MODEM

A Modem Working

In order to achieve its purpose---to communicate with other computers over a standard, connected telephone line---a modem must be able to interface both with a computer processor's digital data and the audible signals to be sent over the telephone line. To fill this role, modems use a built-in processor, sometimes assisted by the computer's main processor, to "modulate" the digital data into sound that can be sent across a standard telephone line. By modulating the data, the modem converts digital information, stored as a series of ones and zeroes known as binary code, into audible tones; one specific tone represents a "1" and another specific tone represents a "0" data value.

A Wired Modem Is Connected to the Telephone Network

To communicate with another computer or host, wired modems make use of a connected telephone line to access the remote machine over the public switched telephone network (PSTN). Modems use a standard command set---initiated in the 1980s by the Hayes corporation, and thusly known as the "Hayes command set"---to take telephone lines off hook, send the appropriate dual-tone multifrequency (DTMF) sounds needed to dial a telephone number, and connect with a remote machine that may answer the phone.

A Modem Demodulates

When the remote modem answers the incoming call, it sends an audible signal known as a line acceptance tone, or LAT to indicate to the caller that it has reached a computer. The two modems then exchange basic information (in a process known as a "handshake") such as the compression method in use, error-checking abilities, and data transfer speed, then begin transferring information from machine to machine. Once this process is complete, the two modems exchange modulated data. To be useful, however, the modem must demodulate the incoming sounds into binary data which the computer can read. Like the modulation process, this conversion is performed by the modem's on-board processor or, on some machines, is assisted by the computer's main processor.