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It is a little known fact that the first telefax service was actually 11 years before the invention of the telephone. In 1865 there was a fax line between Paris and Lyon. Since then, there have been wireless faxes, telephone faxes, computer interface faxes, etc. Today, there are basically two types of facsimile machines in commercial telephone fax use, digital and analog. For further information on analog and digital fax machines, see the Wikipedia definition for fax machines.

Group 1 and 2 faxes are called analog faxes, these faxes are sent in the same way you would send a frame of analogue television. Group 1 faxes are regulated and have to adhere to a guideline that is recommended, six minutes to transmit a whole page. Group 2 faxes also adhere to a recommended guideline as well although Group 2 faxes is three minutes to transmit an entire page. Both groups adhere to a scan line guideline of 96 scan lines per inch.

Group 3 and 4 faxes are called digital faxes and the data is transmitted in an altogether different way. The format for the faxes is a digital format that uses digital compression methods that greatly decrease transmission times. They also have a finer quality picture because Group 3 faxes have guidelines set up to 500 scan lines per inch. A Fax over your IP can produce transmission rates that are near instantaneous. Group 4 faxes have many of the same guidelines as Group 3 but they have to operate on a ISDN circuit. Digital fax machines come with a CCD chip with photo sensitive pixels to print the image.

In this day and age, internet fax services are popping up online. Fax capabilities are rated by group, class, compliance to guidelines, and data transmission rate. Sometimes even computer modems are categorized in fax classes.