Fax Machine Capabilities

What your fax machine can do

Not all fax machines were created equal over the years, as many had different transmission rates and resolution qualities. Fax machines are made to conform to the International Telecommunication Union (ITU), which sets standards for telecommunication devices.

Fax machines are put into four group categories. Group One machines take six minutes to send each page, and they have a vertical resolution of 98 scan lines per inch. Although you may still own one of these models, they are no longer available on the market. Group Two fax machines take three minutes to send a page, and they have a vertical resolution of 100 scan lines per inch. There are still some of these machines around because they will work with group three machines, but they also are no longer produced.

Group Three fax machines are the type that most people now use, as their page transmission time is less than a minute. Still faster are the Group Four machines, which are designed to operate on high-speed data networks, such as the Integrated System Digital Network (ISDN). These units can send a fax at speeds as quick as three seconds a page. The horizontal and vertical resolutions on these units will vary according to the machine and the setting. The different resolutions are:

  • Horizontal - 100 scan lines per inch
    Vertical - 100 scan lines per inch
  • Horizontal - 200 or 204 scan lines per inch
    Vertical - 98 or 100 scan lines per inch (standard)
    Vertical - 196 or 200 scan lines per inch (fine)
    Vertical - 391 or 400 scan lines per inch (super fine)
  • Horizontal - 300 scan lines per inch
    Vertical - 300 scan lines per inch
  • Horizontal - 400 or 408 scan lines per inch
    Vertical - 391 or 400 scan lines per inch

Group Four fax machines are built to operate over 64 Kbps digital ISDN circuits, and their resolution is determined by the ITU's T.6 recommendation, which is a superset of the T.4 recommendation.

The data transmission rate of fax machines also varies, as different telephone line modulation techniques are used. The sending and receiving fax machines must agree on a common resolution, page width and page length before transmitting. The standard requires that all Group Three models must support at least standard resolution and A4 size. W hen the two machines settle on an information transfer circuit, they will use the highest data rate that both machines can support. This is usually a minimum of 14.4 Kbps for a Group Three fax machine and up to 33.6 Kbps for a super Group Three machine.

When a fax machine transmits a document, it compresses the information to decrease the amount of data that has to be sent. Group Three fax machines use three different types of compression techniques: Modified Huffman (MH), Modified Read (MR), and Modified Modified Read (MMR).

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