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The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Experiments have even been performed in the use of modems over the medium of two cans connected by a string. The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone System (POTS), and once received on the other side, converts those sounds back into 1s and 0s. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". Far more exotic modems are used by internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optic fibers. Most intercontinental data links now use optic modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second.
HistoryModems in the United States were first introduced as a part of the SAGE air-defense system in the 1950s, connecting terminals located at various airbases, radar sites and command-and-control centers to the SAGE director centers scattered around the US and Canada. SAGE ran on dedicated communications lines, but the devices at either end were otherwise similar in concept to today's modems. IBM was the primary contractor for both the computers and the modems used in the SAGE system. A few years later a chance meeting between the CEO of American Airlines and a regional manager of IBM led to a "mini-SAGE" being developed as an automated airline ticketing system. In this case the terminals were located at ticketing offices, tied to a central computer that managed availability and scheduling. The system, known as SABRE, is the ancestor of today's Sabre system.
AT&T monopoly in the United StatesFor many years, AT&T maintained a monopoly in the United States on the use of its phone lines, allowing only AT&T-supplied devices to be attached to their network. For the growing group of computer users, AT&T introduced two digital sub-sets in 1958. One is the wideband device shown in the picture to the right. The other was a low-speed modem which ran at 200 baud. In the summer of 1960 the name Data-Phone was introduced to replace the earlier term "digital subset". The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960. In 1962 the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase shift keying (PSK). The 201A operated half-duplex at 2000 bit/s over normal phone lines, while the 201B provided full-duplex 2400 bit/s service on four-wire leased lines, the send and receive channels running on their own set of two wires each.
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