The First Modem
Modem (from MOdulator-DEModulator) is a device that modulates an analog carrier signal to encode digital data, and also demodulates such a carrier signal to decode the transferred data. The goal is to create a signal that can be broadcast easily and decoded to reproduce the original digital data. Modems can be utilized across any means of transmitting analog signals, from driven diodes to radio.
The most common example is a voiceband modem that changes the digital 1s and 0s of a PC into sounds that can be transferred over the telephone lines of Plain Old Telephone Systems (POTS), and once picked up on the other side, changes those 1s and 0s back into a pattern used by a USB, Ethernet, serial, or network connection. Modems are typically classified by the quantity of information they can transmit in a given time, usually measured in bits per second, or "bps". They can also be classed by Baud, the number of times the modem varies its signal state per second. Baud isn't the modem's speed in bits, but in symbols. The baud rate changes, depending on the modulation formula used.
Modems matured out of teletype machines, which in turn arose from automated telegraphs. News wire services of the 1920s employed multiplex equipment that met the definition, but the modem purpose was incidental to the multiplexing function, so they are not generally included in the history of modems. George Stibitz connected a New Hampshire teletype to a computer in New York City by phone lines in 1940. Modems in the United States were part of the SAGE air-defense system in the 1950s, linking terminals at several airbases, radar sites, and command-and-control centers to the SAGE director centers dispersed around the U.S. and Canada. SAGE operated on devoted communications lines, but the devices at each end were otherwise related in concept to today's modems.
In 1962, the first commercial modem was fabricated - the Bell 103 by AT&T. The Bell 103 was likewise the first modem with full-duplex transmission, and featured data rates up to 300 bits per second. Soon after the Bell 103, there came the Bell 212, which achieved speeds of 1200 bits per second. It also used a process of modulation called phase-shift keying (PSK). This was a step ahead from the frequency-shift keying (FSK) method that the Bell 103 utilized.
Over the next fifteen years, the campaigns were to make the modems transfer data at higher rates. In order to achieve this, the telephone system needed some improvement. As it was, due to mutual disturbance of signals being weakened at assorted rates through the system, there was blurring of data symbols. To correct for this, equalizers needed to be applied to the telephone lines. The automatic adaptive equalizer was invented in 1965 at Bell Laboratories by Robert Lucky. While equalizers had been utilized for awhile, they called for human intervention to be aligned appropriately. With the advent of the automatic adaptive equalizer, data could be transferred at high rates, as was craved. Modem technology also bettered in this time, and by 1980, there were modems that could transfer up to 14.4 kilobits per second over four-wire engaged lines.
As years passed, dial-up modems' technology improved and the data rates were multiplied. In 1994 data rates doubled to 28.8 Kbps. In 1996 a new technology came out, enabling data rate of 56 Kbps. Again, new technologies emerged in the late 1990s, and improved significantly the modems accessible speed. Today's V.42, V.42bis and V.44 standards permit the modem to transfer information quicker than its standard rate would imply. For example, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 == 320 kbit/s using pure text. However, the compression ratio tends to change due to noise on the line, or due to the transfer of already-compressed files.
Many technologies, such as ADSL that processes over telephone lines, and broadband cable modems that work on the TV cables, are called "broadband modems". These newer modems enable connection speeds through modern Internet Providers that is at least 10 to 200 times faster than the old dial-up connections.