The Bell 202 modem was an early (1976)

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

standard developed by the Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second,

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 bps and 300 bps 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 North America, Bell 202 AFSK modulation is used to transmit

Caller ID Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call i ...

information over POTS lines in the

public telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...

. It is also employed in some commercial settings. 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 kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

widely used in the

process industries Process manufacturing is a branch of manufacturing that is associated with formulas and manufacturing recipes,

. Surplus Bell 202 modems were used by amateur radio operators to construct the first packet radio stations, despite its low signalling speed. A modified Bell 202 AFSK modulation, a common physical layer for

AX.25 AX.25 (Amateur X.25) is a data link layer protocol originally derived from layer 2 of the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks. AX.25 v2.0 and later occupies t ...

, remains the standard for amateur VHF operation in most areas. Notably,

Automatic Packet Reporting System Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. Data can include object Global Positioning System (GPS) coordinates, weather st ...

(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 system Within supply chain management and manufacturing, production control is the activity of monitoring and controlling any particular production or operation. Production control is often run from a specific control room or operations room. With invent ...

s, 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

Manchester encoding In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no D ...

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 The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...

V.23 ''V.'' is the debut novel of Thomas Pynchon, published in 1963. It describes the exploits of a discharged United States Navy, U.S. Navy sailor named Benny Profane, his reconnection in New York City, New York with a group of pseudo-bohemianism, b ...

communications standard defines a similar modulation scheme.

References

{{Reflist, 33em
BELL202 Specification Interface
Modems

Related technology

See also

References