April 1997
How Do Fax Machines Work?
A technical introduction to the modern fax.
T he best way to understand fax is to build a fax machine. If this sounds too difficult, build a model. It's easy, so easy in fact that PBS did it on one of their kids' TV shows. All you have to do is tie two rolling pins together and connect them together with string (remember the old string-and-tin-can-telephone trick).
After you've done this, roll the pins back and forth with paper taped to them and send taps down the line when you want to make dots and timing signals to keep the rolling pins in synch. And presto, you get a faxed copy at the other end. Believe it or not, this is how fax started out, rolling on big drums.
Modern faxes work the same way: one device scans dots and sends them to another device. That device reprints the dots on a "synchronized" page. While real faxes use more sophisticated technology than kitchen utensils and string, they still face the same engineering challenges (timing, signal quality, length of transmission time, dot resolution, sheet size, etc).
Here's the technical low-down on modern fax:
1. Start. You stick a piece of paper in a fax machine, it advances slightly to the scan head. The fax already knows the paper's width. The scanner will translate the paper's printing to digital bits and bytes after you make the call to the other fax machine.
2. Dial. With the kitchen-fax-model, you speak into the string-phone to talk to the person on the other end. In the real world we must deal with switching. When you dial a phone, a number of call progress signals may be generated to indicate how things are going in the dialing/ switching process. Ring-back, busy, fast busy and operator intercept tones, made by signals of analog current, indicate the stages of call progress.
All fax machines, computer-based or otherwise, interpret call progress signals. Call Progress Monitor also takes place during the the call. This looks to see if the call has been terminated and checks if the line still on the hook (called disconnect supervision). Regardless of call mechanics, you have to connect (and stay connected) to the receiving fax machine to proceed.
3. Connect, handshake. When two fax devices connect, they form a live analog phone circuit between them. The fax machines emit the familiar fax "mating call" - a 1100 Hz CNG (calling) tone signal every three seconds from the calling fax, followed by a 2100 Hz CED tone (Calling Station Identification) from the receiving fax machine.
4. Capability exchange. Once two fax machines exchange CNG and CED tones, pre-message procedure gets the answering machine to announce its capabilities first in HDLC data frames Why? The receiving fax machine's abilities limit the fax transaction. No matter how wonderful the sending fax machine is, your machine is limited by the receiver's abilities.
Using the kitchen analogy, the two machines then agree upon the faxing and signaling conventions (i.e. saying "dot" as the signal to mark, which direction to roll the pins, etc).
The two modems signal back and forth, "training" each other and testing the quality of the phone connection. They start at either 9,600bps or 14.4kps and move to lower rates (7200, 4800, 2400 bps) if they can't successfully exchange TCF (Training Check Frame) and CFR (Confirmation to Receive) frames.
This setup and training process takes between 10-15 seconds. Call completion over POTS adds another three to five seconds. You pay for these seconds. And they add up if you do a lot of faxing. ISDN faxes go through faster but take longer to set up (20 seconds).
5. Synch, scan and send data. A fax is made of many black dots (1.87 million to be exact). After fax machines negotiate a data rate, they get in "synch," and send dots (data) from one piece of paper to another.
While transmitting synch data (think of this as error-checking and knowing where end-of-line positions are), the sending fax gathers dot data by running the image past its scanner. Most fax machines have two resolutions for scanning: "standard" 204 (horizontal) x 98 (vertical) dots per inch, and "fine," which doubles the vertical resolution to 192 dpi. Most fax machines have some way of switching between the two resolutions. Standard is usually the default. Resolutions are often rounded (e.g. 200 x 200 or 200 x 100 dpi); ink jet and laser printers do 300 x 300 dpi and 600 x 600 dpi.
Grayscale faxing is done by dithering -- dot patterns, fooling your eyes into seeing gray -- but really still just black dots. Standard dots are vertical rectangles (sometimes referred to as pils). Why vertical rectangles? Because fax is optimized for text, and text fits in vertical rectangles. "Fine" resolution gains extra pils by slowing down the rate of paper advance and redundantly scanning lines. Fine resolution documents take longer to transmit because the data files are larger. This pushes up your phone costs.
Faxes' dot information can be cached in a memory buffer when sending, or stored for later transmission. If you cache a fax, you scan the sheet(s) first, then the fax calls up and attempts to send.
Because Group 3 faxes (the most common type) use digital data (ones and zeroes) to encode the fax. Fax machines can compress the fax data with a variety of math algorithms. Compression removes repetitious information like white space by substituting code words/equations.
There are three types of compression -- Modified Huffman (MH), Modified Read (MR, more compressed) and Modified Modified Read (MMR, the most compressed). These compress based on information the previous line(s), called reference line(s), sending on only data about the changes.
Most fax machines do MH and MR; MMR compatibility varies by machine. Compression is good: if each dot were a bit, each page would be 234 KB (1.87 million dots divided by eight bits per byte). On a 9600bps modem a 234 KB-page takes six minutes. Too long! Fortunately compressed files run 20 KB to 80 KB per page (the more black, the higher the size).
After the answering fax modem receives the encoded compressed data it converts the analog signal to a digital, compressed form, which the answering fax uncompresses and decodes. The binary fax data can then be printed as a hard copy finished fax.
6. More pages? Like the handshake phase, the faxes revert to HDLC packet information exchange after sending a page. If more pages are being sent, a Multi-Page Signal (MPS) is sent. The answering fax confirms with a MCF frame, and step 5 is repeated. When the last page has been sent, the sending fax transmits an End Of Message (EOM) frame, which the receiver confirms.
7. Hang Up. For faxes to release the call, the side that last transmitted sends a DCN (Disconnect) frame, and hangs up.
Related Articles:
