Cable Modem Specs....
Cable operators have long believed success in the high-speed data business would require that cable modems be interoperable, low-cost and sold at retail like telephone modems and data network interface cards. This way, MSOs could avoid the capital burden associated with purchasing cable modems and leasing them back to subscribers, and consumers would be able to choose products from a variety of manufacturers.
The Institute of Electronic and Electrical Engineering's (IEEE) 802.14 Cable TV Media Access Control (MAC) and Physical (PHY) Protocol Working Group was formed in May 1994 by a number of vendors to develop international standards for data communications over cable. The original goal was to submit a cable modem MAC and PHY standard to the IEEE in December 1995, but the delivery date slipped to late 1997.
Tired of waiting for the IEEE 802.14, cable operators combined their purchasing power to jump-start the standards process. In January 1996, cable MSOs Comcast, Cox, TCI, and Time Warner -- operating under a limited partnership dubbed Multimedia Cable Network System Partners Ltd. (MCNS) -- issued a request for proposals (RFP) to retain a project management company to research and publish a set of interface specifications for high-speed cable data services by the end of the year. MSOs MediaOne (formerly Continental Cablevision) and Rogers Cablesystems, and CableLabs, also signed on to the MCNS Data Over Cable Service Interface Specification (DOCSIS) RFP. Together, this coalition represents the majority of the North American cable industry, serving 85% of U.S. cable subscribers and 70% of Canadian subscribers.
MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. To date, more than 20 vendors have announced plans to build products based on the MCNS DOCSIS standard.
A Tale of Two Standards: MCNS vs. IEEE
The differing cable modem specifications advocated by 802.14 and MCNS reflect the priorities of each organization. A vendor-driven group, 802.14 has focused on a creating a future-proof standard based on industrial-strength technology. The MSO members of MCNS, on the other hand, are far more concerned with minimizing product costs and time to market. To achieve its objectives, MCNS sought to minimize technical complexity and develop a technology solution that was adequate for its members' needs.
At the physical layer, which defines modulation formats for digital signals, the IEEE and MCNS specifications are similar. The 802.14 specification supports the International Telecommunications Union's (ITU) J.83 Annex A, B and C standards for 64/256 QAM modulation, providing a maximum 36 Mbps of downstream throughput per 6 MHz television channel. The Annex A implementation of 64/256 QAM is the European DVB/DAVIC standard, Annex B is the North American standard supported by MCNS, while Annex C is the Japanese specification. The proposed 802.14 upstream modulation standard is based on QPSK (quadrature phase shift keying) and 16QAM, virtually the same as MCNS.
For the media access control (MAC), which sets the rules for network access by users, 802.14 has specified Asynchronous Transfer Mode (ATM) as its default solution from the headend to the cable modem. MCNS went a different route, using a scheme based on variable-length packets that favors the delivery of Internet Protocol (IP) traffic. Although the MCNS MAC is based on packets and the IEEE specifies fixed ATM cells, both cable modem solutions specify a 10Base-T Ethernet connection from the cable modem to the PC.
IEEE 802.14 committee members say they chose ATM because it best provides the quality of service (QoS) guarantees required for integrated delivery of video, voice, and data traffic to cable modem units. The group saw ATM as a long-term solution that would provide the flexibility to deliver more than just Internet access.
MCNS members didn't buy the argument. Cable operators are clearly focused on delivering high-speed Internet services to consumers and believed ATM would add unnecessary complexity and cost to cable modem systems. By supporting a variable-length packet implementation, MCNS members plan to capitalize on the favorable pricing associated with Ethernet and IP networking technology. |
