# Fax protocols

The International Telecommunications Union (ITU) is the international standards body for telecommunications. In 1968 it published the standard for Group 1 (G1) fax protocol. In 1976 it published Group 2 (G2) protocol; in 1980 it first published Group 3 (G3) protocol.

Group 3 protocol is specified in several standards. T.4 and T.6 specify the image transfer protocols. T.30 specifies the session management procedures that support the establishment of a fax transmission. It allows the two stations to agree on such things such as transmission speed and page size. Since Group 3 protocol is specified for the switched analog network and is an all-digital procedure, it must use modems. The modems are specified in additional ITU standards:

- V.21 (300 b/s) for the T.30 procedures

- V.27ter (4800/2400 b/s) for image transfer

- V.29 (9600/7200 b/s) (optional)

- V.33 (14400/12000 b/s) (optional)

- V.17 (14400/12000/9600/7200 b/s) (optional)

The following International Telecommunications Union (ITU) specifications provide detailed information about facsimile:

- ITU-T T.30: Procedures for Document Facsimile Transmission in the General Switched Telephone Network

- ITU-T T.4: Standardization of Group 3 Facsimile Apparatus for Document Transmission

- ITU-T T.6: Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus