The Basic Idea
Fax machines have been around in one form or another for more than a century -- Alexander Bain patented the first fax design in 1843. If you look back at some of the early designs, you can get a very good idea of how they work today.

Most of the early designs involved a rotating drum. To send a fax, you would attach the piece of paper to the drum, with the print facing outward. The rest of the machine worked something like this:

• There was a small photo sensor with a lens and a light.
• The photo sensor was attached to an arm and faced the sheet of paper.
• The arm could move downward over the sheet of paper from one end to the other as the sheet rotated on the drum.

In other words, it worked something like a lathe.

The photo sensor was able to focus in and look at a very small spot on the piece of paper -- perhaps an area of 0.01 inches squared (0.25 millimeters squared). That little patch of paper would be either black or white. The drum would rotate so that the photo sensor could examine one line of the sheet of paper and then move down a line. It did this either step-wise or in a very long spiral.

To transmit the information through a phone line, early fax machines used a very simple technique: If the spot of paper that the photo cell was looking at were white, the fax machine would send one tone; if it were black, it would send a different tone (see How Modems Work for details). For example, it might have sent an 800-Hertz tone for white and a 1,300-Hertz tone for black.

At the receiving end, there would be a similar rotating-drum mechanism, and some sort of pen to mark on the paper. When the receiving fax machine heard a 1,300-Hertz tone it would apply the pen to the paper, and when it heard an 800-Hertz tone it would take the pen off the paper.

Modern Fax Machines
A modern fax machine does not have the rotating drums and is a lot faster, but it uses the same basic mechanics to get the job done:

• At the sending end, there is some sort of sensor to read the paper. Usually, a modern fax machine also has a paper-feed mechanism so that it is easy to send multi-page faxes.
• There is some standard way to encode the white and black spots that the fax machine sees on the paper so that they can travel through a phone line.
• At the receiving end, there is a mechanism that marks the paper with black dots.

A typical fax machine that you find in an office is officially known as a CCITT (ITU-T) Group 3 Facsimile machine. The Group 3 designation tells you four things about the fax machine:

• It will be able to communicate with any other Group 3 machine.
• It has a horizontal resolution of 203 pixels per inch (8 pixels/mm).
• It has three different vertical resolutions:
  • Standard: 98 lines per inch (3.85 lines/mm)
  • Fine: 196 lines per inch (7.7 lines/mm)
  • Super fine (not officially a Group 3 standard, but fairly common): 391 lines per inch (15.4 lines/mm)
• It can transmit at a maximum data rate of 14,400 bits per second (bps), and will usually fall back to 12,000 bps, 9,600 bps, 7,200 bps, 4,800 bps or 2,400 bps if there is a lot of noise on the line.

The fax machine typically has a CCD or photo-diode sensing array. It contains 1,728 sensors (203 pixels per inch), so it can scan an entire line of the document at one time. The paper is lit by a small fluorescent tube so that the sensor has a clear view.

The image sensor looks for black or white. Therefore, a single line of the document can be represented in 1,728 bits. In standard mode, there are 1,145 lines to the document. The total document size is:

To reduce the number of bits that have to be transmitted, Group 3 fax machines use three different compression techniques:

• Modified Huffman (MH)
• Modified Read (MR)
• Modified Modified Read (MMR)

See this page for a discussion of these compression types. The basic idea in these schemes is to look for "runs" of same-color bits. For example, if a line on the page is all white, the modem can transmit a dozen or so bits rather than the full 1,728 bits scanned for the line. This sort of compression can cut transmission time by a factor of at least two, and for many documents much more. A document containing a significant amount of white space can transmit in just a few seconds.

Receiving the Fax
The bits for the scanned document travel through the phone line and arrive at a receiving fax machine. The bits are decoded, uncompressed and reassembled into the scanned lines of the original document. There are five common ways to print the fax, depending on the type of machine that receives it:

• Thermal paper - When fax machines started infiltrating offices en mass in the 1980s, most of them used thermal paper. The paper is coated with chemicals that react to heat by turning black. Thermal paper has several big advantages:
  • It is very inexpensive to build a thermal printer.
  • Thermal printers have no moving parts except for the paper-feed mechanism.
  • There are no expendables like ink or ribbons because the paper contains the ink.
  • Thermal printers are nearly indestructible.

  The only disadvantage is that the paper discolors over time, and it turns completely black if you leave it in a hot car.

   

• Thermal film - Thermal film uses a page-width ribbon that contains ink that melts onto paper when heated. This is more complicated mechanically than thermal paper but less complicated than an inkjet.

   

• Inkjet - This technique uses the same mechanism as an inkjet printer.

   

• Laser printer - This technique uses the same mechanism as a laser printer.

   

• Computer printer - The fax is actually received by a fax modem (a modem that understands the Group 3 data standards), stored on the computer's hard disk as a graphics file and then sent to the computer's usual printer.