A cable modem is a type of modem that provides access to a data signal sent over the cable television 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 cable television network. They are commonly found in Australia, New Zealand, Canada, Europe, Costa Rica, and the United States. In the USA alone there were 22.5 million cable modem users during the first quarter of 2005, up from 17.4 million in the first quarter of 2004.

Cable modems in the OSI model or TCP/IP model

In network topology, a cable modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The cable modem bridges Ethernet frames between a customer LAN and the coax cable network.

With respect to the OSI model, a cable modem is a data link layer (or layer 2) forwarder, rather than simply a modem.

A cable modem does support functionalities at other layers. In physical layer (or layer 1), the cable modem supports the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. It is to this cable-specific PHY that the name cable modem refers. In the network layer (or layer 3), the cable modem is a IP host in that it has its own IP address used by the network operator to manage and troubleshoot the device. In the transport layer (or layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the application layer (layer 5 or layer 7), the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.

Some cable modem devices may incorporate a router along with the cable modem functionality, to provide the LAN with its own IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit. So, the cable modem function will have its own IP address and MAC address as will the router.

Hybrid Networks

Hybrid Networks developed, demonstrated and patented the first high speed, asymmetrical cable modem systems in 1990. A key Hybrid Networks insight was that highly asymmetrical communications would be sufficient to satisfy consumers connected remotely to an otherwise completely symmetric high speed data communications network. This was important because it was very expensive to provide high speed in the upstream direction, while the CATV systems already had substantial broadband capacity in the downstream direction. Also key was that it saw that the upstream and downstream communications could be on the same or different communications media using different protocols working in each direction to establish a closed loop communications system. The speeds and protocols used in each direction would be very different. The earliest systems used the public switched telephone network (PSTN) for the return path since very few cable systems were bi-directional. Later systems used cable for the upstream as well as the downstream path.