cable modem

Modem used to convert analog data signals to digital form and vise versa, for transmission or receipt over cable television lines, especially for connecting to the Internet. A cable modem modulates and demodulates signals like a telephone modem, but is a much more complex device. Data can be transferred over cable lines much more quickly than over traditional phone lines. Transmission rates are typically around 1.5 megabits per second. Faster transmission is actually possible, but speed is usually restricted by the cable company's (typically slower) connection to the Internet. Cable Internet access is regarded as a replacement for slower dial-up and ISDN services, and is competitive with other broadband modes of delivery (e.g., DSL connections). See also broadband technology.

A modem used to connect a computer to a cable TV service that provides Internet access. Cable modems can dramatically increase the bandwidth between the user's computer and the Internet service provider. Download speeds have reached 6 Mbps and beyond, but the connection is asynchronous. In order to prevent users with lower-cost cable access from hosting high-traffic Web servers, the upload speed is considerably slower, from 10 to 20 times slower. Cable operators also routinely change IP addresses assigned to users to prevent Web hosting (see DDNS).

Connect Via Ethernet
Cable modems connect to the computer via an Ethernet port, which is an always-on connection. Ethernet is a shared medium, and the individual user's speed will vary depending on how many customers are sending or receiving data on that cable segment at the same time. For example, when the kids come home from school around 3pm, many cable users experience a corresponding slowdown. See DOCSIS, cable Internet, Internet appliance and MSN TV.

A Cable Modem System
Internet packets are combined with standard TV programming in a cable modem system. The cable modem termination system (CMTS) is responsible for packet to RF conversion, routing, bridging, filtering and traffic shaping (see CMTS).

(communications, hardware)

cable modem - A type of modem that allows people to access the Internet via their cable television service.

A cable modem can transfer data at 500 kbps or higher, compared with 28.8 kbps for common telephone line modems, but the actual transfer rates may be lower depending on the number of other simultaneous users on the same cable.

Industry pundits often point out that the cable system still does not have the bandwidth or service level in many areas to make this feasible. For example, it has to be capable of two-way communication.

See also: DOCSIS.