The modem, which is an acronym for modulator/demodulator, was invented in the 1950's for military use. Manufactured by the now popular computer company, IBM, modems were used as part of an air-defense system; their purpose was to connect various airbases and control centers. Modems are devices that mix (modulate) and separate (demodulate) signals, allowing one computer to connect to another. They transfer the data over telephone lines by using analog waves and the modem then converts the waves back and forth. The first modems were designed to hold a telephone's receiver in a cradle and had wire connections that went from the cradles to the computer. Today, most modems are either internal or external hardware devices. Before the computer modem, there was the com-port. When an internal modem card is placed inside of a computer, it behaves as a COM2 or COM3 port. It is also possible connect serial mice into one of these ports (Gilbert, 1996). Asynchronous communication is used in the PC COM port. Each byte of data is a separate unit and the computer that is sending the data can pause between any two bytes of the message. However, the receiver of the message may have to catch the data as quickly as it arrives. This is done by the "a synch" data requiring one extra bit worth of time to announce the new byte's beginning and once extra bit worth of time at the end. This is what is known as the "start" and "stop" bits. This means that a 2400 baud modem could transfer only 240 bytes of data per second. Each byte would require a minimum of 10-bit times. This was once called "start-stop" communication, but asynchronous (a sync, for short) is the name (Gilbert, 1995). The modem does not start and stop the bits. They are actually put out as part of the general data compression. The start and stop bits continue to be generated on the wire that connects a COM port to an external modem. The modem COM port is generally configured to use a higher speed between...
Cited: Chute, George M. and Robert D. Electronics in Industry. New York, Toronto, London:
McGraw-Hill Book Company, Inc, 1971
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