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TheBell 202 modem was an early (1976)modem standard developed by theBell System. It specifiesaudio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200bits per second (bit/s),half-duplex. It has separate sets of circuits for 1200 bit/s and 300 bit/s rates.[1] These signalling protocols, also used in third-party modems, are referred to generically asBell 202 modulation, and any device employing it asBell-202-compatible.
InNorth America, Bell 202 AFSK modulation is used to transmitCaller ID information overPOTS lines in thepublic telephone network. It is also employed in some commercial settings.
In addition, Bell 202 is the basis for the most commonly used physical layer for theHART Communication Protocol - acommunication protocol widely used in theprocess industries.
Surplus Bell 202 modems were used byamateur radio operators to construct the firstpacket radio stations, despite its lowsignalling speed. A modified Bell 202 AFSK modulation, a common physical layer forAX.25, remains the standard for amateurVHF operation in most areas. Notably,Automatic Packet Reporting System (APRS) transmissions are encoded this way on VHF. OnHF, APRS usesBell 103 modulation.
The Bell 202 standard was adopted around 1980 as the communications standard for subsea oil and gasproduction 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 This modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this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 ofManchester encoding over the FSK link, to provide simple Modulo-2 RZ (return to zero) bit error detection and suppression improvement over these long distances.
TheITU-T V.23 communications standard defines a similar modulation scheme.
Amateur radio digital modes | ||
|---|---|---|
| Frequency-shift keying (FSK) |  | |
| Multiple frequency-shift keying (MFSK) | ||
| Phase-shift keying (PSK) | ||
| COFDM |
| |
| Non-traditional digital modes | ||