Group 4 Fax Machines

What Group 4 fax is

Group 4 fax (also written G4 fax) is a class of facsimile apparatus intended for all-digital transmission networks rather than the analog telephone line used by Group 3 machines. The class is defined by two core ITU-T Recommendations: T.6, 'Facsimile coding schemes and coding control functions for Group 4 facsimile apparatus,' which specifies the image-coding scheme, and T.563, 'Terminal characteristics for Group 4 facsimile apparatus,' which specifies the terminals themselves. According to Wikipedia, Group 4 faxes are designed to operate over 64 kbit/s digital ISDN circuits. This is a class page: it documents the Group 4 standard and the family of machines built to it, not any single model, and every figure below is drawn from an authoritative standards or reference source.

The T.6 coding scheme (MMR)

Group 4's image compression is the Modified Modified READ (MMR) scheme, defined in ITU-T Recommendation T.6. Wikipedia and the Library of Congress describe MMR as a lossless method of compressing bi-level (black-and-white) images that is fully two-dimensional: it builds on the Group 3 two-dimensional coding of Recommendation T.4 but removes the end-of-line (EOL) codes, treating the page as a continuous bitstream rather than a series of separately delimited lines. Because Group 4 assumes an error-free digital channel, it does not need the resynchronisation codes that Group 3 relies on over noisy analog lines. Wikipedia states that Group 4 typically achieves about a 20:1 (roughly 95%) compression ratio, giving the example of an 8.5 x 11-inch page scanned at 200 dpi shrinking from about 467.5 kB to about 23.4 kB. The MMR coding scheme was adopted in 1984, and the current ITU-T text of Recommendation T.6 is dated November 1988.

Resolution

The standard picture-element density of Group 4 apparatus is 200 pels per 25.4 mm — about 200 dpi — in both the main-scan (horizontal) and sub-scan (vertical) directions, as defined in ITU-T Recommendation T.563. The same recommendation provides for higher optional densities of 240, 300 and 400 pels per 25.4 mm (about 240, 300 and 400 dpi). Wikipedia characterises the Group 4 resolution set as a superset of the Group 3 (T.4) resolutions, so a Group 4 terminal can reproduce the resolutions a Group 3 machine offers and add higher ones. In keeping with the archive's sourcing policy, resolutions above the 200-dpi base are given only where the standard defines them; no value is estimated.

Built for digital networks (ISDN)

The defining difference between Group 4 and earlier fax classes is the carrier it was designed for. Where Group 3 sends a page over the analog public switched telephone network (PSTN) using a voice-band modem, Group 4 was specified for all-digital networks — principally the Integrated Services Digital Network (ISDN). Wikipedia states that Group 4 faxes are designed to operate over 64 kbit/s digital ISDN circuits, a channel that is both faster and, being digital, effectively error-free. That error-free assumption is what allows the T.6 coding to dispense with the line-by-line resynchronisation codes required on analog connections. The archive's discussion of analog fax versus digital fax explores how changing the underlying carrier reshaped the cost and reliability of faxing.

Group 4 compared with Group 3

Group 3 and Group 4 perform the same task — scanning a page at one end and reconstructing it at the other — over different networks and with different standards. A Group 3 machine conforms to ITU-T Recommendations T.4 (coding) and T.30 (the call and session protocol) and runs over ordinary telephone lines using V-series modems; Wikipedia lists the V.27ter, V.29 and V.17 modulations reaching 14,400 bit/s, with later 'Super G3' terminals using ITU-T V.34 half-duplex modulation to reach up to 33,600 bit/s. A Group 4 machine instead uses the T.6 coding scheme and T.563 terminal characteristics over a 64 kbit/s ISDN channel. In compression terms the two are closely related, since MMR (T.6) is the two-dimensional Group 3 scheme with its end-of-line codes removed. The archive's guide to how fax machines work covers the shared scanning-and-printing mechanics that both classes rely on.

Adoption and legacy

Group 4 was intended to supplant Group 3 by carrying documents error-free over digital networks at up to 64 kbit/s. In practice its dependence on an end-to-end ISDN connection limited where it could be used: ISDN service was more expensive and less widely available than the ordinary analog telephone line that any Group 3 machine could use. Industry references note that Group 4 usage stayed largely confined to ISDN, while the great majority of fax machines in service remained Group 3. The Group 3 line was itself extended over time — the 'Super G3' terminals using ITU-T V.34 (half-duplex) raised its speed to 33.6 kbit/s — which further narrowed the practical advantage of moving to Group 4. As a result, Group 4 is best understood as the digital-network fax standard rather than the fax standard most offices actually adopted.

Reference scope

This page records only facts that can be traced to an authoritative source — chiefly the ITU-T Recommendations that define the class (T.6 and T.563) together with encyclopedic and industry references such as Wikipedia, Britannica and the Library of Congress format registry. Because Group 4 is a class and standard rather than a single product, specifications are given at the class level and no per-model figures are invented; any specification that cannot be sourced is omitted rather than estimated. The page quotes no current pricing or availability. The sources consulted are listed below.