What is fax?

The main reason for the success of fax is the standardization of fax machine communication by the ITU-T. The ITU-T have published standards for four types of fax machine. These are known as Groups 1, 2, 3, and 4. Groups 1 and 2 are now considered to be obsolete. Almost all fax machines in use today are ITU-T standard Group 3. Group 4 is intended for all-digital transmission and, although Group 4 machines are available, their usage is limited to ISDN networks.

The Brooktrout fax card uses Group 3 standard to send image data (a picture of the page being sent) using:

• Sampling of the image into a matrix of dots (pixels), each pixel being either on or off. The standard resolution for faxes is about 204 dots per inch horizontally and about 98 dots per inch vertically, giving about 20,000 dots per square inch.
• Compression of the zeros and ones representing the image by a factor of around 15 using a run-length technique This means that rather than sending each dot separately, a code is sent representing the number of zeros and ones that occur together.
• The resulting data (a stream of zeros and ones representing the compressed image) is sent down the telephone wire using a technique identical to that which is used by modems to send computer data.

Before and after sending the image data itself, the two fax machines communicate with each other in order to exchange information such as the resolution of the image and their respective Station IDs. They also go through a training sequence in order to determine what speed of data transfer can be supported on a particular telephone line.