Facsimile (Fax) Machine


Fax (short for facsimile, from Latin fac simile, “make similar”, i.e. “make a copy”) is a telecommunications technology used to transfer copies (facsimiles) of documents, especially using affordable devices operating over the telephone network. The word telefax, short for telefacsimile, for “make a copy at a distance”, is also used as a synonym. Although fax is not an acronym, it is often written as “FAX”. The device is also known as a telecopier in certain industries. When sending documents to people at large distances, faxes have a distinct advantage over postal mail in that the delivery is nearly instantaneous, yet its disadvantages in quality have relegated it to a position beneath email as the prevailing form of electronic document transferral.

Facsimile Transmission, a communication system that copies, sends, and receives documents by way of telephone networks or over Internet connections. Also called faxing, this method of communication allows people to share exact copies of important papers by duplicating and sending them on one end and then receiving and reproducing them on the other.

There are several different indicators of fax capabilities: Group, class, data transmission rate, and conformance with ITU-T (formerly CCITT) recommendations. Fax machines utilize standard PSTN lines and telephone numbers.

A “fax machine” usually consists of an image scanner, a modem, and is also offered as an option for many high-volume workgroup printers and photocopiers.

Monochrome Picture Tube

How Facsimile Machines Work

The standard facsimile machine works like a combination telephone and photocopier. The user places the documents into a document feeder on the sending machine and then dials the telephone number of the receiving fax machine. A gear mechanism pulls the original document over an optical scanner. The scanner records variation between light and dark areas of the document as dots arranged in a series of rows or columns. A photoelectric cell converts the dots into electronic impulses, which are then transmitted to the receiving fax machine via telephone lines. A simple block diagram of the facsimile system is shown in the figure below.

Facsimile (Fax) Machine
A simple block diagram of the FAX system

The receiving fax machine decodes the electrical impulses into a series of dots. It sends the decoded signal to a print mechanism built into the fax machine, which prints a duplicate of the original document. International standards ensure that fax machines around the world are compatible with each other.

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 simply 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.

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 a sound that can be recognized by another computer or 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 me 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.

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.

Although devices for transmitting printed documents electrically have existed, in various forms, since the 19th century, modem fax machines became feasible only in the mid-1970s as the sophistication increased and cost of the three underlying technologies dropped. Digital fax machines first became popular in Japan, where they had a clear advantage over competing technologies like the teleprinter. Over time, faxing gradually became affordable, and by the mid-1980s, fax machines were very popular around the world.

Analogue Formats

Group 1 and 2 faxes were sent in the same manner as a frame of analogue television; with each scanned line transmitted as a continuous analogue signal. Horizontal resolution depended upon the quality of the scanner, transmission line, and printer. Analogue fax machines are obsolete and no longer manufactured. ITU-T Recommendations T.2 and T.3 were withdrawn as obsolete in July 1996.

  • Group 1 faxes conform to the 1TU-T Recommendation T.2. Group 1 faxes take six minutes to transmit a single page, with a vertical resolution of 98 scan lines per inch. Group 1 fax machines are obsolete and no longer manufactured.
  • Group 2 faxes conform to the ITU-T Recommendations T.30 and T.3. Group 2 faxes take three minutes to transmit a single page, with a vertical resolution are of 100 scan lines per inch. Group 2 fax machines are almost obsolete, and are no longer manufactured. Group 2 fax machines can interoperate with Group 3 fax machines.

Digital Formats

Group 3 and 4 faxes are digital formats and take advantage of digital compression methods to greatly reduce transmission times.

Group 3 faxes conform to the ITU-T Recommendations T.30 and T.4. Group 3 faxes take between six and fifteen seconds to transmit a single page (not including the initial time for the fax machines to handshake and synchronize). The horizontal and vertical resolutions are allowed by the T.4 standard to vary among a set of fixed resolutions:

  1. Horizontal: 100 scan lines per inch

Vertical: 100 scan lines per inch

2. Horizontal: 200 or 204 scan lines per inch

(i) Vertical: 100 or 98 scan lines per inch (`Standard’)

(ii) Vertical: 200 or 196 scan lines per inch (‘Fine’)

(iii) Vertical: 400 or 391 (note not 392) scan lines per inch (`Superfine’)

3. Horizontal: 300 scan lines per inch

(1) Vertical: 300 scan lines per inch

4. Horizontal: 400 or 408 scan lines per inch

(i) Vertical: 400 or 391 scan lines per inch (`Ultrafine’)

Group 4 faxes conform to the ITU-T Recommendations T.563, T.503, T.521, T.6, T.62, T.70, T.72, and T.411 to T.417. They are designed to operate over 64 kbps digital ISDN circuits. Their resolution is determined by the T.6 recommendation, which is a superset of the T.4 recommendation.

Fax Over IP (FOIP) can transmit and receive pre-digitized documents at near real-time speeds. 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 like the e-mail to fax service that still uses fax modems at least one way.

In many corporate environments, standalone fax machines have been replaced by “fax servers” and other computerized systems capable of receiving and storing incoming faxes electronically, and then routing them to users on paper or via email (which may be secured). Such systems have the advantage of reducing costs by eliminating unnecessary printouts and reducing the number of inbound analog phone lines needed by an office.

Data Transmission Rate

Several different telephone line modulation techniques are used by fax machines. They are negotiated during the fax-modem handshake, and the fax devices will use the highest data rate that both fax devices support, usually a minimum of 14.4 kbps for Group 3 fax.

ITU StandardReleased DateData Rates (bps)Modulation Method
V.2719884800, 2400PSK
V.2919889600, 7200, 4800QAM
V.17199114400, 12000, 9600, 7200TCM

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