ication. The proposed 802.14 upstream modulation standard is based on QPSK and 16QAM, virtually the same as MCNS (Van Matre, 1999). The MAC sublayer provides the general requirements for many cable modem subscribers to share a single upstream data channel for transmission to the network. These requirements include collision detection and retransmission, timing and synchronization, bandwidth allocation to cable modems at the control of CMTS, error detection, error handling, and error recovery, as well as procedures for registering new cable modems. For the MAC sublayer, 802.14 specified Asynchronous Transfer Mode (ATM) as its default solution from the head-end 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 (Van Matre, 1999). 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.Initially, cable operators were solely 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. However, QoS guarantees were added under DOCSIS version 1.1 (Van Matre, 1999).. The Link-Security Sublayer insures the privacy of cable modem user data by encrypting link-layer data between cable modems and CMTS. Security is a major concern with cable modems beca...
