Telecom Dictionary - Definitions of terms

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Fax Group - Definition and Use

Fax Group - An encoding format used for fax transmission. There are two types: Fax Group 3, also known as G3, and Fax Group 4, also known as G4. Fax Group 3 and 4 are two of the encoding formats for Tagged Image File Format (TIFF) files. The more commonly used format, Fax Group 3, is Recommendation T.4 of the CCIT, now known as the ITU-T (for Telecommunication Standardization Sector of the International Telecommunications Union). Fax Group 3 supports one-dimensional image compression of black and white images, On a standard fax machine, Fax Group 3 uses redundancy reduction to enhance speed and is able to transmit a page in one minute or less. Fax Group 3 can achieve compression ratios of 10:1 for office documents and 15:1 for engineering drawings with a resolution of 200 dots per inch (dpi).

Less frequently used, Fax Group 4 (G4) is ITU-T Recommendation T.6 and supports two-dimensional image compression, compressing the line width as well as the line length. Fax Group 4 can achieve compression ratios of 15:1 for office documents and 20:1 for engineering drawings with a resolution of 400 dpi. Unlike Fax Group 3, Fax Group 4 can use Integrated Services Digital Network (ISDN) for transmission.