How a fax works

A page, a phone call and a conversation between two machines, step by step.

A fax machine reads a page as rows of black or white dots, packs those rows into a compact code and lets a modem play them down a phone line as sound. The machine at the other end listens, decodes and prints the rows back into a page. The rules for the call come from ITU-T Recommendation T.30, the image coding from T.4; together they make up Group 3 fax, still understood by practically every device you can dial. Faxturbo sits at the sending end: you give it a file in the browser, and a gateway turns it into that call.

The call in five steps

Nothing is printed until the two machines have agreed on how to talk, and they check in again after each page.

  1. 1

    Calling and answering

    The caller emits a brief 1,100 Hz beep at regular intervals (CNG). The called device picks up and replies with a continuous 2,100 Hz tone (CED). Both ends now know a machine is on the other side.

  2. 2

    Comparing abilities

    The answering machine announces what it can do in a DIS frame: resolutions, paper width, compression methods, modem speeds. The caller chooses among them with a DCS frame and sends a short test sequence (TCF). A clean result gets approval; a poor one makes both sides drop to a slower speed.

  3. 3

    Transmitting the page

    Rows of dots follow one another in compressed form, modulated at 9,600, 14,400 or up to 33,600 bit/s depending on the deal struck earlier. Control messages use a much slower 300 bit/s so they survive even on a noisy line.

  4. 4

    Confirmation after each page

    When a page ends, the caller states whether another follows and the receiver answers with a confirmation (MCF) or a complaint. With Error Correction Mode (ECM), only the blocks that arrived damaged are sent again.

  5. 5

    Ending the call

    After the last accepted page the caller sends a disconnect signal (DCN) and hangs up. This closing exchange is what lets a service report that a document was really taken in, page after page.

Detail and duration

On an A4 sheet T.4 specifies 1,728 dots per row, about 204 dots per inch horizontally. Only the vertical resolution varies: 98 lines per inch in standard mode, 196 in fine, 391 in superfine. Fine mode renders small type and stamps legibly but needs about twice the data.

Modems define the pace: V.29 reaches 9.6 kbit/s, V.17 14.4 kbit/s, and V.34, used by Super G3 devices, 33.6 kbit/s. A page may take a few seconds or close to a minute. What counts is the content, not the file size in megabytes: a mostly white page compresses superbly, while a grey scan or a photographic halftone compresses poorly.

Why internet telephony disturbs fax

A fax modem expects an unbroken stream of sound that arrives in the right order. Voice over IP cuts audio into tiny packets, each lasting a few tens of milliseconds. In a chat nobody notices a lost packet; in a fax it removes lines from the picture or derails the handshake.

Telephone networks also tune audio for human ears: compressing codecs like G.729, suppression of silence, echo cancellation, jitter buffers. Each of these alters the fine detail that a modem depends on to stay in sync.

T.38, approved by the ITU-T in 1998, is the remedy. A gateway decodes the modem sound at the edge of the IP network, carries only the page data and T.30 signalling in packets, and rebuilds a clean fax signal at the far edge. Faxturbo hands each transmission to an IP-to-fax gateway of this kind, which then dials the recipient's landline over the telephone network. If your own machine fails after moving to a box or fibre line, ask your operator about T.38 first, then about an uncompressed G.711 codec.

Three points to retain

  • The recipient receives an image of the page rather than a document: text cannot be copied or edited. Many offices value that for formal papers.
  • The receiving device approves every page before the next one goes out, so a successful report reflects an answer from the other machine, not an assumption on the sender's side.
  • A fax needs a landline number behind a fax machine, a fax server or a gateway. Mobile phones and email addresses cannot join the T.30 exchange, which is why Faxturbo dials landlines only.

Technical questions

What is that shrill sound on a fax call?
It is the 2,100 Hz answer tone followed by the machines' signalling, which is sent as audio. For a modem those whistles are data; for your ear they are noise.
Does fax support colour?
A T.30 extension allows JPEG colour, yet hardly any equipment implements it. In everyday use every page is reduced to black and white, and greys are rendered as dot patterns, which explains why photos look coarse.
Our fax stopped working after the move to fibre. Where do I look?
The machine now connects through the router and uses a voice-over-IP service. Ask the operator whether T.38 is supported, or else an uncompressed G.711 codec. Limiting the device to 9,600 bit/s can also help.
Is a fax encrypted in transit?
Not over the phone network: T.30 has no encryption and the signal crosses the network as is. Faxturbo protects your upload to our servers with HTTPS, but the last stretch to the recipient's machine has this limit with every provider.

Keep exploring

More pages to help you prepare, send or track a fax.