FaxFair guidesPublished November 1, 2026

How does a fax machine work?

A fax machine works by scanning a page into rows of tiny black and white dots and turning those rows into a compact code. It plays that code down a phone line as sound, and the fax machine at the other end turns the sound back into dots and prints a copy. Before the first page goes, the two machines trade a series of short signals to agree on speed and settings.

The coding, the call and the modem signals are written down by the International Telecommunication Union (ITU). Recommendation T.4 covers how a Group 3 fax machine scans and codes the page, and T.30 covers how the call runs. We read both for this guide, along with the modem recommendations they rely on. Group 3 was recommended in 1980, and the IEEE's record says it "enabled the intercommunication of all facsimiles throughout the world."

One page, start to finish. Steps 1 and 2 happen inside the sending machine, steps 3 and 4 on the phone line, and step 5 inside the machine that receives the fax. The steps follow the page's journey, and in the call itself the handshake in step 3 comes before the page is sent.

Step 1: The scanner reads the page as lines of dots

A Ricoh patent for a compact fax machine, granted in 1986, shows how the parts fit together. A feed mechanism moves the page through a scanning area lit by a lamp. Two lenses focus the lit strip onto a CCD, a light sensor that reads the page while it moves. In the patent's words, the page is "slit-scanned while it is being transported," that is, the sensor sees one thin strip across the page at a time.

T.4 sets how fine that grid is. Each line across the page is cut into 1,728 dots over a standard width of 215 mm, about 8.5 inches, and each dot is either black or white. Lines are spaced 3.85 to the millimeter as standard, about 98 lines an inch. T.4 also allows two finer options, 7.7 and 15.4 lines to the millimeter. The scan runs left to right, and each new line sits just below the one before.

By our arithmetic, a US letter page in standard mode comes to about 1,076 lines, or roughly 1.9 million dots. There is no gray in that grid, so pale pencil and tiny print can drop out. Our cover sheet page has tips on keeping a page readable after faxing.

Step 2: The dots become a short code (T.4)

Sending 1.9 million dots one at a time would be slow. Even at 14,400 bits per second, the top speed of the V.17 fax modem, an uncoded letter page would take over two minutes by our arithmetic. So T.4 sends codes for runs of dots instead of the dots themselves.

Each line becomes a series of code words. In the standard's words, "each code word represents a run length of either all white or all black." Every line starts with a white run, even one of zero dots. Some codes are tiny, and a run of 2 or 3 black dots takes just 2 bits. The table shows how T.4 codes a completely empty line of 1,728 white dots.

How ITU-T T.4 codes one empty line of 1,728 white dots
Part of the line	T.4 code	Bits
White run of 1,728 (make-up code)	`010011011`	9
White run of 0 (terminating code)	`00110101`	8
End of line	`000000000001`	12
Total		29, instead of 1,728

The end-of-line code is eleven zeros and a one. T.4 says this pattern "can never be found within a valid line of data," so a receiving machine can find the next line again, even after a burst of errors.

T.4 also sets a minimum time for each coded line, 20 milliseconds as standard. A short line, like the empty one above, gets padded with zeros called fill to meet it. Two machines can agree on a shorter minimum, down to none at all.

T.4 also has a two-dimensional mode, which codes a line by where its edges sit compared with the line above. At standard resolution it can code at most every other line this way, and the lines in between are coded on their own. The IEEE milestone record says this method, called MR, offered twice the compression of the standard MH coding of the time.

Our history guide covers how Group 3 became the standard that office fax machines still follow.

Step 3: The two machines agree on settings (T.30)

Before a page can go, the machines have to settle on a modem speed, a resolution, a paper width and a coding method, because not every machine can do everything. The settings agreed here decide the resolution and coding of the page. T.30 splits the whole call into five phases, and the settings are agreed in the second.

The five phases of a fax call in ITU-T T.30
Phase	What happens
Phase A, call set-up	The sending machine dials. Its calling tone tells the other end that a fax machine is on the line, and the machine that answers replies with an answer tone.
Phase B, before the page	The answering machine lists what it can do (DIS). The caller picks the settings for this fax (DCS) and sends a 1.5-second test at the chosen speed (TCF). The answer is either "ready" (CFR) or "train again" (FTT).
Phase C, the page	The coded page goes out at the agreed speed.
Phase D, after the page	The caller says another page follows (MPS) or that this was the last (EOP). The receiver confirms the page arrived well (MCF) or says it didn't (RTN).
Phase E, call release	The caller sends DCN, and the call ends.

Those short messages normally travel at just 300 bits per second, using the V.21 modem, inside a standard data wrapper called HDLC. Machines that use the faster V.34 modem can set up the call a different way. Along the way, the answering machine may send its number in a signal called CSI. The calling machine may identify itself in one called TSI.

Every one of these signals makes its own noise on the line. Our guide to the fax machine sound matches each beep, warble and hiss to its signal, in the order you hear them, and explains how a V.34 call starts.

Step 4: The page travels down the phone line as sound

Fax shares its equipment with voice calls, so the page has to travel as sound. That's the modem's job, turning bits into tones the line can carry and back again at the other end. For the page itself, T.30 calls on four ITU modem recommendations, all much faster than the 300 bits per second of the handshake.

Modems that T.30 uses for the fax page, with their speeds
Modem	Speeds for the page, in bits per second
V.27 ter	2,400 and 4,800
V.29	7,200 and 9,600
V.17	7,200, 9,600, 12,000 and 14,400
V.34	Up to 33,600

The caller chooses one that both machines support when it sends its settings. Of the four, V.17 is the one the ITU wrote "only for use in high speed facsimile applications."

Phone lines can be noisy, so Group 3 has an optional error correction mode. With it turned on, the page goes in numbered frames. The receiving machine can list frames in a partial page request (PPR), and the sender sends those frames again.

Step 5: The receiving machine decodes and prints

The machine that answered runs the same steps in reverse. It turns each code word back into a run of white or black dots and rebuilds the line. Then it prints the lines left to right and top to bottom, in the same order they were scanned. The machine in the 1986 Ricoh patent prints with a thermal print head onto a roll of recording paper.

When a page is in, the receiving machine sends its confirmation, and if another page follows, the two machines go straight back to step 4 for it. After the last page, the caller ends the call.

How online fax services fit in

In our reading of the standards, a fax website or app has to speak T.30 too, because the fax machine at the other end does. The service's own equipment takes the place of the sending machine. It turns your PDF or photo into the same lines of dots and holds the same handshake with the fax machine that answers.

Part of the trip can run over the internet. The ITU wrote Recommendation T.38 for calls where "a portion of the transmission path" between two fax machines is an IP network. In the gateway setup T.38 describes, each gateway links the IP network to an ordinary phone call. Once the calls are up on both ends, the standard says, the two fax machines "are virtually linked" and run their handshake with each other as usual. T.38 also covers fax devices that connect to an IP network directly.

If your own fax machine runs on internet phone service, our guide on faxing without a phone line lists the VoIP settings Brother, Canon and Epson describe.

So when the number belongs to a fax machine on an ordinary phone line, the last stretch is still a phone call, and a fax machine has to answer it. A fax sent from FaxFair can fail when the line is busy, no fax machine answers, or the number isn't a fax line.

If yours fails, our guide to why a fax is not going through explains each error message and what to do.

If you don't have a machine, our guide to faxing without a fax machine compares the other ways to send one.

Need to send a fax today without a machine of your own? This website faxes your PDFs or photos to a fax number in the US or Canada, and you don't need an account. For paper pages, the FaxFair iPhone app scans them with the camera.

Send a fax online

Questions

Does a fax machine need the internet to work?

No. A standard Group 3 fax machine needs a phone line, not the internet. T.30 describes fax between two machines over the general switched telephone network, the ITU's name for the ordinary phone network.

Does a fax machine work with VoIP?

It can, but fax was designed for ordinary phone lines. The ITU wrote T.38 so a fax can cross an IP network, the kind internet phone service runs on. T.38 says those paths "should have reasonably low delays" for the best results. If your fax machine is plugged into internet phone service, ask the provider whether it supports fax.

How does an online fax service work?

In our reading of the standards, it does the sending fax machine's job on its own equipment. It turns your file into fax lines, and part of the trip may run over the internet. If the number belongs to a fax machine on an ordinary phone line, the last stretch is a phone call. That machine has to answer for the fax to go through.

How fast does a fax machine send a page?

A fax machine sends a page at 2,400 to 14,400 bits per second on the older modems, or up to 33,600 with V.34. The two machines agree on the speed for each call. How long a page takes depends on what's on it. Because T.4 codes runs of white and black, a mostly blank page makes far less data than a dense one. By our arithmetic, an uncoded letter page at standard resolution would take about 129 seconds at 14,400 bits per second, and coding usually makes it shorter.

Why do faxes look blurry or faint?

Because a standard fax page is a coarse grid of dots that are each pure black or pure white. In standard mode there are about 98 lines to the inch, so gray, color and small print can come out rough or drop out. T.4's optional finer resolutions, 7.7 and 15.4 lines to the millimeter, add more lines when both machines support them.

Sources

The pages we read for this guide.