Bell 103

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What Does Bell 103 Mean?

The Bell 103, which was the first commercial computer modem, is an AT&T standard for asynchronous 300 bps full-duplex modems that use frequency-shift keying (FSK) modulation on dial-up lines. Developed in 1962, the Bell 103 modem facilitates digital data transmission at a speed of 300 bps over regular phone lines. It is a split-channel modem and is ideal for low-demand users that exchange PC files on a frequent basis.


Bell 103 modulation is used in amateur radio, shortwave radio and commercial applications. Its use of audio frequency and low signaling speed makes it compatible with unreliable narrowband links.

Techopedia Explains Bell 103

The Bell 103 uses audio FSK (AFSK) for data encoding. Each station uses different audio frequency pairs. The originating station uses a mark tone of 1270 Hz with a space tone of 1070 Hz, but the answering station uses a mark tone of 2225 Hz with a space tone of 2025 Hz.

Modems use protocols during data exchange. Bell 103 protocols are as follows:

  • Microcom Networking Protocol (MNP) Levels 1-4: Deployed in the 1980s as an industry standard due to high demand, this protocol enables error-free and asynchronous data transmission.
  • MNP Level 5: This protocol incorporates the first four MNP levels with a data compression algorithm.
  • V.42 and V.42bis: These protocols are internationally recognized for data compression and error control.

Although the Bell 103 modem is rarely used in modern computing, the Bell 103 encoding scheme is still used by devices known as Bell 103 or compatible Bell 103 modems. Additionally, higher speed modems still have Bell 103 emulation capability, providing a fallback method for low-speed data transmission under high-speed but inadequate channel conditions.


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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.