The Bell 202 modem was an early (1976)

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standard developed by the

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. It specifies

audio frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. The technology is used for c ...

(AFSK) to encode and transfer data at a rate of 1200

bits per second In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...

(bit/s),

half-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

. It has separate sets of circuits for 1200 bit/s and 300 bit/s rates. These signalling protocols, also used in third-party modems, are referred to generically as ''Bell 202 modulation'', and any device employing it as ''Bell-202-compatible''. * Bell 202 AFSK uses a 1200 Hz tone for ''mark'' (typically a binary ''1'') and 2200 Hz for ''space'' (typically a binary ''0''). In

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, Bell 202 AFSK modulation is used to transmit

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information over POTS lines in the public telephone network. It is also employed in some commercial settings. MX614P

In addition, Bell 202 is the basis for the most commonly used physical layer for the HART Communication Protocol - a

communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

widely used in the process industries. Surplus Bell 202 modems were used by

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operators to construct the first

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stations, despite its low signalling speed. A modified Bell 202 AFSK modulation, a common physical layer for AX.25, remains the standard for amateur VHF operation in most areas. Notably, Automatic Packet Reporting System (APRS) transmissions are encoded this way on VHF. On HF, APRS uses Bell 103 modulation. 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 modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this technique. The 202 standard permitted useful techniques such as multi-dropping of slave modems to allow multiple nodes to be connected to the host via a single modem channel. Other techniques have included superposition of signal on power conductors, and distances in excess of 80 km were achieved in subsea applications using these techniques. This has been enhanced through the use of

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over the FSK link, to provide simple Modulo-2 RZ (return to zero) bit error detection and suppression improvement over these long distances.

Related technology

The ITU-T V.23 communications standard defines a similar modulation scheme.

References

BELL202 Specification Interface
{{Digital modes Modems Bell System modem standards

Related technology

See also

References