g product costs and were in an extreme hurry to get into the high-speed data market. To achieve its objectives, MCNS sought to minimize technical complexity and develop a technology solution that was adequate for its members' needs (Van Matre, 1999). Under the MCNS DOCSIS specifications, to enable transparent transfer of Internet Protocol messages across a cable system, three of the protocol layers of the International Organization for Standardizations (ISO) 7 Layer Open System Interconnect (OSI) Reference Model are used. These three layers are the Network Layer, Data Link Layer and the Physical Layer. The functions of each layer are described below (Salent,1999).Network LayerThe Network Layer uses the Internet Protocol (IP), which enables IP traffic to be seamlessly delivered over the cable modem platform to end-users. Data Link LayerThe Data Link Layer is comprised of three sublayers: a Logical Link Control (LLC) Sublayer, which conforms to Ethernet standards, a Link-Security Sublayer that supports the basic needs of privacy, authorization, and authentication, and a Media Access Control Sublayer, suitable for cable system operation, that supports variable-length protocol data units (PDU).Physical LayerThe Physical Layer which defines the upstream and downstream modulation format. There is minimal coupling between physical and higher layers which accommodates the incorporation of future physical layer technologies. 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 specif...
