How fax works

Feed a page into one machine and it prints out on another. This is the story of the call in between.

Updated on:

A fax does not carry your document. It carries a picture of it. The page is scanned into rows of black and white dots, compressed without losing any of them, and handed to a modem that turns the data into sound on a regular phone line. The machine on the other end listens, decodes the sound and prints the rows in the same order. The ITU-T T.30 recommendation governs the call and T.4 governs the image coding. The pair is known as “Group 3,” the language that almost every fax machine and online fax service still speaks.

Five steps in every fax call

Nothing is sent on faith. The machines settle the terms first, then verify each page as it arrives.

  1. 1

    Ring and answer

    The sending machine dials, then repeats a short 1,100 Hz beep every few seconds (the calling tone, CNG). The machine that answers responds with a long 2,100 Hz tone (the answer tone, CED). Each side now knows it is talking to a machine, not a person.

  2. 2

    Trading specs

    The receiving machine lists what it can handle: resolutions, paper widths, compression schemes and maximum speeds (the DIS frame). The sender chooses from that list (DCS) and sends a brief test burst, the training check. If it comes through clean, the transfer goes ahead. If it is garbled, both machines step down to a lower speed and retry.

  3. 3

    Sending the image

    The page goes out one scan line at a time in compressed form. The modem runs at 9,600, 14,400 or as much as 33,600 bits per second, depending on the line and the agreement. Control messages move slowly, at 300 bits per second, so they get through even on a noisy connection.

  4. 4

    Checking each page

    Once a page ends, the sender signals whether another follows. The receiver replies with a positive confirmation (MCF) or reports a fault. When Error Correction Mode is active, only damaged blocks go out again instead of the whole page.

  5. 5

    Hanging up

    Once the last page is acknowledged, the sender signals the end (DCN) and the call drops. A machine’s activity log, or an online service’s delivery record, can then show that the pages were accepted.

Sharpness and time per page

A standard scan measures 1,728 dots across the page, which is roughly 204 per inch. The setting you actually pick is vertical density: 98 lines per inch for standard, 196 for fine, 391 for superfine. Choose fine for tiny type, stamps and signatures, keeping in mind that it nearly doubles the amount of data.

The modem standard caps the speed. V.29 peaks at 9.6 kbit/s and V.17 at 14.4 kbit/s, while “Super G3” units on V.34 get up to 33.6 kbit/s. A page needs somewhere between a few seconds and almost a minute, driven by what is on it rather than by file size. A mostly blank page compresses to next to nothing, whereas a gray scan or halftone photo does the reverse and drags on.

Why fax and internet phone service don’t get along

A fax modem needs a steady stream of audio that arrives in order. Voice over IP chops sound into packets just a few tens of milliseconds long and reassembles them afterward. In a phone call, a lost packet is a tiny glitch. In a fax, it can shift a line in the image or wreck the handshake.

Phone networks also tune audio for human ears. Codecs like G.729 throw away detail, silence suppression cuts the quiet parts, echo cancellers rewrite the waveform, and jitter buffers shift the timing. A modem reads data from minute changes in the signal, so after all that processing it is simply guessing.

The remedy is T.38, ratified in 1998. Rather than relaying the audio, a gateway at the edge of the IP network decodes the fax, carries the page data and T.30 messages as packets, and regenerates a clean signal at the other end. When a fax machine stops working after a move to internet phone service, find out from your carrier whether T.38 is enabled. If it isn’t, request the uncompressed G.711 codec.

Three takeaways

  • The recipient gets an image of your page. The text can’t be copied or edited, which is part of why signed paperwork still goes by fax.
  • The receiving machine acknowledges every page before the next begins. A “delivered” status reflects a real answer from the far end, not the sender’s assumption.
  • Whatever answers must be a landline attached to a fax machine, a gateway or a fax server. Neither a cell phone nor an email address can join in the T.30 exchange.

Common technical questions

What is that screeching I hear when I dial a fax number?
That is the answer tone followed by the handshake messages, which are data encoded as sound. A modem understands them perfectly. To us it is just a shriek.
Can a fax carry color?
The standard defines a JPEG-based color mode, yet hardly any equipment supports it. In reality each page is flattened to black and white, with grays imitated by patterns of dots, which explains why photos come out grainy after faxing.
Our fax machine quit after we switched to fiber. What should I check?
Chances are the machine is now plugged into the router and runs over voice-over-IP. Ask your carrier if T.38 or the uncompressed G.711 codec is available. Sometimes it also helps to cap the machine’s speed at 9,600 bit/s.
Are fax transmissions encrypted?
No. T.30 includes no encryption, so the signal travels over the phone network in the clear. CaptainFax protects your file with HTTPS while it is sent to our platform, but the last stretch to the destination machine is bound by that protocol limitation, as it is for all fax services.

Keep exploring

A few pages that go well with the one you just read.

Related questions