Group 3 Fax Machines (ITU-T T.4 / T.30)

What Group 3 fax is

Group 3 (often written G3) is the class of facsimile terminals defined by ITU-T Recommendation T.4, with the call-control procedures specified in ITU-T Recommendation T.30. It is the standard for sending scanned black-and-white document images as digital data over the analog general switched telephone network (PSTN); the same terminals may also operate over leased circuits and ISDN. A Group 3 machine scans a page into a bitmap, compresses it, negotiates a connection with the receiving terminal, and transmits the compressed image through a built-in voiceband modem, which the far end decodes and prints. Because interoperability is fixed by the ITU-T recommendations rather than by any single manufacturer, machines from different vendors can exchange documents — so this page describes the class and its governing standards rather than an individual unit.

Standardization and history

Group 3 was standardized in 1980 by the CCITT, the ITU committee whose telecommunication standardization work is now carried out by the ITU-T. The IEEE's Engineering and Technology History Wiki records the 1980 international standardization of G3 facsimile as an engineering milestone, and reference histories note that Recommendation T.4 brought interoperability to digital fax and cut transmission to under a minute per page — a large improvement over the earlier analog Group 1 and Group 2 machines, which needed about six and three minutes per page respectively. Combining digital image coding with ordinary telephone lines made Group 3 the dominant office fax standard from the 1980s onward, and T.4 remains an in-force ITU-T recommendation.

How a page is scanned and compressed

A Group 3 terminal samples each scan line at 1728 picture elements across the nominal 215 mm (A4) coded line width, which corresponds to roughly 203–204 dots per inch horizontally. Standard (normal) vertical resolution is about 98 lines per inch (3.85 lines per millimetre) and fine mode about 196 lines per inch (7.7 lines per millimetre); an optional superfine mode roughly doubles the vertical density again, to about 391 lines per inch. The resulting bitmap is compressed before transmission. The baseline scheme is one-dimensional Modified Huffman (MH) run-length coding defined in T.4; T.4 also allows a more efficient two-dimensional Modified READ (MR) option, and many Group 3 terminals additionally support Modified Modified READ (MMR) — the two-dimensional coding defined in Recommendation T.6 for Group 4 — as a higher-compression option.

The T.30 session and modem handshake

Recommendation T.30 governs what happens on the line. When a call connects, the two terminals exchange handshake tones and capability frames to agree on the page size, the resolution and coding to use, and the highest modem speed both machines support; T.30 then controls the page-by-page image transfer, the boundaries between pages, and the orderly termination of the call. The low-speed signaling for this negotiation is carried using V.21-based modulation, while the compressed image itself is sent with faster half-duplex voiceband modems. Group 3 image transfer commonly uses V.27ter (2400 and 4800 bit/s), V.29 (7200 and 9600 bit/s), and V.17 (up to 14.4 kbit/s), with the terminals falling back to a slower shared rate when line quality does not support the fastest one.

Super G3 and interoperability

The fastest tier of Group 3, marketed as "Super G3," uses ITU-T V.34 (half-duplex) modulation to raise the image-transfer rate to as much as 33.6 kbit/s, roughly halving the time needed to send a page compared with a 14.4 kbit/s V.17 machine. Super G3 stays part of the same standard family: during the T.30 handshake a Super G3 terminal and an older Group 3 terminal simply negotiate down to the fastest speed and coding both support, so the two remain interoperable. The Group 3 procedures have also been adapted for carrying fax traffic over IP networks rather than only analog phone lines.

Printing the received image

The Group 3 standards define how a page is scanned, coded, and transmitted, but not how the receiving machine puts the image on paper, so output technology varied by product generation. Early and low-cost Group 3 machines printed onto heat-sensitive thermal roll paper using a thermal printhead, while later "plain-paper" fax machines and multifunction devices reused laser (electrophotographic) or inkjet marking engines to print received faxes onto ordinary cut sheets. Separating the transmission standard from the marking method is why a single Group 3 protocol could be implemented across thermal, laser, and inkjet hardware from many manufacturers.