisioned, and actively transmit. DOCSIS also specifies requirements for the cable plant as shown in Table 13-2. Although DOCSIS 1.0 compliant cable modems have been widely deployed, limitations of the DOCSIS 1.0 specification, particularly in the area of QoS and upstream signaling have prompted the release of subsequent standards. The DOCSIS 1.1 specification includes more detailed provisions for QoS, the differentiation of traffic streams, sophisticated multicast transmission, and other important components. The recent DOCSIS 2.0 specification primarily addresses the physical layer aspects of the DOCSIS networks and includes specifications for advanced physical layer communications, which can improve network throughput under certain conditions, and with new CMTS and cable modem hardware. Figure 13-5 depicts connecting the CMTS to the cable modem through the HFC.

Despite the apparent superiority of more recent DOCSIS specifications, at the time of this writing the vast majority of DOCSIS cable modems deployed are capable of DOCSIS 1.0 communication only. This is in part because of the large installed base of modems that existed prior to the finalization of more recent specifications. However, DOCSIS 1.1 cable modems are being deployed in increasing numbers and eventually the bulk of deployed modems will be based on newer specifications.

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