MondialFax

How a fax works

From the scanned page to the printed document at the other end, step by step.

A fax turns a page into a black-and-white image, compresses that image, then transmits it as sound over a phone line. The receiving machine reverses the process and prints the page. The whole exchange is governed by ITU-T T.30, published in 1980 and still in force: it describes how the two devices recognise each other, agree on a speed, transmit each page and acknowledge it.

The five phases of a transmission

A fax call is not a plain data transfer: it is a dialogue, in which every page is agreed upon.

  1. 1

    Call setup

    The sending device dials. The receiver answers with a tone identifying it as a fax — the characteristic whistle you hear if you pick up by mistake.

  2. 2

    Capability negotiation

    The two machines announce what they can do: resolution, page size, compression scheme, maximum speed. They settle on the lowest common denominator, then probe the line to pick a workable rate.

  3. 3

    Page transmission

    The image is sent line by line, compressed. The signal is modulated into audible sound: that is why a fax travels over an ordinary phone line, and only over one.

  4. 4

    Page confirmation

    The receiver states whether the page arrived intact. On error the page can be resent — confirmation is per page, not per document.

  5. 5

    Call release

    Once the last page is confirmed, both devices hang up. It is that final confirmation that makes a reliable acknowledgment of receipt possible.

Resolution and duration

Two resolutions dominate: standard, around 204 × 98 dots per inch, and fine, around 204 × 196. The second doubles vertical definition — so the legibility of small type — and doubles the volume to transmit. MondialFax sends in high resolution.

Speeds range from 9.6 kbit/s on the oldest modems to 33.6 kbit/s on recent ones. A page of text therefore takes ten seconds to a minute. What counts is not the file size but the amount of black on the page: a dense document takes longer than a nearly empty one.

Fax and VoIP

Fax was designed for a continuous analogue circuit, where the signal arrives in order and without interruption. A VoIP line cuts the conversation into packets, which may arrive late, out of order, or not at all. For a human voice, the ear compensates. For a fax modem, every missing packet corrupts the image.

On top of that come treatments designed for voice and harmful to fax: echo cancellation, silence suppression, comfort-noise injection. They degrade a signal the modem expects intact.

The standard answer is the T.38 protocol: the gateway demodulates the fax at the edge of the IP network, carries the page data as packets, then remodulates it on the way out. The analogue signal never crosses the internet. Not every carrier offers it, and it is the first thing to check when a fax stops going through after a line change.

What to take away

  • A fax transmits an image, not text: the recipient receives a photograph of your page, not a reusable file.
  • Confirmation happens page by page, which is what makes the acknowledgment reliable — and why fax survives in legal and medical practice.
  • A fax only reaches a landline number wired to fax equipment: not a mobile, not an email address.

Frequently asked questions

Why does a fax make that screeching noise?
Because the image is transmitted as sound. What you hear is the modulated signal: the calling tone, the negotiation between the two machines, then the page data itself.
Can a fax be sent in colour?
The standard provides a colour mode, but it is barely implemented. In practice a fax is transmitted in black and white: shades are converted into dots, which is why photographs come out halftoned.
Why has my fax stopped working since I changed carrier?
Usually because your line moved to VoIP without T.38 support. The fax signal tolerates neither packet loss nor echo cancellation. That is the first thing to check with your carrier.
Is a fax encrypted?
The T.30 protocol itself provides no encryption: over an analogue line the transmission is in the clear. Online services do encrypt the leg between you and their servers — MondialFax does — but the last segment to the fax machine remains that of the telephone network.

Continue exploring

The most useful pages to carry on with your fax.