Fax transmission time calculator: how long a fax takes, page by page

How the transmission time calculator works

The calculator adds up the phases of a Group 3 fax call as ITU-T Recommendation T.30 defines them. A call begins with dialling, ringing and the exchange of the calling and called tones, then a handshake in which the receiver announces what it can do and the sender chooses the speed, then a training sequence; the calculator budgets 20 seconds for all of that. Each page is then sent as compressed image data at the negotiated modem rate, and after each page the sender asks the receiver whether it got the page and waits for the answer, which costs about 4 seconds of signalling and retraining per page. After the last page the sender signals the end of the procedure, waits for the final confirmation and disconnects, another 3 seconds. Total time is setup plus pages times the per-page time plus teardown, and the range shown is that figure plus or minus 20 percent.

The per-page data time comes from the page’s size in bits divided by the modem rate. Under ITU-T Recommendation T.4 a fax line is 1,728 picture elements across a 215 mm width; a 297 mm page holds 1,145 lines at the standard 3.85 lines per millimetre and 2,290 at the fine 7.7, rounded. Those raw bits are compressed before transmission with T.4’s Modified Huffman or Modified READ coding, and how well they compress depends entirely on what is on the page: the calculator assumes roughly 20:1 for a light page of text, 10:1 for dense text, 6:1 for a form with boxes and logos, and 2.5:1 for a photograph or a scanned image with grey tones dithered into dots. Those ratios are typical results, not a standard, and they are the largest source of uncertainty in the estimate. Error correction mode adds about 10 percent on a clean line; a VoIP line caps the rate at 14,400 bit/s and adds 15 percent for retrains, for reasons explained further down.

What happens during a fax call

ITU-T T.30 divides a fax call into five phases, lettered A to E, and every fax machine and fax gateway made since 1980 follows them. Knowing the phases is what turns “it is still sending” from a worry into a timeline.

1. Phase A, call establishment. The sender dials and, while the line rings, emits the CNG calling tone, a 1,100 Hz beep every three seconds that tells whoever answers that a fax machine is calling. The receiver answers with the CED called-station tone at 2,100 Hz. This is the part a person hears if they pick up a fax line by mistake.
2. Phase B, pre-message procedure. The receiver sends its Digital Identification Signal, DIS, listing the speeds, resolutions, paper sizes and coding schemes it supports. The sender replies with a Digital Command Signal, DCS, choosing one combination, and then sends a training sequence at the chosen modem speed. If training fails, the sender drops to a lower speed and tries again; each drop costs seconds.
3. Phase C, message transmission. The compressed image data of one page is sent at the negotiated rate. This is the only phase whose length depends on the page itself.
4. Phase D, post-message procedure. After the page the sender sends Multi-Page Signal, MPS, if more pages follow, or End of Procedure, EOP, after the last. The receiver answers with Message Confirmation, MCF, if it decoded the page acceptably, or Retrain Positive or Retrain Negative if it wants the next page at a different speed or the same page again. The MCF is the receiver’s acknowledgement, and it is what a transmission receipt is built on.
5. Phase E, call release. The sender sends Disconnect, DCN, and both ends hang up.

Faxvio shows a fax as Delivered only when its carrier reports that the MCF for the last page arrived, and marks it Could not be delivered — pages returned when the call ended without one. That is the whole reason the status can stay at Transmitting for minutes: the app is waiting for the same signal a fax machine waits for. What Delivered means in Faxvio is explained on the home page.

Which modem speeds do fax machines use?

Group 3 fax uses a family of ITU-T modem Recommendations, and the two machines on a call negotiate the fastest one both support and the line will carry. The receiver lists its capabilities in DIS; the sender picks in DCS; training confirms the line can actually sustain the choice, and if it cannot, the sender steps down. That is why the speed printed on a fax machine’s box is a ceiling, not a promise.

V.27ter and V.29 are the mandatory pair every Group 3 machine carries, at 2,400 and 4,800 bit/s and at 7,200 and 9,600 bit/s respectively. V.17 raised the ceiling to 14,400 bit/s in 1991 and is the rate most calls settle on. V.34, marketed as Super G3, reaches 33,600 bit/s and roughly halves the page time, but only when both machines support it and the path between them is a real analogue circuit, which over modern networks it seldom is.

Why a page takes 30 to 60 seconds at 14,400 bit/s

A fine-resolution page is about four million raw bits. At 14,400 bit/s that would be over four and a half minutes uncompressed, and nobody would fax anything. T.4’s coding shrinks it by describing each scan line as runs of white and black rather than pixel by pixel, which is why the time depends on the page: a letter with wide margins is mostly long white runs and compresses twentyfold, while a scanned photograph, dithered into a pattern of alternating dots, barely compresses at all. The calculator’s default, a page of dense text, comes to 395,712 bits after compression and 27 seconds on the line before error correction, plus the 4 seconds of MPS and MCF around it.

Providers that bill by the minute put a number on this. eFax’s published international rate page states that its per-page rates apply to pages that take less than 60 seconds to transmit and that this covers approximately 90 percent of all fax pages; that is the provider’s own figure, but it matches what the arithmetic above predicts for text at 14,400 bit/s. Faxvio bills by the page, not the minute, so a slow page costs the same as a fast one; the time matters only for how long you wait for the receipt.

Computed from the calculator’s model; real calls vary with the page content, the receiving machine and the line, so read each figure as the middle of a range about 20 percent either side.

What error correction mode does to the time

Error correction mode, defined in Annex A of T.30, splits each page into blocks of frames with checksums. The receiver reports which frames arrived damaged and the sender retransmits only those, so a line hit that would otherwise print as a streak across the page is repaired invisibly. The cost is time: on a clean line ECM adds framing overhead and a short exchange per block, which the calculator prices at about 10 percent, and on a noisy line every retransmission adds to the call, sometimes doubling it. Fax carriers that route over the internet generally advise switching ECM off on lines that show repeated retransmissions, because a longer call over an unstable path is more likely to drop than a slightly imperfect page is to be rejected.

Why a VoIP line is slower than the modem’s number

Almost every fax sent from a phone app, Faxvio included, leaves the carrier as a voice-over-IP call and is converted to an analogue signal only at the far end. Two mechanisms carry fax over IP. T.38 is a Recommendation of its own: the gateway demodulates the fax signal and sends the image data and the T.30 signalling as packets, and the gateway nearest the receiver remodulates it. G.711 pass-through instead treats the fax tones as ordinary audio and sends them through the voice codec uncompressed. T.38 is the more robust of the two, and it is what carriers try first when they detect a fax tone; if the far end does not answer the T.38 request, the call continues in G.711.

Both mechanisms cost speed. The widely deployed version 0 of T.38 does not carry V.34, so a Super G3 machine on a T.38 call falls back to V.17 and 14,400 bit/s at best, which is why the calculator caps the rate over VoIP. Packet loss or jitter on the path shows up to the modems as line noise, and their response is to retrain at a lower speed or, under ECM, to retransmit blocks; the calculator adds 15 percent for that. Telnyx’s support documentation on T.38 and G.711 fax goes further and recommends limiting calls to 9,600 bit/s, using normal rather than fine resolution and turning ECM off on unstable routes, on the grounds that a shorter call is a more reliable one. Choose VoIP in the calculator and 9,600 bit/s to see what that advice costs in time.

International faxes

Distance does not change the page time; the two machines agree a rate and the bits take as long as they take. What an international call adds is in Phase A: longer dialling and ringing, and on some routes a gateway or two more between the carrier and the receiving machine, each a place where packets can be delayed. A fax to Japan or Korea from a US carrier typically spends a few more seconds in setup and is otherwise identical. Faxvio sends to 37 countries at the same page price, and the timing model here applies to all of them.

What to do while the app says Transmitting

Nothing. Leave the app, or leave the phone; the call runs from the carrier’s gateway, not from the device, and the result arrives as one notification. Faxvio prints the time of the last status update under the state, so a fax that has said Transmitting for four minutes with an update forty seconds ago is a ten-page document doing what the table above predicts. A status that has not changed in 20 minutes is labelled stale, which means the carrier has stopped reporting and the app is waiting on its poller rather than a live event. If the call ends without the last MCF, the fax is marked as returned and the pages go back to your balance; you can send it again, and the free retry is the returned pages.

Bottom line

A fax of 3 pages of dense text in fine resolution takes about 2 min 6 s at 14,400 bit/s over an ordinary line, of which roughly 23 seconds is call setup and release and about 34 seconds is each page with its confirmation. Under ITU-T T.30 every page must be acknowledged by the receiving machine before the next is sent, and that acknowledgement, the MCF after the last page, is what Faxvio waits for before it stamps a receipt Delivered. Text pages take 30 to 60 seconds at 14,400 bit/s, images can take two minutes, a VoIP path caps the speed at 14,400 and adds retrains, and none of it changes the price, because Faxvio charges by the page, not by the minute.