Group 3 Protocols

Group 3 Protocols

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Group 3 Protocol refers to a standard used in fax transmissions established by the International Telecommunication Union (ITU). ITU-T Recommendation T.4 and T.30 are often used for Group 3 (G3) fax machines.

Group 3 faxes are digital, and use the public switched telephone network (PSTN) to transmit monochrome images with a resolution of 200 or 100 dots per inch (dpi). They became popular in the 1980s and remain the most common standard for fax machines as of my knowledge cutoff in 2021.

The Group 3 standard includes procedures for compressing the image data to enable faster transmission. The two main methods are Modified Huffman (MH) and Modified READ (MR), with MR generally achieving better compression ratios.

Group 3 faxes can transmit a single A4 or letter-size page in about a minute under optimal conditions, although actual speeds can vary depending on the complexity of the document, the quality of the phone line, and other factors.

Note that the Group 3 standard was succeeded by the Group 4 standard, which uses digital network (like ISDN or B-ISDN) for transmission and can provide higher resolution and faster speeds. However, because Group 4 fax machines require a digital line, they are less common in environments where only analog phone lines are available.

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