A modem, short for modulator-demodulator, is a device that lets digital data travel over a communication channel built for analog signals, most famously the ordinary telephone line. The modulator turns a stream of bits into an analog waveform that the phone network can carry, and the demodulator at the far end recovers the original bits from that waveform. Because the public telephone network already reached nearly everywhere, the modem became the bridge that connected computers to each other over distances long before dedicated data networks existed.
The telephone channel was never designed for data, and engineers had to characterize what it could actually carry. A 1960 Bell System Technical Journal paper, “Capabilities of the Telephone Network for Data Transmission” by A. A. Alexander, R. M. Gryb, and D. W. Nast, reported a nationwide field-testing program that measured error performance, net loss, bandwidth, envelope delay, and noise on the switched message network. Work like this established the voice-band channel’s limits and informed the design of the data sets that AT&T would sell.
The Bell 103 data set, introduced by AT&T in the early 1960s, became the canonical early modem. It used frequency-shift keying, in which a “mark” bit and a “space” bit are sent as two slightly different audio tones, and it ran full-duplex by assigning each direction its own pair of tones: the originating end around 1,070 and 1,270 Hz, the answering end around 2,025 and 2,225 Hz. The result was 300 bits per second in each direction over an unconditioned phone line. AT&T’s own Bell System technical references, such as the data set interface specifications catalogued in publications like PUB 40000, documented the electrical and signaling details that equipment makers had to match.
Modems were governed by published standards so that equipment from different makers could interoperate. The Bell 103 and the later 212A set early de facto standards in North America, while the international CCITT (later ITU-T) V-series recommendations governed higher speeds worldwide. Over the following decades modems climbed from 300 bps through 1,200, 2,400, 9,600, 14,400, and 28,800 bps to the roughly 56,000 bps of the late 1990s, using ever more sophisticated modulation to wring more bits out of the same voice channel.
The modem made dial-up networking a mass phenomenon. Bulletin board systems, early online services, and ultimately consumer access to the internet all rode on modems that turned a household phone line into a data link on demand. The characteristic handshake tones of a connecting modem became a familiar sound of the early networked era, and the dial-up modem remained the dominant way ordinary people reached the internet until broadband displaced it.