A modem (modulator-demodulator) is a device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals 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. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
History
News wire services in the 1920s, used multiplex devices that satisfied the definition of a modem. However the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs.
Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used[1]), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.
The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires.
The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.
| Connection | Modulation | Bitrate [kbit/s] | Year released |
|---|---|---|---|
| 110 baud Bell 101 modem | FSK | 0.1 | 1958 |
| 300 baud (Bell 103 or V.21) | FSK | 0.3 | 1962 |
| 1200 modem (1200 baud) (Bell 202) | FSK | 1.2 | |
| 1200 modem (600 baud) (Bell 212A or V.22) | QPSK | 1.2 | 1980[9][10] |
| 2400 modem (600 baud) (V.22bis) | QAM | 2.4 | 1984[9] |
| 2400 modem (1200 baud) (V.26bis) | PSK | 2.4 | |
| 4800 modem (1600 baud) (V.27ter) | PSK | 4.8 | [11] |
| 9600 modem (2400 baud) (V.32) | QAM | 9.6 | 1984[9] |
| 14.4k modem (2400 baud) (V.32bis) | trellis | 14.4 | 1991[9] |
| 28.8k modem (3200 baud) (V.34) | trellis | 28.8 | 1994[9] |
| 33.6k modem (3429 baud) (V.34) | trellis | 33.6 | 1996[12] |
| 56k modem (8000/3429 baud) (V.90) | digital | 56.0/33.6 | 1998[9] |
| 56k modem (8000/8000 baud) (V.92) | digital | 56.0/48.0 | 2000[9] |
| Bonding modem (two 56k modems) (V.92)[13] | 112.0/96.0 | ||
| Hardware compression (variable) (V.90/V.42bis) | 56.0–220.0 | ||
| Hardware compression (variable) (V.92/V.44) | 56.0–320.0 | ||
| Server-side web compression (variable) (Netscape ISP) | 100.0–1,000.0 |
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