Transmission time
Fax transmission time: what really determines it
We count in pages; the line counts in black lines. Two documents of the same length do not take the same time to go through, and the gap has nothing to do with the number of sheets.
Short answer
Transmission time depends on three things: the initial negotiation between the two machines, the line speed finally agreed, and the density of the image being sent. Page count only multiplies the time of one page; it does not explain that time. A dense page of scanned text takes several times as long as an exported page of comparable content.
What happens before the first line
Before any image, the two machines exchange their capabilities: accepted resolution, compression mode, possible speed. This negotiation takes a fixed amount of time, independent of the document, and settles on a compromise speed — the one the line actually sustains, not the one the sender would like. A noisy line, or a path crossing voice over IP, forces a lower speed, and the same page then takes appreciably longer.
Image density, not file size
Fax encoding compresses runs of dots of the same colour. A page with wide white margins reduces to very little data; a page covered in fine text, grey screens or scanning noise produces far more, line after line. That is why the size of the PDF predicts nothing: a heavy but clean file can go through quickly, while a light, noisy scan drags. Image density after conversion to black and white is the only useful indicator.
Why a failure does not resume where it stopped
The page is the unit of transmission: it is sent in one go, then confirmed by the receiving machine. If synchronisation is lost mid-page, it is not the missing portion that is resent but the whole page. A dense document that fails near the end of a page therefore costs the time of that entire page, and a full retry means placing the call and negotiating again. This is the detail behind sends that take twice as long as expected, and it never appears in estimates given in minutes per page.
Frequently asked questions
- How long does ten pages take?
- There is no answer in minutes that holds everywhere: the total is the call setup time plus the sum of each page's duration, each depending on its density and on the agreed speed. Ten exported, airy pages go through far faster than ten scanned ones, with no setting changed.
- How can a send be shortened?
- Start from an exported PDF rather than a scanned one, remove needless grey backgrounds and solid black areas, and check that the document is straight and free of scanning noise. Each of these reduces the amount of black to transmit, which is the only thing the line actually carries.
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