The head-end portion of a CATV network designed to support high speed data, as described in the Data over Cable Service Interface Specification (DOCSIS). Matching DOCSIS cable modems (CMs) in the CMTS and the customer premises support high speed, full duplex (FDX) data communications over a hybrid fiber/coax (HFC) system. The CMTS supports a packet data connection to an IEEE 802.3 10/100-Mbps Ethernet port on a router. In terms of the OSI Reference Model, the system runs the Internet Protocol (IP) at the Network Layer in Ethernet frames at the Data Link Layer. Associated with the CMTS are various servers for security, address translation, data caching, and video caching.A CMTS can support as many as 2,000 cable modem users on a single 6-MHz channel (8 MHz in Europe), with issues of congestion for shared bandwidth becoming more severe as the number of active users increases. The modem on the customer premises is in the form of a set-top box, which supports traditional coax connections to multiple TV sets and a 10/100BaseT Ethernet connection to a PC or to a hub serving multiple PCs. See also 10BaseT, 100BaseT, 802.3, bandwidth, caching, coaxial cable, Data Link Layer, DOCSIS, Ethernet, HFC, IP, Network Layer, optical fiber, OSI Reference Model, server, and set-top box.