modem

A modem, short for modulator-demodulator, is a device that enables digital data to be transmitted over analogue or other physical media. At the sending end it modulates digital signals into analogue signals suitable for the transmission medium; at the receiving end it demodulates the analogue signal back into digital data. Modems operate at the physical layer of a network and are often used at the edge of a local network to connect to an Internet service provider.

Historically, home Internet began with dial-up modems over the public switched telephone network. Early modems transmitted using audio-frequency tones with speeds from a few hundred bits per second up to several kilobits per second. The 1990s and 2000s saw standardized families such as V.34, V.90, and V.92, which increased speeds and improved connection continuity. Later, broadband technologies replaced dial-up in many regions.

Modern variants include broadband modems for DSL, cable, fiber, and wireless networks. A DSL modem connects via telephone lines (ADSL/VDSL). A cable modem connects via coaxial cable following DOCSIS standards. Fiber deployments use an optical-network terminal (ONT) that converts light signals to Ethernet. Wireless modems (or cellular data modems) connect to mobile networks. In homes and offices the modem is frequently combined with a router into a single device called a gateway.

Operation and standards: modulation schemes such as QAM, PSK and OFDM enable higher data rates; error correction and multiplexing improve reliability. The term modem is sometimes used loosely for devices that include both modem and routing functions. While dial-up modems are largely obsolete for Internet access, modems remain essential at the edge of many networks, with modern speeds measured in hundreds of megabits to gigabits per second.