How fax works

A page goes in one machine and comes out of another. Here is what happens in between.

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A fax machine does not transmit a document; it transmits a picture of it. The page is scanned into rows of black or white dots, shrunk with a lossless compression scheme, and handed to a modem that converts the bits into audible tones on an ordinary phone line. The far machine listens, decodes the tones and prints the rows back in sequence. ITU-T T.30 defines the rules of the exchange, and T.4 covers the image coding. Together they make up “Group 3”, the dialect that almost every fax device and online fax service still speaks.

The five acts of a fax call

Nothing is sent blindly. The two machines agree on terms first, then confirm each page as it lands.

  1. 1

    Call and answer

    The calling machine dials and plays a brief 1,100 Hz beep every few seconds (the calling tone, CNG). The machine that picks up replies with a long 2,100 Hz note (the answer tone, CED). Both sides now know there is a machine on the line, not a person.

  2. 2

    Comparing capabilities

    The receiver lists what it can do: resolutions, paper widths, compression methods, top speeds (the DIS frame). The sender chooses from that menu (DCS) and sends a short test burst, the training check. If the test arrives intact, transmission is cleared; if not, both fall back to a slower speed and retry.

  3. 3

    The picture itself

    The page moves line by line in compressed form. According to line quality and what was negotiated, the modem works at 9,600 bps, 14,400 bps or as high as 33,600 bps. Control messages, by contrast, go out slowly at 300 bits per second so that they survive even a noisy link.

  4. 4

    Confirming each page

    After every page the sender announces whether another follows. The receiver answers with a positive confirmation (MCF) or reports trouble. Where Error Correction Mode is active, only the damaged blocks are sent again instead of the whole page.

  5. 5

    Disconnect

    When the last page has been acknowledged, the sender signals the end (DCN) and both sides hang up. At that point a machine journal, or an online service’s delivery record, can state that the pages were accepted.

Detail, speed and time per page

A standard A4 scan is 1,728 dots across, roughly 204 per inch. What you actually choose is the vertical resolution: 98 lines per inch is standard, 196 is fine and 391 is superfine. Fine mode suits small type, stamps and signatures, but it about doubles the data to transmit.

Modem generations set the ceiling. V.29 tops out at 9.6 kbit/s and V.17 at 14.4 kbit/s, while “Super G3” devices on V.34 can reach 33.6 kbit/s. A page takes anywhere from a few seconds to nearly a minute, depending on what it contains rather than the file size. A mostly white page compresses to almost nothing; a grey scan or halftone photo is the reverse and takes far longer.

Why internet telephony struggles with fax

A fax modem relies on a smooth, ordered stream of audio. Voice over IP slices that audio into packets a few tens of milliseconds long and reassembles them at the other end. In a conversation, a dropped packet is a tiny click. In a fax it can shift a row of the picture or derail the handshake.

Phone networks also tune the audio for human ears. Codecs such as G.729 discard detail, silence suppression cuts quiet stretches, echo cancellers rewrite the signal and jitter buffers shuffle timing. A modem decodes information from very small signal changes, so all of this leaves it guessing.

T.38, approved in 1998, is the fix. Rather than carrying the sound, a gateway decodes the fax where the IP network begins, transports the page data and T.30 messages as packets, and rebuilds a clean signal at the far end. If a machine starts failing after a move to an internet line, ask the provider whether T.38 is enabled. If not, ask for the uncompressed G.711 codec as a fallback.

Worth keeping in mind

  • What arrives is an image of your page. Nobody can copy text out of it or edit it, one reason offices still favour fax for signed paperwork.
  • The receiving machine acknowledges every page before the next one goes. A “delivered” status therefore reflects a reply from the far end, not an assumption on the sender’s side.
  • The destination must be a landline wired to a fax machine, a gateway or a fax server. A mobile handset or an e-mail inbox cannot take part in the T.30 conversation.

Technology questions, answered

What is that shrill sound when I call a fax line?
You are hearing the answer tone and then the handshake messages, which are data encoded as audio. A modem reads them fluently. A human ear hears a squeal.
Do faxes support colour?
The specification includes a JPEG-based colour mode, but almost no device implements it. In practice, each page becomes black and white and greys are imitated with dot patterns. That is why photographs look coarse once faxed.
Our fax stopped working after we moved to fibre. Where do I start?
The machine most likely now connects through the router and rides on a voice-over-IP line. Ask your provider whether T.38 is supported, or whether you can use the uncompressed G.711 codec. Capping the machine’s speed at 9,600 bit/s helps in some cases.
Is a fax transmission encrypted?
No. T.30 defines no encryption, and the signal crosses the phone network in the clear. CaptainFax protects your file with HTTPS up to our platform, but the last stretch to the destination machine carries that protocol limit, as it does for every fax service.

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