Bell 103

Bell 103 was an early standard for frequency-shift keying (FSK) modulation used in modems to transmit data over telephone lines. Introduced by Bell Labs, it defined the frequencies used to represent binary 0 and binary 1, allowing for asynchronous serial communication. It supported a maximum data rate of 300 bits per second (bps).

Specifically, Bell 103 defined the originate modem as using 1070 Hz for the mark (binary 1) and 1270 Hz for the space (binary 0) when transmitting, and 2025 Hz for the mark and 2225 Hz for the space when receiving. The answer modem used the reverse frequencies: 2025 Hz/2225 Hz for transmitting and 1070 Hz/1270 Hz for receiving. This frequency separation allowed for full-duplex communication, meaning data could be transmitted and received simultaneously.

The Bell 103 standard was widely adopted and became a foundation for early modem technology, enabling the growth of dial-up internet access and other data communication applications before being superseded by faster and more efficient modulation techniques. Its simplicity and widespread adoption made it a crucial element in the development of modern communication networks.