A fax modem is a device that permits documents to be sent over standard, or analog, telephone lines. When a document is scanned using fax machine software in a computer, a series of digital, or numeric, signals are created that represent the document image. Analog telephone lines cannot send these signals directly, but a facsimile (fax) modulator/demodulator (modem) converts the signals and sends them over telephone networks to another machine.
Before high-speed Internet connections and email became common in the late twentieth century, most paper documents were sent through mail delivery. As businesses looked for faster ways to transmit information, manufacturers began developing fax technology. Early fax machines incorporated a scanner that prepared the image for sending, and a telephone headset was placed in a cradle that dialed the receiving machine and transmitted the image.
The cradle was the fax modem that converted the image signals into an analog signal, similar to a voice, which could be sent over the telephone line. A machine connected at the other end would ring like a telephone, and then answer with a tone indicating it was ready to receive a document. Faxes could be printed immediately through a printer, or devices contained storage units, similar to an answering machine, that could store the documents until requested by the user.
Later machines included an internal modem that was built into the machine. A telephone wire connection was required to a wall socket that was connected to the telephone system. By the second half of the 20th century, fax machines were commonplace, and considered a secure way to transmit documents and other images. Device improvements, along with telephone company options like call waiting, allowed documents to be sent at the same time as a telephone call.
As computers became more common for business and residential use in the 1970s, a new technology was needed for sending documents directly from them. The fax modem was created to fulfill this need, because broadband or high-speed Internet was not yet common. Most computer users dialed an Internet service provider (ISP) over a telephone line that gave them a connection for a monthly fee. A fax modem could use this Internet connection, usually only requiring a separate telephone line purchased by the user.
Fax modems could be issued a separate telephone number, similar to stand-alone machines, or shared a line with a telephone. A user could create a document in their computer, access software that linked to the modem, and send the document to another fax machine or modem. Software designers normally tried to make their products operate like telephones, to minimize customer confusion.
Even though broadband Internet connections became more common in the late 20th century, fax technology continued to be valuable for some professions. Into the 21st century, medical offices and insurance companies continued to rely on fax machines and modems for secure transmissions. An email sent from a computer via the Internet was not considered a secure form of communication, but one fax modem could dial another and transmit medical information without using the Internet.