modembin.timeline

Radioteletype brought typed communication over radio waves, replacing Morse code operators with mechanical teleprinters. Messages were encoded using the 5-bit Baudot code, a character set designed for machines, not people, transmitted as frequency-shift keyed audio tones.

AT&T's long-distance trunk lines used a single 2600 Hz tone for in-band signaling. A sustained tone told the switch the line was idle. Phreakers discovered that playing 2600 Hz into a live call seized the trunk, giving them operator-level control of the network.

Slow-scan television transmits still images over audio channels. Originally designed for ham radio, later used by NASA to send images from the moon. Each image is encoded as a swept audio tone, pixel by pixel, taking 8 to 114 seconds depending on the mode.

The Bell 103 was the first commercially available modem, transmitting data at 300 baud over ordinary phone lines using frequency-shift keying. Its distinctive warbling handshake became the sound of early networked computing, from ARPANET terminals to bulletin board systems.

Dual-tone multi-frequency replaced pulse dialing with a 4×4 matrix of audio frequencies. Each key press sends two simultaneous tones, one from a low group, one from a high group, making it fast, reliable, and nearly impossible to fake with a single whistle.

Before DTMF existed for consumers, the phone network used multi-frequency tones internally to route calls between switches. Blue boxes replicated these MF tones, letting phreakers dial anywhere in the world for free. Woz and Jobs built and sold them before founding Apple.

The Bell 202 quadrupled throughput to 1200 baud by trading full-duplex for half-duplex operation. It found its way into point-of-sale terminals, alarm systems, and metering equipment, and decades later became the physical layer for Caller ID delivery.

Caller ID sends the calling party's name and number between the first and second ring, encoded as Bell 202 FSK data. The phone decodes a structured message containing the date, time, phone number, and up to 15 characters of caller name.