How Does a Telephony Modem Work?

A Modem Modulates Data

For use in telephony, a modem must modulate data into sound. As the computer to which the modem is connected prepares to connect and transmit information, it sends information to the modem to be modulated. When the modem receives this data, it works in conjunction with the processor on the computer to turn the data into sound suitable for transmission over a voice-capable telephone line.

With the data successfully converted into audible sounds, the telephony modem is ready to make a connection. In manually controlled modems, commands standardized by the Hayes corporation (known as "Hayes Commands") can be issued to take the get the modem's attention (AT), turn the modem speaker on (M1) and set its volume to level three (L3), set the dial speed (S11), dial in tones (DT), and automatically wait for an answer. At the other end, a command can be issued to answer (ATA) and a connection is automatically established (while these commands can be manually issued at any time, telephony modems in frequent use generally use software for automated operation). When the called modem answers, it sounds a tone known as a "Line Acceptance Tone" (LAT) that indicates the modem transfer capabilities to the calling modem; the calling modem begins negotiating the connection, establishing information such as the maximum transfer speed and error check protocols during the connection process known as a modem "handshake."

One critical piece of information established during the handshake process is the data integrity verification known as error correction. A variety of error correction protocols is available, and each works differently, but all protocols verify the integrity of the received data based on a programmed algorithm and, if the received data fails the error check, requests the particular data be resent.

As the called modem receives incoming data that has passed error correction processes, it demodulates the sounds into binary data useful to the computer. Depending on the capabilities of the modem, it may also compensate for line noise and other variables as it demodulates data, and it may rely on either its own performance or work in conjunction with the computer's processor to achieve optimum performance.