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A modem (a portmanteau word constructed from modulator and demodulator) is a device that modulates an analog carrier signal ( sound), to encode digital information, and that 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. Primarily used to communicate via telephone lines, modems can be used over any means of transmitting analog signals, from driven diodes to radio.
Modems 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. IBMThis article is about the International Business Machines Corporation; see IBM (disambiguation) for other uses of this abbreviation. International Business Machines Corporation IBM or colloquially, Big Blue (incorporated June 15, 1911, in operation since 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 AirlinesAmerican Airlines AA is the major airline in the United States. It is headquartered in Fort Worth, Texas, and operates scheduled flights throughout the United States, as well as flights to Latin America, Western Europe, and Japan. Since 1982, AA has been 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 ticketting offices, tied to a central computer that managed availability and scheduling. The system, known as SabreSabre (an acronym for Semi-Automatic Business Research Environment) is a computer reservations system used by airlines, railways, hotels, and other travel companies. The system was created by American Airlines and IBM in the 1950s, after AA president C., is the distant parent of today's SABRE system.
By the early 1960sCenturies: 19th century 20th century 21st century Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s Years: 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 Events and trends The 1960s was a turbulent decade of change around commercial computer use had bloomed, due in no small part to the developments above, and in 1962Events January January 1 Western Samoa becomes independent from New Zealand January 3 Pope John XXIII excommunicates Fidel Castro January 4 New York City introduces a train that operates without a crew on-board January 8 Leonardo da Vinci's Mona Lisa is e AT&TAT&T formerly an abbreviation for American Telephone and Telegraph Corporation is an American telecommunications company, publicly listed on the New York Stock Exchange under the ticker symbol T . AT&T provides voice, video, data, and Internet telecommuni released the first commercial modem, the Bell 103 . Using frequency-shift keyingFrequency-shift keying (FSK is frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the " mark frequency" a, where two tones are used to represent the 1's and 0's of digital data, the 103 had a transmission rate of 300 bit/s. Only a short time later they released the Bell 212The Bell 212 is a light utility helicopter and generally has a crew of two and can carry up to 12 passengers. It used by the U. Government in various branches, in various Civilian applications, and by other countries. Type: BELL 212 (UH-1N) Helicopter Typ, switching to the more reliable phase-shift keying system and increasing the data rate to 1200 bit/s. The similar Bell 201 system used both sets of signals (send and receive) on 4-wire leased lines for 2400 bit/s operation.
The next major advance in modems was the Hayes Smartmodem , introduced in 1981 by Hayes Communications. The Smartmodem was a simple 300 bit/s modem using the Bell 103 signaling standards, but attached to a small controller that let the computer send commands to it to operate the phone line.
Prior to the Smartmodem, modems almost universally required a two-step process to activate a connection: first, manually dial the remote number on a standard phone handset, then plug the handset into a modem-attached acoustic coupler, a device with two rubber cups for the handset that converted between the audio signals and the electrical modem signals. With the Smartmodem, the acoustic coupler was eliminated by plugging the modem directly into a modular phone set or wall jack, and the computer was "smart" enough to bypass the phone and dial the number directly. These changes greatly simplified installation and operation of bulletin board systems (BBS).
Modems stayed at about these rates into the 1980s. A 2400 bit/s system very similar to the Bell 212 signalling was introduced in the US, and a slightly different, and incompatible, one in Europe. By the late 1980s most modems could support all of these standards, and 2400 bit/s was becoming common. A huge number of other standards were also introduced for special-purpose situations, commonly using a high-speed channel for sending, and a lower-speed channel for receiving. One typical example was used in the French Minitel system, where the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1200 bit/s for reception, and 75 bit/s for sending commands back to the servers.
These sorts of solutions were useful in a number of situations where one side would be sending more data than the other. In addition to a number of "medium-speed" standards like Minitel, four US companies became famous for high-speed versions of the same concept. Microcom Systems introduced their MNP , Hayes their Ping Pong , USR had their HST protocol, and Telebit used software to increase performance. In all of these cases the high-speed line was set to 9600 bit/s, and the low-speed line to between 75 and 300 bit/s. Each company carved out a niche in the market, Telebit was huge in the universities due to their direct support of UUCP prototols in the modem itself, Microcom became common in commercial settings, and USR was huge among BBS operators (as they could download Fidonet messages more quickly), but the Hayes standard never caught on. In all of these cases there was a well defined high-speed and low-speed direction, but such a split was not so obvious for users who were uploading and downloading files in the same session, and these solutions were rarely used by them.
Operations at these speeds pushed the limits of the phone lines, and would have been generally very error-prone. This led to the introduction of error correction systems built into the modems, made most famous with Microcom's MNP systems. A string of MNP standards came out in the 1980s, each slowing the effective data rate by a smaller amount each time, from about 25% in MNP1, to 5% in MNP4. MNP5 took this a step further, adding compression to the system, thereby actually increasing the data rate - in general use the user could expect an MNP modem to transfer at about 1.3 times the normal data rate of the modem. MNP was later "opened" and became popular on a series of 2400 bit/s modems, although it was never widespread.
Another common feature of these high-speed modems was the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus two USR modems would be able to connect at 9600 bit/s, but when another user with a 2400 bit/s modem called in, the USR would "fall back" to the common 2400 bit/s speed. Without such a system the operator would be forced to have multiple phone lines for high and low speed use.