 - Now known as CableLabs Certified Cable Modems, DOCSIS (Data Over Cable Service Interface Specifications) is a standard interface for cable modems, the devices that handle incoming and outgoing data signals between a cable TV operator and a personal or business computer or television set. DOCSIS 1.0 was ratified by the International Telecommunication Union (ITU-TS) in March of 1998. Although "DOCSIS" continues to be used, the newer name emphasizes that the standard is now being used to certify the products of cable modem makers. Cable modems conforming to DOCSIS are now being marketed.
Cable operators whose existing customers have non-standard cable modems can handle them by adding backwards-compatible support to the DOCSIS card at the cable operator's end. As DOCSIS continues to evolve to new versions, existing modems can be upgraded to the newer versions by changing the programming in the cable modem's EEPROM memory. DOCSIS-compliant cable modems are being integrated into set-top boxes for use with television sets. DOCSIS must also support or converge with the high definition television (HDTV) standard. The set-top box itself follows a standard known as OpenCable.
DOCSIS specifies modulation schemes and the protocol for exchanging bidirectional signals over cable. It supports downstream-to-the-user data rates up to 27 Mbps (megabits per second). Since this data rate is shared by a number of users and because many cable operators will be limited by a T1 connection to the Internet, the actual downstream data rate to an individual business or home will be more like 1.5 to 3 Mbps. Since the upstream data flow has to support much smaller amounts of data from the user, it's designed for an aggregate data rate of 10 Mbps with individual data rates between 500 Kbps and 2.5 Mbps.
Cisco and Microsoft have endorsed DOCSIS. They are collaborating on a DOCSIS-compliant cable hybrid fiber-coax (HFC) system, called the Multimedia Cable Network System (MCNS), that will deliver services to residential, commercial, and educational customers.
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29 Apr 2004
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