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 calling party dials and then sends a brief 1,100 Hz signal every three seconds (CNG). The answering device answers and transmits an unbroken 2,100 Hz tone (CED): it is two pieces of equipment talking, not two people.

  2. 2

    Catalogue of capabilities

    The receiver declares its capabilities (DIS frame): supported resolutions, paper widths, compression methods, and transmission speeds. The sender then picks from those options (DCS frame) and sends a brief training burst (TCF). If the outcome is clean: move forward. If quality is weak: drop to a slower rate and retry.

  3. 3

    Image transmission

    The page is transmitted row by row of dots, compressed. The modem converts it into audio, at 9,600, 14,400 or 33,600 bit/s based on what was agreed. The command signals, meanwhile, remain at 300 bit/s: over a poor-quality line, they need to stay legible.

  4. 4

    Check after each page

    After each completed page, the sender indicates whether others follow; the receiver answers "received correctly" (MCF) or signals an issue. With error correction (ECM), just the corrupted blocks get retransmitted — the full page is never resent.

  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 at 1,728 dots across, around 204 dots per inch horizontally. Vertical resolution depends on the mode chosen: standard delivers 98 scan lines to the inch, fine delivers 196, and superfine 391. Moving from standard to fine sharpens small characters and stamps and roughly doubles the data volume.

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 needs a steady, uninterrupted signal with no breaks. Voice over IP, by contrast, slices audio into tiny fragments lasting only a few tens of milliseconds. During a voice call, a delayed or dropped packet is nearly imperceptible; in a fax transfer, however, it scrambles a line of the scanned page or halts the negotiation.

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 standard solution is available: T.38, issued by the ITU-T back in 1998. A gateway transforms the modem's audio into digital data at the point where the telephone network meets the IP network, carries both the page content and T.30 signalling as packets, and reconstructs a clean fax signal on the far end. Your machine started failing after a line change? Check with your operator that T.38 is active on the line; failing that, switching the line to plain G.711, without compression, often clears things up.

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.
  • A T.30 exchange only works if a fixed line leads to equipment that speaks fax, be it a standalone machine, server software or a gateway. Mobile numbers and email addresses cannot take part in that exchange.

What people often ask

Where does that hiss come from when someone picks up during a fax?
The session starts with the 2,100 Hz answer tone, after which handshake frames are sent as audio. The modem reads these signals as data, whereas a person listening hears only noise.
Is it possible to transmit color over a fax line?
A standard extension allows color in JPEG, yet virtually no device supports it. In reality, every page is rendered in plain black and white; shades convert into a dot pattern, which accounts for the grainy appearance of photographs.
Since switching to fibre, my faxes fail: what should I check?
Your device now communicates with the broadband router, i.e. voice over IP. Your provider can tell you if T.38 is enabled on the line; if it isn't, have them switch you to G.711 without compression. Limiting the fax machine to its 9,600 bit/s mode sometimes helps too.
Is the journey encrypted?
No: T.30 ignores encryption, and the signal moves across the telephone line without safeguarding it. Faxeroo protects the PDF's journey to its servers through HTTPS; the last stretch, up to the machine being called, is still subject to that protocol's constraints — just like every other provider.

Keep reading

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

See also