Bell 202 modem
Encyclopedia
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) and at a rate of 1800 bits per second full duplex 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.
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 This modulation standard was retained until around 2000, when it was superseded by faster FSK and PSK modulation methods, although it is still utilised for extension of existing control systems that are already configured for this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.
V.23 communications standard defines a similar modulation scheme.
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
developed by Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
. It specifies audio frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
(AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex (one-way) and at a rate of 1800 bits per second full duplex using differential phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
(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
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
(and perhaps elsewhere), Bell 202 AFSK modulation is used to transmit Caller ID
Caller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
information over POTS lines
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
in the public telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. It is also employed in some commercial settings.
Surplus Bell 202 modems were used by amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operators to construct the first packet radio
Packet radio
Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...
stations, despite its low signalling speed
Data signaling rate
In telecommunication, data signaling rate , also known as gross bit rate, is the aggregate rate at which data pass a point in the transmission path of a data transmission system.Notes:#The DSR is usually expressed in bits per second....
. The modified Bell 202 AFSK modulation, AX.25
AX.25
AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks....
, remains the standard for amateur VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
operation in most areas. Notably, Automatic Packet Reporting System (APRS) transmissions are encoded this way on VHF. On HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
, APRS uses Bell 103
Bell 103 modem
The Bell 103 modem or Bell 103 dataset was the second commercial modem for computers, released by AT&T in 1962. It allowed digital data to be transmitted over regular unconditioned telephone lines at a speed of 300 bits per second. It followed the introduction of the 110 baud Bell 101 dataset in...
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.
Related technology
The ITU-TITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
V.23 communications standard defines a similar modulation scheme.