How Fax Machines Work

Most of the early fax machine 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:

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.

On the next page, we'll talk about 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:

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:

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:

1,728 pixels per line * 1,145 lines = approximately 2,000,000 bits of information

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

See Electronics Plus: Facsimile Theory 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.

On the next page, we'll talk about receiving faxes. 

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:

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.

Find out how to use a fax machine on the next page.

Even though traditional fax machines are fast being replaced by e-mail and Internet fax services, it's still important to know how to use this workplace workhorse. Here are some basic instructions for sending and receiving a fax.

Sending a fax:

Now the fax machine will scan each of the document pages into its memory. After all of the pages have been scanned, you'll hear a series of fax tones. These tones signal the "handshake" between the sending and receiving fax machines, establishing a communications link. Wait for a few minutes as the fax is sent. If the fax machine has a small display screen, look for a confirmation that the fax went through. Some fax machines will also print out a short confirmation report.

Here's how to receive a fax:

On the next page, we'll talk about how to troubleshoot common fax machine problems.

Fax machines are complicated pieces of equipment with delicate sensors, motors and finely calibrated moving parts. As much as you might want to kick your office fax machine when there's a paper jam, there are other solutions. Here are some troubleshooting tips for the most common fax problems: image quality, paper jams and connectivity issues.

The most common image quality problems with faxes are pages that come out too dark, too light or are unreadable due to streaks, splotches and spots. For dark and light pages, it could be as simple as changing the darkness or density setting on the receiving machine [source: FineStar Imaging]. If the receiver does a test print and the page comes out clean, then the problem is most likely with the sender's machine.

The sender should open up the document feed area of the fax machine and clean all the surfaces and moving parts with a slightly damp cloth [source: Sands Office Equipment, Inc.] If the fax machine is one of those all-in-one gadgets that doubles as a scanner and copier, clean off the scanner glass as well with a little glass cleaner.

If the test print comes out with lines and splotches, then the problem is on the receiving end. The most common cause of dirty copies is spilled or leaking toner or ink. Laser printers and fax machines use large toner cartridges containing dry, black, powdery toner [source: Automation Consulting and Supplies]. If you open up the printer door on the fax machine and see a bunch of dry toner lying around, remove the toner cartridge and give it a shake. If toner falls out of the cartridge, you should buy a new one.

Inkjet printers and fax machines use liquid ink, not dry toner [source: Automation Consulting and Supplies]. If you see drips or puddles of wet ink in the printing area, you'll want to replace the ink cartridge and clean the entire area well.

Paper jams are another common fax machine problem. If you have a small fax machine, there are only two areas where paper could get jammed: the document feeder and the printing area. Larger machines have more gears and moving parts where paper can get stuck, but luckily those machines often come with sensors to tell you where the jam is located.

The key to removing jammed paper from a fax machine is to always pull in the direction that the paper naturally travels [source: FineStar Imaging]. Yanking paper out in the opposite direction could damage small wheels and gears or leave scraps of shredded paper in the machine [source: FineStar Imaging].

If the fax machine signals an error, check the manual or call the manufacturer to see what it means. If you get a lot of communications errors, make sure the phone line is plugged in all the way and that you hear a clear dial tone through the phone receiver. If not, you might want to call the telephone company and have them check your line for interference.

On the next page, we'll talk about portable fax machines.

As long as fax remains an essential part of doing business, every office will have a fax machine. But what if an essential part of your business is not being in the office? How are you supposed to send and receive faxes from the road? E-mail and Internet fax technology have been a boon for road warriors, but some people still like to have that piece of paper in their hand. That's where portable fax machines come in.

As of this writing, only one company sells a truly portable, wireless, self-contained fax machine. The Greta GSM Fax and Printer by Possio is essentially a big cell phone that doubles as a fax machine, scanner and copier. Weighing a little over two pounds and measuring 11 inches by 6 inches, the device contains a GSM card that allows it to send and receive cell phone voice and data calls. It also comes with a 50-foot roll of paper for printing out approximately 50 fax pages.

To send a fax, you feed your document page by page into the slot scanner. Then you dial your destination fax number and press send. To receive a fax, the machine simply needs to be powered on and within range of a cellular signal. While GSM is the standard in Europe, coverage in the United States is still catching on and could be spotty.

Portable fax machines like the Greta could also be helpful for their mobile printing, scanning and copying functions. Even if you use Internet fax, you can use the Greta to easily scan paper documents to send as e-mail attachments.

Portable fax machines would be particularly useful for salesmen, real estate agents, lawyers, truck drivers and anyone else who spends a considerable amount of time away from the office.

For more information about fax machines and related topics, check out the links on the next page.