Cable Modem (CM)

Also called a Subscriber Unit (SU). Just as its analog predecessor that we have used for years to connect computers through the PSTN to ISPs and the Internet, the cable modem, instead of modulating and analog tones, transmits and receives RF carrier signals over a coaxial cable and converts them back into Internet Protocol (IP) packets, thus enabling voice and data to be delivered over a cable TV (CATV) network.

Despite this superficial similarity with analog modems, however, cable modems are a bit more complicated. They include a tuner to separate the data signal from the TV broadcast stream (integrated with a diplexer to allow for both upstream and downstream signals) as well as components from network adapters, bridges, and routers so you can connect to multiple computers. Network management software runs on cable modems so your friendly local cable company can monitor its operations, and encryption is used to prevent your data from being intercepted by someone else.

At the headend, or central distribution point of the CATV system is where can be found the Cable Modem Termination System (CMTS), the central device for connecting the CATV network to the Internet or some other data network. At the headend, signals are combined with those from satellites and local sources, then rebroadcast through the CATV network.

One CMTS has a downstream frequency to cable modems in the 42 to 850 MHz range depending on plant capabilities, and can normally supply service to about 2,500 simultaneous Cable Modem...