PDF of this tutorialFax systems must be tested in order to ensure compliance with industry standards and to verify compatibility with the existing installed base of fax devices—many of which deviate from the standards in a variety of ways.
Fax operating procedures are defined by standards published by the International ITU. Committees made up of representatives from fax machine manufacturers, telecommunications companies, and government agencies develop these standards, some of which are listed below:
Modulation Standards—Bit Encoding Methods for Transmission of Images and Control Data- V.21—defines modulation used for transmission of control messages at 300 bps in conventional fax
- V.17, V.29, V.27 ter—defines modulation used for transmission of image data at rates up to 14.4 kbps
- V.34 fax—defines modulation and procedures used for transmission of images at rates up to 33.6 kbps
- V.8—defines modulation and procedures for initial data interchange to set up V.34 fax
- T.30—defines procedures and message sequences used for fax communications
Data Compression Standards—Procedures Used to Compress Images for Fax Transmission
- T.4—defines algorithms used for one-dimensional and two-dimensional data compression
- T.6—defines algorithms used for image compression for error correction mode
The intent of these standards is to ensure that all fax devices follow a common procedure in order that devices manufactured by different companies can communicate reliably with one another. Unfortunately, standards compliance varies from manufacturer to manufacturer. ITU standards are recommendations, not laws, and are subject to interpretation (or misinterpretation), and sometimes manufacturers for various technical reasons deliberately compromise the standards. Extensive testing is required to ensure standards compliance and smooth handling of common deviations.
