Groups 1 and 2
Groups 1 and 2 are based on 300-baud communication rates. They pertain to analog devices and do not include a modem. Group 1 transmits one page in six minutes. Group 2 transmits one page in three minutes.
Group 3 is for digital equipment and can use the same modem for data and fax. Not all modems in this group are compatible. Group 3 is comprised of several subclasses as shown in the following table.
|V.21 Channel 2||300|
Group 4 allows the highest resolution to date (400 by 400, up to 1200 dpi). These speeds are for use with digital telephone circuits, ISDN, or leased lines.
Fax Over IP (FoIP)
Fax Over IP (FoIP) can transmit and receive pre-digitized documents at near realtime speeds using ITU-T recommendation T.38 to send digitised images over an IP network using JPEG compression. T.38 is designed to work with VoIP services and often supported by analog telephone adapters used by legacy fax machines that need to connect through a VoIP service.
Scanned documents are limited to the amount of time the user takes to load the document in a scanner and for the device to process a digital file. The resolution can vary from as little as 150 DPI to 9600 DPI or more. This type of faxing is not related to the e-mail to fax service that still uses fax modems at least one way.