Fax, under the bonnet

The real journey of a page, second by second, from one machine to another.

Faxing converts each page to a pattern of black-and-white dots, compresses that image, then has a modem carry it as sound. The modem at the other end decodes the audio, and the page is rebuilt. The call follows ITU-T recommendation T.30 — the old CCITT — published in 1980; its companion T.4 covers image coding. Together they define “Group 3” fax, which nearly all fax machines and online services understand today.

Pages in this topic

“Fax in everyday life” covers several pages, each tackling a distinct question. Choose the page that fits your situation.

The call, broken into five phases

Before the first image dot, the two machines negotiate; after each page, they take stock.

  1. 1

    Call and introduction

    The caller dials the number and emits a short 1,100 Hz beep every three seconds (CNG). The machine called picks up and answers with a continuous 2,100 Hz tone (CED): two machines, not two people, are on the line.

  2. 2

    Catalogue of capabilities

    The receiver lists what it can do (DIS frame): accepted resolutions, paper width, compression modes, available speeds. The sender selects from that catalogue (DCS frame) and submits a short test (TCF). Intact test: green light. Degraded: start again at a lower speed.

  3. 3

    Image transmission

    The page goes out line of dots after line of dots, compressed. The modem turns it into sounds, at 9,600, 14,400 or 33,600 bit/s depending on the agreement reached. The command signals, for their part, stay at 300 bit/s: on a mediocre line, they have to remain readable.

  4. 4

    Check after each page

    Each page completed, the sender says whether more remain; the receiver replies “received correctly” (MCF) or reports a problem. With error correction (ECM), only the damaged blocks are sent again, never the whole page.

  5. 5

    End of communication

    The last page acknowledged, the sender emits the disconnect signal (DCN) and the line goes quiet. The device log — or an online service's log — can then note an actual delivery, page by page.

Resolution and speed: what sets the minute

Under T.4, an A4 page is scanned 1,728 dots wide, about 204 dots per inch horizontally. Vertically it depends on the mode: 98 lines per inch in standard, 196 in fine, 391 in superfine. Going from standard to fine clarifies small characters and stamps, roughly doubling the volume of data.

Speed: 9.6 kbit/s for a V.29 modem, 14.4 kbit/s for a V.17, 33.6 kbit/s for “Super G3” using V.34 modulation. Between a few seconds and nearly a minute per page. What decides it is not the weight of the PDF but its content — an airy page assembles into little data; a greyish scan or a halftone image demands far more.

Fax and VoIP: a fragile understanding

A fax modem demands a continuous, ordered signal, without interruption. Voice over IP, for its part, chops sound into short packets of a few tens of milliseconds. In a conversation, a late or lost packet is barely noticeable; in a fax, it tears a line of the image or breaks the handshake.

Voice networks add processing that helps speech and harms fax: echo cancellation, silence detection, compressed codecs such as G.729, badly managed jitter. The modem, which reads the tiniest variations in the signal, loses its bearings.

A standardised answer exists: T.38, adopted by the ITU-T in 1998. The gateway converts the modem's audio into digital data where the telephone network meets the IP side, transports page content and T.30 signalling as packets, and rebuilds a clean fax signal on the receiving side. A machine that fails after a change of line? Ask the operator first about T.38 activation; if that doesn't work, an uncompressed G.711 codec often sorts things out.

Three points to remember

  • The receiver gets an image of your pages, not a file: nothing to select, nothing to edit — one of fax's strengths for official documents.
  • The destination machine acknowledges each page before the next one leaves: an “OK” report therefore reflects a response from the receiver, not a guess by the sender.
  • The destination number has to be a landline linked to one of three things: a fax machine, a fax server, or a gateway. Mobiles and email addresses cannot sustain a T.30 exchange.

What people often ask

Where does that hiss come from when someone picks up during a fax?
It is the 2,100 Hz answer tone, then the handshake frames broadcast as audio. The modem hears data in it; the human ear hears a hiss.
Can colour be sent by fax?
An extension of the standard provides for colour in JPEG, but almost no machine implements it. In practice, every page becomes plain black and white; greys turn into a dot screen, which is why photographs come out grainy.
Since switching to fibre, my faxes fail: what should I check?
Your machine now talks to the broadband box, so to voice over IP. Check with your provider that the line supports T.38 — otherwise ask for an uncompressed G.711 codec. Limiting the machine to 9,600 bit/s also sometimes fixes it.
Is the journey encrypted?
No: T.30 ignores encryption, and the signal crosses the telephone line in clear. Faxeroo protects the PDF's journey to its servers over HTTPS; the last segment, as far as the machine called, remains subject to that limit of the protocol — as with every provider.

Keep reading

Pages chosen to help you prepare a send, track it, or keep proof of it.

See also