Short for
facsimile machine, a
device that can send or receive pictures and
text over a telephone line. Fax machines work by
digitizing an image -- dividing it into a grid of
dots. Each dot is either on or off, depending on whether it is black or white. Electronically, each dot is represented by a
bit that has a value of either 0 (off) or 1 (on). In this way, the fax machine translates a picture into a series of zeros and ones (called a
bit map) that can be transmitted like normal
computer data. On the receiving side, a fax machine
reads the incoming data, translates the zeros and ones back into dots, and reprints the picture.
The idea of fax machines has been around since 1842, when Alexander Bain invented a machine capable of receiving signals from a telegraph wire and translating them into images on paper. In 1850, a London inventor named F. C. Blakewell received a patent for a similar machine, which he called a copying telegraph.
But while the idea of fax machines has existed since the 1800s, fax machines did not become popular until the mid 1980s. The spark igniting the fax revolution was the adoption in 1983 of a standard protocol for sending faxes at rates of 9,600 bps. The standard was created by the CCITT standards organization and is known as the Group 3 standard. Now, faxes are commonplace in offices of all sizes. They provide an inexpensive, fast, and reliable method for transmitting correspondence, contracts, résumés, handwritten notes, and illustrations.
A fax machine consists of an optical scanner for digitizing images on paper, a printer for printing incoming fax messages, and a telephone for making the connection. The optical scanner generally does not offer the same quality of resolution as stand-alone scanners. Some printers on fax machines are thermal, which means they require a special kind of paper.
All fax machines conform to the CCITT Group 3 protocol. (There is a new protocol called Group 4, but it requires ISDN lines.) The Group 3 protocol supports two classes of resolution: 203 by 98 dpi and 203 by 196 dpi. The protocol also specifies a data-compression technique and a maximum transmission speed of 9,600 bps.
Some of the features that differentiate one fax machine from another include the following:
As an alternative to stand-alone fax machines, you can also put together a fax system by purchasing separately a fax modem and an optical scanner. You may not even need the optical scanner if the documents you want to send are already in electronic form.
