Bell 202 Modem

The Bell 202 modem was an early modem developed by Bell System. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex (one-way) using differential phase-shift keying (DPSK) modulation. 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 (and perhaps elsewhere), Bell 202 AFSK modulation is used to transmit Caller ID information over POTS lines in 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, despite its low signalling speed. The modified Bell 202 AFSK modulation, 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 Controls, a spin-out company from Ferranti. This modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this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 80km were achieved in subsea applications using these techniques. 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.

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