The Bell 103 modem was the first commercial modem for computers, released by AT&T in 1962. It allowed digital data to be transmitted over regular telephone lines at a speed of 300 bits per second.

The Bell 103 modem used audio frequency-shift keying to encode data. Different pairs of audio frequencies were used by each station:

• The originating station used a mark tone of 1,275 Hz and a space tone of 1,070 Hz.
• The answering station used a mark tone of 2,225 Hz and a space tone of 2,025 Hz.

Although original Bell 103 modems are no longer in common use, this encoding scheme is referred to generically as "Bell 103 modulation", and any device employing it as "Bell 103-compatible" or "a Bell 103 modem".

For many years, higher-speed modems retained the ability to emulate the Bell 103, allowing a fallback method for data to be communicated at low speed if channel conditions deteriorated.

Applications

Bell 103 modulation is still in use today, in shortwave radio, amateur radio, and some commercial applications. Its low signalling speed and use of audio frequencies makes it suitable for noisy or unreliable narrowband links.

For example, the CHU shortwave station in Ottawa, Canada transmits a Bell 103-compatible digital time code every minute. Bell 103 modulation is also the standard for amateur packet radio in the HF (shortwave) bands.

Related technology

The ITU-T V.21 communications standard defines a very similar modulation scheme. Commercial 300 baud modems typically support both formats.