If
you are reading this article on your computer at home, chances are that it arrived
via a modem.
The
Origin of Modems
The word modem is a contraction of the words modulator-demodulator.
A modem is typically used to send digital data over a phone line. The sending
modem modulates the data into a signal that is compatible with the phone
line, and the receiving modem demodulates the signal back into digital
data. Wireless modems are also frequently seen converting data into radio signals
and back.
Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below:
In a configuration like this, a dumb terminal at an off-site office or store could "dial in" to a large, central computer. The 1960s were the age of time-shared computers, so a business would often buy computer time from a time-share facility and connect to it via a 300 bit-per-second (BPS) modem.
A dumb terminal is simply a keyboard and a screen. A very common dumb terminal at the time was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user typed a character on the terminal, the modem sent the ASCII code for the character to the computer. The computer would then send the character back to the computer so it would appear on the screen.
When personal computers started appearing in the late 1970s, bulletin board systems became the rage. A person would set up a computer with a modem or two and some BBS software, and other people would dial in to connect to the bulletin board. The users would run terminal emulators on their computers to emulate a dumb terminal.
People got along at 300 BPS for quite awhile. The reason this speed was tolerable was because 300 BPS represents about 30 characters per second, and that is much faster than a person can type characters or read. Once people started transferring large programs and images to and from bulletin board systems, however, 300 BPS became intolerable. Modem speeds went through a series of steps at two year or so intervals:
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