Particularly common models from the 1970s were the Novation CAT and the Anderson-Jacobson, spun off from an in-house project at Stanford Research Institute (now SRI International). This led to a market for 103A-compatible modems that were mechanically connected to the phone, through the handset, known as acoustically coupled modems. Before 1968, AT&T maintained a monopoly on what devices could be electrically connected to its phone lines. The Novation CAT acoustically coupled modemįor many years, the Bell System (AT&T) maintained a monopoly on the use of its phone lines, allowing only Bell-supplied devices to be attached to its network. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, the ASR33, and the IBM 2741. 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. It provided full-duplex service at 300 bit/s over normal phone lines. The famous Bell 103A dataset standard was also introduced by AT&T in 1962.
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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 running on their own set of two wires each. They were synchronous modems using two-bit-per-baud phase-shift keying (PSK). In 1962, the 201A and 201B Data-Phones were introduced. The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960.
In the summer of 1960, the name Data-Phone was introduced to replace the earlier term digital subset. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.
SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. 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), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. In 1943, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bits/second. Modems grew out of the need to connect teleprinters over ordinary phone lines instead of more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs. News wire services in 1920s used multiplex devices that were modems by definition, but the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. By contrast, the original ITU V.22 standard, which was able to transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.Ģ.1 Increasing speeds (V.21, V.22, V.22bis)Ģ.2 Increasing speeds (one-way proprietary standards)Ģ.3 4,800 and 9,600 bit/s (V.27ter, V.32)Ģ.4.2 V.61/V.70 Analog/Digital Simultaneous Voice and DataĢ.5 Using digital lines and PCM (V.90/92) For example, the ITU V.21 standard used audio frequency shift keying with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. Modems can alternatively be classified by their symbol rate, measured in baud. Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps), or bytes per second (B/s). These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data. The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel. Modems can be used over any means of transmitting analog signals, from light emitting diodes to radio. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. A modem (modulator-demodulator) 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.
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