Modem: Difference between revisions

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[[Image:BellSystemDataphone1957.jpg|thumb|200px|1957 AT&T Dataphone]]Modems were first introduced as a part of the [[Semi Automatic Ground Environment|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 (computer system)|Sabre]], is the distant parent of today's SABRE system. The [[AT&T]] Dataphone was another pioneering early data-transfer service using telephone lines to transmit business information, such as inventory levels.

AT&T introduced two digital sub-sets in 1958. One is the wideband device shown in the picture above. The other was a low-speed modem which ran at 200 baud.

By the early 1960s commercial computer use had bloomed, due in no small part to the developments above, and in 1962 [[AT&T]] released the first commercial modem, the [[Bell 103]]. Using [[frequency-shift keying]], 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 212]] modem, 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 line]]s for 2400 bit/s operation.

In Summer [[1960]] the name Data-Phone was introduced to replace the earlier term "digital subset". The 202 Data-Phone which provided half-duplex asynchronous service 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. The 201A operated half-duplex at 2000 bps over dial lines. The 201B provided full-duplex 2400 bps service on four-wire leased lines.

The 103A was introduced in [[1962]]. It provided full-duplex service at up to 300 baud over dial lines. Frequency shift keying was used with the call originator transmitting at 1070/1270 Hz and the answering modem transmitting at 2025/2225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, [[ASR33]] and the [[IBM 2741]]. When third-party modems were allowed on the US telephone network after the [[Carterfone decision]],[[ acoustic coupler|acoustically coupled ]] 103A compatible modems were introduced by companies such as Anderson Jacobson and many others. AT&T reduced modem costs by introducing the originate only 113D and the answer only 113B/C modems.

In December [[1972]] Vadic introduced the VA3400. This device was remarkable because it provided full duplex operation at 1200 bps over the dial network. It used different frequency bands for transmit and receive. Two bit per baud PSK was used as in the AT&T 201 modems. The VA3400 included an asynchronous interface so that accept data clocked by start and stop bits rather than a modem-supplied clock. This allowed dial connection of 1200 baud of [[Computer terminal | glass TTYs]] to remote computers. Acoustic coupling was possible with the Vadic frequency assignments.

In November [[1976]] AT&T introduced the 212A modem to compete with Vadic. It was similar in design but used the lower frequency for transmit from originating modem. According to Vadic, this made the 212 protocol incompatible with acoustic coupling. The 212A was capable of operating in 103A mode. In [[1977]],

Vadic responded with the VA3467 triple modem. This was an answer-only modem sold to computer center operators. It supported both 1200 bps protocols and 103A modulation.

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. The basic [[Hayes command set]] remains the basis for computer control of most modern modems.