Bell 202 modem
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The Bell 202 modem was an early modem developed by AT&T. It used audio frequency-shift keying to encode and transfer data at a rate of 1,200 bits per second, half-duplex (one-way). Other third party modems that support the same signalling and are referred to generically as "Bell 202 modulation", and any device employing it as "Bell 202-compatible" or "a Bell 202 modem".
- Bell 202 uses 1,200 Hz for the mark (typically a binary '1') tone and 2,200 Hz for the space (typically a binary '0') tone.
In North America (and perhaps elsewhere), Bell 202 modulation is used to transmit Caller ID information on the public telephone network. It is also employed in some commercial settings.
Surplus Bell 202 modems were used by amateur radio operators to construct the first packet radio stations, and despite its low signalling speed, Bell 202 modulation remains the standard for amateur VHF operation in most areas. Notably, Automatic Position Reporting System transmissions are most often encoded this way.
Related technology
The ITU-T V.23 communications standard defines a similar modulation scheme.