Fax Group is 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.