Cable modem

A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a hybrid fibre-coaxial (HFC) and radio frequency over glass (RFoG) infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high bandwidth of a HFC and RFoG network. They are commonly deployed in Australia, Europe, Asia and the Americas.

Internet Experiment Note (IEN) 96 (1979) describes an early RF cable modem system. From pages 2 and 3 of IEN 96:

The Cable-Bus System

The MITRE/Washington Cablenet system is based on a technology developed at MITRE/Bedford. Similar cable-bus systems are in operation at a number of government sites, e.g. Walter Reed Army Hospital, and the NASA Johnson Space Center, but these are all standalone, local-only networks.

The system uses standard Community Antenna Television (CATV) coaxial cable and microprocessor based Bus Interface Units (BIUs) to connect subscriber computers and terminals to the cable. ... The cable bus consists of two parallel coaxial cables, one inbound and the other outbound. The inbound cable and outbound cable are connected at one end, the headend, and electrically terminated at their other ends. This architecture takes advantage of the well developed unidirectional CATV components. The topology is dendritic (i.e. branched like a tree).
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