Editing Bell 202

{{Short description|Modem standard developed by Bell System in 1976}}
{{More citations needed|date=February 2022}}
[[File:DEC DF112 modem.jpg|thumb
|The [[Digital Equipment Corporation|DEC]] DF112 from the mid-1980s used the Bell 202 protocol.]]
The '''Bell 202 modem''' was an early (1976) [[modem]] standard developed by the [[Bell System]]. It specifies [[frequency-shift keying|audio frequency-shift keying]] (AFSK) to encode and transfer data at a rate of 1200 [[Bit rate|bits per second]] (bit/s), [[Duplex (telecommunications)#Half-duplex|half-duplex]]. It has separate sets of circuits for 1200 bit/s and 300 bit/s rates.<ref>{{cite book |last1=Helliwell |first1=John |title=On-Line with Smart Modems |date=October 1984 |publisher=PC Magazine |page=121 |url=https://books.google.com/books?id=msSKK7cn2K4C&dq=%22Bell+202+modem%22&pg=PA120 |access-date=July 13, 2021}}</ref>   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&nbsp;Hz tone for ''mark'' (typically a binary ''1'') and 2200&nbsp;Hz for ''space'' (typically a binary ''0'').

In [[North America]], Bell 202 AFSK modulation is used to transmit [[Caller ID]] information over [[Plain old telephone service|POTS lines]] in the [[Public Switched Telephone Network|public telephone network]]. It is also employed in some commercial settings.

[[File:MX614P.png|thumb|right|A single chip implementation of the Bell 202 protocol, c. 2007.]]
In addition, Bell 202 is the basis for the most commonly used physical layer for the [[Highway Addressable Remote Transducer Protocol|HART Communication Protocol]] - a [[List of automation protocols|communication protocol]] widely used in the [[Process manufacturing|process industries]].

Surplus Bell 202 modems were used by [[amateur radio]] operators to construct the first [[packet radio]] stations, despite its low [[data signaling rate|signalling speed]]. A modified Bell 202 AFSK modulation, a common physical layer for [[AX.25]], remains the standard for amateur [[Very high frequency|VHF]] operation in most areas. Notably, [[Automatic Packet Reporting System]] (APRS) transmissions are encoded this way on VHF. On [[High frequency|HF]], APRS uses [[Bell 103 modem|Bell 103]] modulation.

The Bell 202 standard was adopted around 1980 as the communications standard for subsea oil and gas [[production control system]]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&nbsp;km were achieved in subsea applications using these techniques. This has been enhanced through the use of [[Manchester code|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.

==Related technology==
The [[ITU-T V.23]] communications standard defines a similar modulation scheme.

==See also==
* [[Caller ID]]
*[[Highway Addressable Remote Transducer Protocol|HART Communication Protocol (''Highway Addressable Remote Transducer'')]] - a [[List of automation protocols|communication protocol]] used in the [[Process manufacturing|process industries]].

==References==
{{Reflist|33em}}
* [http://www.softelectro.ru/bell202_en.html BELL202 Specification Interface ]

{{Digital modes}}

[[Category:Modems]]
[[Category:Bell System modem standards]]