How faxing works

What happens, second by second, while a page travels from one fax machine to another.

Faxing means turning a page into a grid of black and white dots, shrinking that image with compression, then handing it to a modem that converts it into an audio signal. At the far end, a second modem decodes the signal and the machine rebuilds the page. The call itself follows ITU-T (formerly CCITT) Recommendation T.30, first issued in 1980; image coding is handled by its companion, T.4. Together they define "Group 3" fax, the language still spoken by virtually every fax machine and online fax service.

A fax call in five stages

Before a single pixel moves, the two machines hold a conversation. Then, after every page, they check in with each other.

  1. 1

    Dialling and recognition

    The sender dials and emits a short 1,100 Hz beep, repeated every three seconds (CNG). The called machine picks up and answers with a steady 2,100 Hz tone (CED). From that moment, each side knows it is dealing with a machine, not a person.

  2. 2

    Swapping capabilities

    The receiver sends its technical ID card (the DIS frame): supported resolutions, paper width, known compression schemes, possible speeds. The sender picks from that list (the DCS frame), then fires off a short training burst (TCF). If it arrives clean, the receiver gives the go-ahead; if not, they try again at a lower speed.

  3. 3

    Sending the image

    The page goes out row of dots by row of dots, in compressed form. The modem turns it into sound at 9,600, 14,400 or up to 33,600 bit/s, depending on what was agreed. Control signals, by contrast, travel at just 300 bit/s so they stay readable even on a poor line.

  4. 4

    Page check

    At the end of each page, the sender says whether more are coming; the receiver replies "received fine" (MCF) or flags a problem. With Error Correction Mode (ECM), only damaged blocks are resent, not the whole page.

  5. 5

    Hang-up

    Once the final page is accepted, the sender transmits a disconnect signal (DCN) and the call ends. The machine's log, or an online service's, can then record a confirmed delivery, page by page.

Image sharpness and transmission time

T.4 sets the scan width at 1,728 dots for an A4 page, roughly 204 dots per inch across. What changes is the vertical density: 98 lines per inch in standard mode, 196 in fine mode, 391 in superfine. Moving from standard to fine makes small print and rubber stamps crisp, at the cost of roughly twice the data.

On speed, V.29 modems top out at 9.6 kbit/s, V.17 at 14.4 kbit/s, and "Super G3" machines using V.34 modulation reach 33.6 kbit/s. Expect anything from a few seconds to almost a minute per page. File size is not what matters; content is. A page with lots of white space compresses very well, a greyish scan or a dithered photo far less so.

Why VoIP gets in the way of fax

A fax modem expects a continuous signal that arrives in order, with no gaps. Internet telephony chops sound into small packets a few tens of milliseconds long. A late or missing packet goes unnoticed in a conversation; in a fax, it tears a line out of the image or breaks the negotiation.

Voice networks also apply processing that helps speech but hurts fax: compressed codecs such as G.729, silence suppression, echo cancellation, poorly smoothed jitter. The modem reads subtle variations in the signal and simply loses its bearings.

The standard fix is T.38, approved by the ITU-T in 1998. Rather than carrying the modem's audio, the gateway decodes it where the IP network begins, ships the page data and T.30 signals inside packets, then regenerates a clean fax signal at the other end. If a fax machine stops working after a line migration, ask your provider first whether T.38 is enabled; failing that, an uncompressed G.711 codec often helps.

Three things to remember

  • Your recipient gets a picture of your document, not a file: the text cannot be selected or edited, which is part of what makes fax attractive for official papers.
  • Every page is accepted by the receiving device before the next one starts, so an "OK" transmission report rests on an actual reply from the receiver, not on a guess by the sender.
  • The destination has to be a landline connected to a fax machine, fax server or gateway. A mobile phone or an email address cannot take part in the T.30 dialogue.

Your questions about the technology

What is the screeching noise when you pick up a fax call?
That is the 2,100 Hz answer tone, followed by negotiation frames sent as audio. To the modem, those sounds are data; to a human ear, they are just a shriek.
Can a fax machine send colour?
An extension to the standard allows JPEG-coded colour, but very few devices support it. In practice, every page is converted to pure black and white; greys become a pattern of dots, which is why photos look grainy.
My fax has failed since we switched to fibre. What should I check?
Your fax machine now plugs into the router, so it runs over a voice-over-IP service. Ask your provider whether the line supports T.38 or, if not, an uncompressed G.711 codec. Capping the machine at 9,600 bit/s sometimes helps too.
Is the transmission encrypted?
No. T.30 has no encryption mechanism, and the signal travels unprotected over the phone network. FaxTelecom encrypts your PDF over HTTPS on its way to our servers; the final leg, to the recipient's fax machine, is still subject to this protocol limitation, as it is with any service.

Where to go next

Hand-picked pages to help you prepare, send or prove your fax.

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