Fax Machines Take a Digital Picture of the Document
Before a fax machine can send a document, it must first convert the document into a digital image. Although some advanced fax machines can actually recognize certain characters, most facsimile machines simple scan one page of the document at a time, converting the page into a digital image in much the same way as a photocopier takes a picture of the document to be copied.
The Fax Machine Modulates the Document Data
Once the fax machine has converted the document into a digital image, it needs to get the image ready to send across a telephone line. The machine prepares to send the document by modulating the digital image into sound that can be recognized by another computer or fax machine.
The Fax Machine Calls Another Fax Machine
When the document is ready to be sent, the fax machine sends an "off hook" signal to the attached telephone line followed by the appropriate dual-tone multi-frequency (DTMF) sounds necessary to dial the receiving fax machine. The receiving machine answers with a special tone known as a line acceptance tone (LAT); this tone indicates to the sending machine that another machine has answered. The sending machine then initiates a data transfer session with the fax machine or computer that answered the phone.
The Fax Machine Sends the Document and Disconnects
Once the sending and receiving fax machines have connected to one another, the sending machine sends the fax as a series of audible tones. The receiving machine listens to these tones, demodulates the data into a digital picture identical to the one scanned by the sending machine, then either stores or prints the received images. When the sending fax machine signals that the last page has been sent, the receiving machine acknowledges that the fax was fully received and disconnects the telephone call. Some sending fax machines subsequently print a report indicating that the fax was sent successfully.
