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How Fax Works

Standard Fax Machine

This section describes some basic concepts, and the interaction with QFAX components. You may choose to skip this chapter.

When you send a fax using a standalone fax machine, it goes through a number of steps.

  1. You prepare a paper copy of the fax, using paper and a pen, computer etc.
  2. The fax machine accepts your document and:
    1. Dials the remote machine and negotiates parameters for transmission.
    2. Scans your document and converts it into a digital format. This digital data is compressed using a complex algorithm into a CCITT G3 format compressed pixel bit map.
  3. The G3 format data is transmitted over the phone line using a modem built into the fax machine.
  4. The machines sign off when complete

When a fax machine receives a fax, the following happens:

  1. The machine answers the phone.
  2. The machine (modem) establishes contact with the sender and negotiates parameters for reception of the fax.
  3. As the G3 format data arrives, it is unpacked into a pixel bit map and printed.
  4. The machines sign off when complete

Fax Software Components

QFAX currently consists of the following components:

  1. text2g3 This program takes a text file (created with the editor of your choice) and converts it into a CCITT G3 format (compressed bitmap) file. You can include graphics in your document using some simple control sequences. This is analogous to the fax machine scanner.
  2. qfax. This program transmits the CCITT G3 format (compressed bitmap) file using your modem. This program instructs the modem to dial the remote fax, supplies parameters for negotiation with the remote machine, configures your modem and sends the data. It reports the success of the fax transmission. The qfax program can also receive G3 format files via your modem. It instructs your modem to answer the phone, configures your modem for negotiation with the remote machine and receives the G3 data. If reports the result of the fax reception and maintains a log file.
  3. qfv. A PE program that displays faxes on the screen or prints them using either HP pcl (i.e. LaserJet/DeskJet), Epson 24 bit (LQ850 and similar/compatible), Epson 9 or Postscript printers. Some `file-format' devices (pbm, pic, gif) are also provided.
  4. printfax. This program allows you to print G3 format files (either prepared by text2g3 or received using qfax) to a HP pcl (i.e. LaserJet/DeskJet), Epson 24 bit (LQ850 and similar/compatible), Epson 9 or Postscript printer. Some `file-format' devices (pbm, pic, gif) are also provided.

The mechanism of sending and receiving faxes using a conventional fax machine may involve some degradation of the original quality due to the scanning of the picture by the sending machine and its printing on (typically) a low quality thermal printer in the receiving fax machine. If you transmit and receive by computer, you retain the quality of the original fax (about 200 dpi). In addition, you have the option of encoding your faxes at a higher resolution than most low end fax machines default to. Using fax software and a laser or inkjet printer will give a high quality plain paper system.


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