Col, Compression> (G4) The CCITT fax protoCol whiCh uses data Compression and allows a variety of file types (e-mail, piCtures, PostSCript, etC.) to be transmitted over digital (ISDN) telephone lines. The Group 4 protoCol was published by CCITT in 1993. Full details of the protoCol are available from ITU-T. See also Group 3. (1998-09-10)