Bell 202

It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second (bit/s), half-duplex.

In North America, Bell 202 AFSK modulation is used to transmit Caller ID information over POTS lines in the public telephone network.

A modified Bell 202 AFSK modulation, a common physical layer for AX.25, remains the standard for amateur VHF operation in most areas.

The Bell 202 standard was adopted around 1980 as the communications standard for subsea oil and gas production control systems, pioneered by the then FSSL (Ferranti Subsea Systems Ltd.) Controls, a spin-out company from the former TRW - Ferranti joint venture in the UK.

This has been enhanced through the use of Manchester encoding over the FSK link, to provide simple Modulo-2 RZ (return to zero) bit error detection and suppression improvement over these long distances.

The DEC DF112 from the mid-1980s used the Bell 202 protocol.
A single chip implementation of the Bell 202 protocol, c. 2007.