Fax Technicalities & Audio Samples

This page is dedicated purely to those who want to know a little more about the key facts behind G3 Fax and hear or see real samples of fax calls of various sorts.

Fax Technicalities

Fax, short for facsimile, is a way of transmitting printed material over a distance, be it over wires (e.g. telephone) or radio. The earliest forms of fax were analog, very similar to the present day weather fax over HF radio, essentially using frequency shift keying to represent the analog brightness of a spot on the document being scanned. These faxes did operate over telephone lines, with some proprietary units (such as the Wirephoto and UPI Model 16-S) and later units standardized by the ITU as Group 1 or Group 2 depending on their send rates. These units are hard to find and are no longer used.

The more modern sort of fax that is used today are predominantly Group 3 (G3 for short), Super G3 or Group 4 machines. These marked the advent of digital fax transmission with G3 specifying documents scanned as a black and white bitmap at mostly 100x100dpi (standard) or 200x200dpi (fine) modes, with other modes (up to 400x400dpi) available but less commonly supported. Super G3 supported up to 600x600dpi, with some machines even supporting colour faxes.

The pages were sent as scan lines, and while photocopy document centres and computer programs typically emitted faxes with a fixed page size to standard paper sizes (e.g. A4 or Letter), older thermal fax machines often put out an imprecise number of lines due to feeding errors, skew and could also be used to reproduce very long documents (up to a meter on the Sharp FO-365 when I was still using it).

Transmission rates were 2400 – 9600bps (1988), 12 – 14.4kbps (1991) for regular G3, and up to 28.8kbps (1994), up to 33.6kbps (1998) for Super G3 with modulation modes specified as V.27ter, V.29, V.17, V.34 and V.34bis. With G3 faxes, Error Corrected Mode (ECM) was optional, and only supported by machines with sufficient processing time and memory. Many older plain paper and thermal direct printing fax machines were not capable of the highest speeds (e.g. 9600bps only) and had no ECM capabilities, supporting only basic 1D compression. Super G3 terminals mandate the use of ECM when in a Super G3 V.34 mode connection.

Scanned images were compressed. The most basic form of 1D compression is MH (Modified Huffman) which works on single lines only, with 2D compression in MR (Modified Read) allowing for commonalities between adjacent lines (two in standard, four in fine) to be encoded. The number of lines were limited to reduce the impact of data corruption from impacting on the rest of the page.

ECM capable links were also allowed to use MMR (Modified Modified Read) which removed the restriction on number of adjacent lines as it was expected that no uncorrected errors would occur. Later machines, especially those which are “internet enabled” T.38 capable, or Super G3 also had JBIG compression (1999) for even better compression. Some manufacturers also implemented their own compression modes (e.g. Matsushita Whiteline Skip) which only operate between machines from the same vendor.

A fax call is separated into five phases:

  • Phase A – Call Establishment
  • Phase B – Pre-message Procedure
  • Phase C – In-message Procedure
  • Phase D – Post-message Procedure
  • Phase E – Call Release

In Phase A, the originating modem picks up the line, dials the number and waits. The answering modem receives a ring, and picks up the line.

In Phase B, tones and low-speed data are used to communicate machine capabilities and IDs between both originating and answering modems.

In Phase C1, a training signal is sent to verify the selected transmission parameters. The receiving modem either acknowledges or requests for a retrain. Once the transmission rate has been set, we move to Phase C2 where the actual image data is sent.

The call reaches Phase D once the page has finished and the modem signals to the other party that the page has finished (and optionally, more pages are to come). The answering modem can reply with a page acceptance and continue, or page rejected (i.e. resend) and retrain. If more pages are to be sent and the multipage signal is used, the calling modem returns to phase C2. If not, the call waits idle and restarts from Phase B. If no more pages are to be sent, the calling modem signals an End of Transmission.

Phase E is where both modems hang up after completing the T.30 protocol.

Such a call is essentially half-duplex as only one machine is speaking at any given time. However, Super G3 calls differ, with the call being more like a modem data call with no jumping back and forth between modulations which increases the speed of transmission, and a continuously established two way channel.

Fax calls did not have to be placed solely for fax purposes. In fact, it was common in some cases to make a voice call and then proceed with sending a fax on the same call after having communicated some details, and with the right timing, it could be possible to resume talking over the same call after a fax had completed. This is hence where the “press the start button on your fax machine now” originates on various fax-back systems, where you would call-up, listen to a phone-tree menu, make a selection and then the system would send you a fax back on that same call. Even computer software had “manual receive now” options so as to force the modem to answer the line immediately, should a fax be inadvertently answered as voice, or a voice call was to turn into fax. As call costs became less, fax-back systems sometimes took a phone number and called that number solely to send a fax, and some document centre machines didn’t even have a handset for communicating voice before a fax is sent.

A vast majority of dial-up modems have basic faxing capabilities owing to their chipset supporting Fax Classes 1, 1.0, 2, 2.0 or 2.1. The available fax capabilities can be queried by issuing AT+FCLASS=?. Because of the long period to standardization, many older modems have quirks in their fax implementation, some of which are based on draft standard releases. Fax Class 1 is almost universally supported, and requires the host to do almost all of the T.30 work, whereas Class 2.0 modems tend to offload more of the work to the modem, however, this also limits some of the flexibility in supporting different modes. Fax Class 2 generally varies modem-to-modem and is likely to result in quirks, whereas 1.0 is an ITU T.31 recommendation but may not be supported in older modems. Fax Class 2.1 signifies support for Super G3 V.34 connections and is supported by very few modems. To fax using the computer requires faxing software to drive the modems. Such software was sometimes bundled with modems and is of various levels of quality. My favourite, by far, was Symantec WinFax Pro, however, this has become abandonware.

This is a basic overview of the fax system, with the full specification in the ITU Recommendations, specifically ITU T.30.

Fax on the Line

Many of you may have never actually heard the exchange that occurs between G3 fax machines because you were using machines which were configured to turn off the speaker as soon as Phase B negotiations began. The most you would have heard was … <ring ring> … <beeeeeeeeeeep> <silence>. What follows is recorded audio samples with spectrogram and brief explanation of what each part of the call is. Unfortunately, owing to not having any Super G3 capable equipment, there are no examples of Super G3 calls.

A Normal Fax Call of 2 Pages using the Multi Page Signal


Audio: MPS-call.wav

This is the most ordinary regular G3 fax that traverses the phone lines. In this case, we have a calling modem emitting a 1.2khz beep, and an answer modem emitting a solid 2.1khz tone (which should also disable any echo cancellers on the line which can interfere with the call).

The answer modem gives its capabilities and station ID first (Phase B), the calling modem acknowledges and gives its capabilities and station ID followed by sending the training data for the selected rate. The answer modem acknowledges the training data as received correctly (i.e. the phone line is clear enough to send at 14.4kbit/s) and the sending modem goes ahead with sending the first page. Alternatively, where there are problems, the answer modem can answer with a negative indicating the training data was not received correctly, try again at a different (usually lower) rate.

Once the page is completed, the sending modem sends a signal which says that the first page has finished but there are more pages to come. The answer modem replies with page accepted, please continue, to which the calling modem sends the second page. Alternatively, if there is a problem, the answer modem could have responded with a page rejected (to which the sending machine should re-send the same page, although many are not capable of it) and retrain negative (or for a marginal page, page accepted and retrain negative, so the page should not be resent) so as to force the sending modem to select a different (usually lower) rate through a process similar to before.

Once the second page is completed, the sending modem signals the completion of the page, the answer modem accepts the page, and the calling modem replies to indicate the end of transmission so the call can be released. Both modems hang up. In the case of one page, the MPS and negotiations in the middle are omitted.

A Slower Fax Call of 2 Pages without the use of Multi Page Signal


Audio: non-MPS-call.wav

This call is very similar to the one above, but this one is a slower way of sending two pages. This actual call is the reply call from Telstra Faxstream (FOLDS) test and is a perfectly legal fax. After the end of the first page, instead of signalling more pages to come, it signals the end of the first transmission but does not reply with end of transmission.

This indicates to the receiving fax that the first document has finished sending but there is another document waiting to come. In which case, the whole answer sequence begins again after a pause, which consumes time, but this is a legal but slightly less efficient way to send multiple pages. This may be necessary as the document itself has two modes, the first page in normal (100x100dpi) and the second page in fine (200x200dpi) as the capabilities are negotiated in Phase B.

A Modern Fax Call of One Page with ECM and v.34 Answer


Audio: ECM-call.wav

This was a call to a friend’s machine, sending a single page with ECM. The receiving machine is V.34 SuperG3 capable and can be distinguished by the phase reversals (breaks) in the 2100hz answer tone, but we are only sending in regular G3 mode.

The call sounds somewhat similar to the regular MPS type call but instead of actually dealing with pages, we are dealing with blocks of data. A certain amount of data is sent across the line, up to a certain amount (or up to a page, whichever is less) before the sending modem stops and asks the answer modem whether everything made it through correctly.

In this case, the answer modem replies with a few errors to be corrected, to which the sending modem immediately resends the bad blocks and checks again to see if everything is satisfactory. This cycle can be repeated several times, as ECM is persistent up to a point, although a call can be dropped if the bad lines are not repaired after a few retries. Once the first block is confirmed to be fine, the transmission of the first page continues through to completion, this time with no bad lines in the second block of data.

An Odd Call where All Rates Fail


Audio: all-rates-failed.wav

This is a very strange call and is rarely heard but I was able to accidentally stimulate and capture such a call. In this call, all available rates fail to negotiate, likely due to some misconfiguration, but it gives us a chance to hear almost all of the available modulations for G3 faxing in the one call.