The T.4 standard recommends a series of capabilities that fax devices use to transmit documents back and forth. Fax devices that use digital technology have been standardized by the International Telecommunications Union (ITU) as Group 3 devices. These devices conform to Recommendations T.30 and T.4. The T.30 Recommendation was established in 1984 by the ITU and defines the procedures that are required for document transmission between two fax devices in the public switched telephone network (PSTN). The standard defines capabilities including compression methods, pixels per horizontal scanning lines, and horizontal and vertical resolutions.
For resolutions, the T.4 specification denotes a series of resolutions that include standard resolution (204 x 98 dots per inch), fine resolution (204 x 196 dots per inch) and superfine (204 x 396 dots per inch). Later, the standard was amended to include 400 x 400 dots per inch (Ultrafine). Each device in a communication handshakes with the other device and the highest resolution supported by both devices is chosen as the resolution that the transmission will use.
In addition to specifying the resolution, the ITU-T T.4 standard provides three compression methods for Group 3 fax that are now in common use. The ITU T.4 Recommendation defines a one-dimensional compression method known as Modified Huffman (MH) and a two-dimensional method known as Modified READ (MR; READ is short for Relative Element Address Designate). In 1984, a more efficient compression method known as Modified Modified READ (MMR) was defined in the ITU-T T.6 Recommendation that was originally defined for use only with Group 4 facsimile, so this compression method has been commonly called Group 4 compression. In 1991, the ITU standardized the MMR method for use in Group 3 fax devices and has since then it has been widely utilized on Group 3 devices. The various compression methods minimize the amount of data required to be transmitted between fax devices to transfer documents. More recently, T.4 was amended to support the Joint Bi-level Image Experts Group (JBIG) format, a lossless image compression method that was designed for especially for fax and is expected to eventually replace the other methods. In the fax handshaking process, the two fax devices figure out which is the most efficient modulation method (in order of efficiency, it goes JBIG, MMR, MR and MH) that they both support and that is the method used in the transmission.