How does a fax machine work?

A fax machine is three machines in one box. A scanner reads your page as dots. A modem turns the dots into sounds that can travel down an ordinary phone line. And a printer at the other end turns the sounds back into dots on paper.

The page you send goes in at the back. Rubber feed rollers pull it past a contact image sensor (CIS): a bar as wide as the page with a strip of LEDs, tiny rod lenses, and a row of light sensors. Each moment, it sees one thin line across the paper. The cameras in CameraClear use the same kind of sensors, just in a grid instead of a line.

The page you receive comes off a roll of thermal paper. A thermal print head with a row of tiny heaters presses it against a rubber platen roller. Wherever a heater warms up, the paper turns black. No ink at all.

The modem and controller board does the thinking, a stepper motor moves the paper in exact small steps, and a small speaker lets you hear the call. The whole machine plugs into the same phone socket as a telephone.

A fax doesn't see letters or pictures. It sees dots. The page slides over the scanner bar, and the bar reads one thin scan line across it: 1,728 spots along 215 mm, about 8 per millimetre, or 204 per inch.

Each spot reflects some light back to its sensor. Paper reflects a lot, ink very little. The machine compares each reading with a threshold: darker than the line means black, lighter means white. There is no grey. One bit per dot.

Then the paper steps forward and the next line is read. In standard mode the steps are 3.85 lines per mm (about 98 per inch), so an A4 page becomes about 1,144 lines. Fine mode takes twice as many, 7.7 per mm (196 per inch), for small print. That is 204 × 98 or 204 × 196 dots per inch, set by the Group 3 fax standard, ITU-T T.4.

Photos are a problem: with no grey, a fax fakes it with a halftone, a pattern of tiny dots, like a newspaper photo.

An A4 page in standard mode is about 2 million dots. Sending every dot at 9,600 bits a second would take over three minutes. But most of a page is white, and neighbouring dots are usually the same colour.

So a fax doesn't send dots. It sends runs: "312 white, 4 black, 20 white…". Each line starts with a white run and then takes turns. This is run-length encoding.

The run lengths are then turned into bits with a code book called Modified Huffman, from the Group 3 standard, T.4. Runs that happen often get short codes: 2 to 7 white dots take just 4 bits, and 2 or 3 black dots take just 2 bits. Rare runs get longer codes. Long runs use a make-up code for each block of 64 dots, plus a short terminating code for the rest.

A completely white line of 1,728 dots becomes a make-up code and a terminating code: 17 bits, plus a 12-bit end-of-line marker. That is why a letter squeezes 10 to 20 times, while a photo made of tiny halftone dots can come out even bigger than the raw dots.

A phone line was built for voices. It carries sound from about 300 to 3,400 Hz and nothing else. So the fax's modem turns bits into sound, and the other modem turns sound back into bits.

Every fax call starts with a handshake, set by the standard T.30. The caller beeps a calling tone at 1,100 Hz that says "I am a fax". The answering machine replies with a long answer tone at 2,100 Hz. Then they swap short messages at a slow, safe 300 bits a second, using two tones (1,650 Hz for 1, 1,850 Hz for 0): that is the warble. "I can do fine mode and 14,400." "OK, let's use that."

Next comes training: 1.5 seconds of test data at full speed. If it arrives clean, the receiver says "go ahead". Then the page itself pours through as a rushing hiss. A fast modem changes the phase and loudness of its tone 2,400 times a second, and each change carries several bits. V.29 carries 4 bits per change for 9,600 bit/s; V.17 carries 6 for 14,400 bit/s. WaveClear explains the waves and tones themselves.

At the other end, the modem turns the sound back into bits, and the machine decodes the run codes back into lines of black and white dots.

A classic fax prints on thermal paper. The thermal print head is a bar with a row of 1,728 tiny heaters, one for every dot across the page. For each line, the heaters under black dots get a pulse of current lasting about a millisecond and warm up. The paper's coating holds a colourless dye and a developer; where they get hot enough, about 100 °C, they melt together and turn black. Then a stepper motor turns the platen roller one line forward.

Thermal paper needs no ink, but it curls, fades in sunlight and can't be written on well. From the 1990s, plain-paper faxes used an inkjet or laser engine instead (see PrinterClear).

Phone lines crackle. A flipped bit turns one run code into another, the colours slip sideways and that line prints as a streak, until the next end-of-line marker puts things right. Error correction mode (ECM) fixes this: the data is sent in numbered frames of 256 bytes, each with a checksum. The receiver lists the bad frames, and the sender sends just those again.

Today most phone calls travel over the internet as packets. That is hard on a fax. If the modem sound is simply chopped into packets (VoIP), one lost packet is a gap in the sound, and a fast modem loses its place. The page fails.

The fix is T.38, agreed in 1998. A gateway near each machine listens to the fax, turns the sound back into data, and sends the data in packets, each carrying copies of the packets before it. A single lost packet no longer matters. Another route, fax to email, delivers the page as an attachment.

Why use fax at all? A fax goes to a phone number, not an inbox. It gives the sender a confirmation report that the other machine took the page, which offices treat as proof. Doctors in the US still fax records, often because of worries about privacy rules for email. The UK's health service was said to be the world's biggest buyer of fax machines in 2017, before it banned new ones. In Japan, a 2021 push to stop faxing in government met hundreds of objections from ministries. Courts and government offices in many countries still accept faxed forms.

Compared with email or a scanned PDF, a fax is black and white at 204 × 196 dpi at best, but it goes to a phone number and gives a delivery report. Email carries exact text and colour, but proves delivery only if the reader agrees; a scanned PDF is a colour picture of the page, usually sharper than a fax.

In India, the late 1980s and 1990s were the age of yellow STD/ISD/PCO booths. For many small businesses without their own line, the neighbourhood booth was where you made trunk calls, and many booths also sent and received faxes: order forms, price lists and bank papers. Mobile phones and email later emptied the booths.