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Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer or other output device. The original document is scanned with a fax machine (or a telecopier), which processes the contents (text or images) as a single fixed graphic image, converting it into a
bitmap In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...

bitmap
, and then transmitting it through the telephone system in the form of audio-frequency tones. The receiving fax machine interprets the tones and reconstructs the image, printing a paper copy. Early systems used direct conversions of image darkness to audio tone in a continuous or analog manner. Since the 1980s, most machines modulate the transmitted audio frequencies using a digital representation of the page which is compressed to quickly transmit areas which are all-white or all-black.


History


Wire transmission

Scottish inventor
Alexander Bain Alexander Bain (11 June 1818 – 18 September 1903) was a Scottish philosopher and educationalist in the British school of empiricism and a prominent and innovative figure in the fields of psychology Psychology is the science of min ...
worked on chemical mechanical fax type devices and in 1846 was able to reproduce graphic signs in laboratory experiments. He received British patent 9745 on May 27, 1843 for his "Electric Printing Telegraph".
Frederick Bakewell Frederick Collier Bakewell (29 September 1800 – 26 September 1869) was an English physicist who improved on the concept of the facsimile machine introduced by Alexander Bain in 1842 and demonstrated a working laboratory version at the 18 ...
made several improvements on Bain's design and demonstrated a telefax machine. The
Pantelegraph The pantelegraph (Italian: ''pantelegrafo''; French: ''pantélégraphe'') was an early form of facsimile machine transmitting over normal telegraph lines developed by Giovanni Caselli, used commercially in the 1860s, that was the first such dev ...

Pantelegraph
was invented by the Italian physicist
Giovanni Caselli Giovanni Caselli (8 June 1815 – 25 April 1891) was an Italian priest, inventor, and physicist. He studied electricity and magnetism as a child which led to his invention of the pantelegraph The pantelegraph (Italian: ''pantelegrafo''; French: ...

Giovanni Caselli
. He introduced the first commercial telefax service between Paris and Lyon in 1865, some 11 years before the invention of the
telephone A telephone is a telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anyt ...

telephone
.The Hebrew University of Jerusalem – Giovanni Caselli biography
In 1880, English inventor
Shelford Bidwell Shelford Bidwell FRS (6 March 1848 – 18 December 1909) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early med ...
constructed the ''scanning phototelegraph'' that was the first telefax machine to scan any two-dimensional original, not requiring manual plotting or drawing. An account of Henry Sutton's "telephane" was published in 1896. Around 1900, German physicist
Arthur Korn Arthur Korn (May 20, 1870, Breslau, Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, Ge ...

Arthur Korn
invented the '' Bildtelegraph'', widespread in continental Europe especially following a widely noticed transmission of a wanted-person photograph from Paris to London in 1908, used until the wider distribution of the radiofax. Its main competitors were the ''Bélinographe'' by
Édouard Belin Édouard Belin (5 March 1876 – 4 March 1963) was a French photographer and inventor. In 1907 Belin invented a phototelegraphic apparatus called the Bélinographe (télestéréographe)—a system for receiving photographs 396x396px, ...

Édouard Belin
first, then since the 1930s the ''
Hellschreiber The Hellschreiber, Feldhellschreiber or Typenbildfeldfernschreiber (also Hell-Schreiber named after its inventor Rudolf Hell) is a facsimile '', a famous illuminated manuscript, is on view to both the public and to scholars only in the form of ...
'', invented in 1929 by German inventor
Rudolf Hell Rudolf Hell (19 December 1901 – 11 March 2002) was a German inventor. He was born in Eggmühl, Germany. From 1919 to 1923 he studied electrical engineering in Munich. He worked there from 1923 to 1929 as assistant of Prof. Max Dieckmann, w ...
, a pioneer in mechanical image scanning and transmission. The 1888 invention of the
telautograph An early telautograph The telautograph, an analog precursor to the modern fax machine, transmits electrical impulses recorded by potentiometers at the sending station to servomechanisms attached to a pen at the receiving station, thus reproducing a ...
by
Elisha Gray Elisha Gray (August 2, 1835 – January 21, 1901) was an American electrical engineer Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electri ...

Elisha Gray
marked a further development in fax technology, allowing users to send signatures over long distances, thus allowing the verification of identification or ownership over long distances. On May 19, 1924, scientists of the AT&T Corporation "by a new process of transmitting pictures by electricity" sent 15 photographs by telephone from Cleveland to New York City, such photos being suitable for newspaper reproduction. Previously, photographs had been sent over the radio using this process. The Western Union "Deskfax" fax machine, announced in 1948, was a compact machine that fit comfortably on a desktop, using special
spark printer Spark printing is an obsolete form of computer printing File:Printer.ogv, A video showing an Inkjet printing, inkjet printer while printing a page. In computing, a printer is a peripheral machine which makes a persistent representation of g ...
paper.G. H. Ridings
A Facsimile transceiver for Pickup and Delivery of Telegrams
(January 1949); page 17-26.


Wireless transmission

As a designer for the
Radio Corporation of America The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric General Electric Company (GE) is an American Multinati ...
(RCA), in 1924, Richard H. Ranger invented the wireless photoradiogram, or transoceanic radio facsimile, the forerunner of today's "fax" machines. A photograph of President
Calvin Coolidge Calvin Coolidge (born John Calvin Coolidge Jr.; ; July 4, 1872 – January 5, 1933) was an was an American lawyer and politician, who became the 30th president of the United States The president of the United States (POTUS) is the hea ...

Calvin Coolidge
sent from New York to London on November 29, 1924, became the first photo picture reproduced by transoceanic radio facsimile. Commercial use of Ranger's product began two years later. Also in 1924, Herbert E. Ives of
AT&T AT&T Inc. is an American multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a ...
transmitted and reconstructed the first color facsimile, a natural-color photograph of silent film star
Rudolph Valentino Rodolfo Alfonso Raffaello Pierre Filiberto Guglielmi di Valentina d'Antonguella (May 6, 1895 – August 23, 1926), known professionally as Rudolph Valentino and nicknamed The Latin Lover, was an Italian actor based in the United States who ...

Rudolph Valentino
in period costume, using red, green and blue color separations.Sipley, Louis Walton (1951). ''A Half Century of Color''. Macmillan. Beginning in the late 1930s, the Finch Facsimile system was used to transmit a "radio newspaper" to private homes via commercial AM radio stations and ordinary radio receivers equipped with Finch's printer, which used thermal paper. Sensing a new and potentially golden opportunity, competitors soon entered the field, but the printer and special paper were expensive luxuries, AM radio transmission was very slow and vulnerable to static, and the newspaper was too small. After more than ten years of repeated attempts by Finch and others to establish such a service as a viable business, the public, apparently quite content with its cheaper and much more substantial home-delivered daily newspapers, and with conventional spoken radio bulletins to provide any "hot" news, still showed only a passing curiosity about the new medium.Schneider, John (2011)
"The Newspaper of the Air: Early Experiments with Radio Facsimile"
theradiohistorian.org. Retrieved 2017-05-15.
By the late 1940s, radiofax receivers were sufficiently miniaturized to be fitted beneath the dashboard of
Western Union The Western Union Company is an American multinational financial services Financial services are the economic services provided by the finance Finance is a term for the management, creation, and study of money In a 1786 James ...

Western Union
's "Telecar"
telegram Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...

telegram
delivery vehicles. In the 1960s, the
United States Army The United States Army (USA) is the land Land is the solid surface of Earth that is not permanently submerged in water. Most but not all land is situated at elevations above sea level (variable over geologic time frames) and consists ma ...
transmitted the first photograph via satellite
facsimile '', a famous illuminated manuscript An illuminated manuscript is a formally prepared document A document is a written Writing is a medium of human communication Communication (from Latin ''communicare'', meaning "to share") is the a ...
to
Puerto Rico Puerto Rico (; abbreviated PR; tnq, Boriken, ''Borinquen''), officially the Commonwealth of Puerto Rico ( es, link=yes, Estado Libre Asociado de Puerto Rico, lit=Free Associated State of Puerto Rico) is a Caribbean island and Unincorporated ...

Puerto Rico
from the
Deal Test Site The Deal Test Site (now Joe Palaia Park) is located in Ocean Township, New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is ...
using the Courier satellite. Radio fax is still in limited use today for transmitting weather charts and information to ships at sea.


Telephone transmission

In 1964, Xerox Corporation introduced (and patented) what many consider to be the first commercialized version of the modern fax machine, under the name (LDX) or Long Distance Xerography. This model was superseded two years later with a unit that would truly set the standard for fax machines for years to come. Up until this point facsimile machines were very expensive and hard to operate. In 1966, Xerox released the Magnafax Telecopiers, a smaller, facsimile machine. This unit was far easier to operate and could be connected to any standard telephone line. This machine was capable of transmitting a letter-sized document in about six minutes. The first sub-minute, digital fax machine was developed by
Dacom Dacom, Inc. was founded in 1966 by two ex-LockheedLockheed (originally spelled Loughead) may refer to: Brands and enterprises * Lockheed Corporation, a former American aircraft manufacturer * Lockheed Martin, formed in 1995 by the merger of Lo ...
, which built on digital data compression technology originally developed at
LockheedLockheed (originally spelled Loughead) may refer to: Brands and enterprises * Lockheed Corporation, a former American aircraft manufacturer * Lockheed Martin, formed in 1995 by the merger of Lockheed Corporation and Martin Marietta ** Lockheed Mart ...
for satellite communication.''The implementation of a personal computer-based digital facsimile information distribution system''
– Edward C. Chung,
Ohio University Ohio University (officially The Ohio University and often referred to and stylized as OHIO) is a Public university, public research university in Athens, Ohio. The first university chartered by an Act of Congress and the first to be chartered in ...
, November 1991, page 2
''Fax: The Principles and Practice of Facsimile Communication'', Daniel M. Costigan, Chilton Book Company, 1971, pages 112–114, 213, 239 By the late 1970s, many companies around the world (especially Japanese firms) had entered the fax market. Very shortly after this, a new wave of more compact, faster and efficient fax machines would hit the market. Xerox continued to refine the fax machine for years after their ground-breaking first machine. In later years it would be combined with copier equipment to create the hybrid machines we have today that copy, scan and fax. Some of the lesser known capabilities of the Xerox fax technologies included their Ethernet enabled Fax Services on their 8000 workstations in the early 1980s. Prior to the introduction of the ubiquitous fax machine, one of the first being the
Exxon Exxon is the brand A brand is a name, term, design, symbol or any other feature that identifies one seller's good or service as distinct from those of other sellers. Brands are used in business, marketing, and advertising for recognition ...
Qwip in the mid-1970s, facsimile machines worked by optical scanning of a document or drawing spinning on a drum. The reflected light, varying in intensity according to the light and dark areas of the document, was focused on a
photocell Photodetectors, also called photosensors, are sensors In the broadest definition, a sensor is a device, module, machine, or subsystem whose purpose is to detect events or changes in its environment and send the information to other electro ...

photocell
so that the current in a circuit varied with the amount of light. This current was used to control a tone generator (a
modulator In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic partic ...
), the current determining the frequency of the tone produced. This audio tone was then transmitted using an
acoustic coupler In telecommunications, an acoustic coupler is an Network interface device, interface device for coupling (electronics), coupling electrical signals by acoustical means—usually into and out of a telephone. The link is achieved through converti ...
(a speaker, in this case) attached to the microphone of a common . At the receiving end, a handset's speaker was attached to an acoustic coupler (a microphone), and a
demodulator Demodulation is extracting the original information-bearing signal from a carrier wave upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal ...
converted the varying tone into a variable current that controlled the mechanical movement of a pen or pencil to reproduce the image on a blank sheet of paper on an identical drum rotating at the same rate.


Computer facsimile interface

In 1985,
Hank Magnuski Henry "Hank" Stanley Magnuski is an American engineer and was the co-founder and CEO of GammaLink, an early pioneer in PC-to-fax technology. He also founded Internet Video Services, a video service provider; MediaMart, an electronic commerce site; ...
, founder of GammaLink, produced the first computer fax board, called GammaFax. Such boards could provide voice telephony via Analog Expansion Bus.


In the 21st century

Although businesses usually maintain some kind of fax capability, the technology has faced increasing competition from
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
-based alternatives. In some countries, because
electronic signature An electronic signature, or e-signature, is data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to e ...
s on contracts are not yet recognized by law, while faxed contracts with copies of signatures are, fax machines enjoy continuing support in business. In
Japan Japan ( ja, 日本, or , and formally ) is an island country An island country or an island nation is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the land of an in ...

Japan
, faxes are still used extensively as of September 2020 for cultural and graphemic reasons. They are available for sending to both domestic and international recipients from over 81% of all
convenience stores A convenience store, convenience shop, or corner store is a small retail Retail is the process of selling consumer goods or Service (economics), services to customers through multiple distribution channel, channels of distribution to earn ...
nationwide. Convenience-store fax machines commonly print the slightly re-sized content of the sent fax in the electronic confirmation-slip, in
A4 paper ISO 216 is an International Organization for Standardization, international standard for paper sizes, used around the world except in North America and parts of Latin America. The standard defines the "A", "B" and "C" series of paper sizes, incl ...
size. Use of fax machines for reporting cases during the
COVID-19 pandemic The COVID-19 pandemic is an ongoing global pandemic A pandemic (from , , "all" and , , "local people" the 'crowd') is an of an that has spread across a large region, for instance multiple or worldwide, affecting a substantial numbe ...

COVID-19 pandemic
has been criticised in Japan for introducing data errors and delays in reporting, slowing response efforts to contain the spread of infections and hindering transition to work from home. In many corporate environments, freestanding fax machines have been replaced by
fax server A fax server is a system installed in a local area network (LAN) server that allows computer users whose computers are attached to the LAN to send and receive fax messages. Alternatively the term ''fax server'' is sometimes used to describe a pr ...

fax server
s and other computerized systems capable of receiving and storing incoming faxes electronically, and then routing them to users on paper or via an
email upThe email_address.html"_;"title="at_sign,_a_part_of_every_SMTP_email_address">at_sign,_a_part_of_every_SMTP_email_address Electronic_mail_(email_or_e-mail)_is_a_method_of_exchanging_messages_("mail")_between_people_using_electronic_dev ...

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. The once ubiquitous fax machine has also begun to disappear from the small office and home office environments. Remotely hosted fax-server services are widely available from VoIP and e-mail providers allowing users to send and receive faxes using their existing e-mail accounts without the need for any hardware or dedicated fax lines. Personal computers have also long been able to handle incoming and outgoing faxes using analog modems or
ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous Digital data, digital Transmission (telecommunications), transmission of voice, video, data, and other network services over the digitalised circui ...
, eliminating the need for a stand-alone fax machine. These solutions are often ideally suited for users who only very occasionally need to use fax services. In July 2017 the United Kingdom's
National Health Service The National Health Service (NHS) is the umbrella term for the publicly funded healthcare systems of the United Kingdom (UK). Since 1948, they have been funded out of general taxation. There are three systems which are referred to using the " ...
was said to be the world's largest purchaser of fax machines because the digital revolution has largely bypassed it. In June 2018 the
Labour Party Labour Party or Labor Party may refer to: Angola *MPLA, known for some years as "Popular Movement for the Liberation of Angola – Labour Party" Antigua and Barbuda *Antigua and Barbuda Labour Party Argentina *Labour Party (Argentina) Armenia ...
said that the NHS had at least 11,620 fax machines in operation and in December the
Department of Health and Social Care The Department of Health and Social Care (DHSC) is the British government departments, UK government department responsible for government policy on health and adult social care matters in England, along with a few elements of the same matters ...
said that no more fax machines could be bought from 2019 and that the existing ones must be replaced by secure email by March 31, 2020.
Leeds Teaching Hospitals NHS Trust Leeds Teaching Hospitals NHS Trust is an NHS hospital trust A hospital trust, also known as an acute trust, is an NHS trust An NHS trust is an organisational unit within the National Health Service in England and Wales, generally serving either a ...
, generally viewed as digitally advanced in the NHS, was engaged in a process of removing its fax machines in early 2019. This involved quite a lot of e-fax solutions because of the need to communicate with pharmacies and nursing homes which may not have access to the NHS email system and may need something in their paper records. In 2018 two-thirds of Canadian doctors reported that they primarily used fax machines to communicate with other doctors. Faxes are still seen as safer and more secure and electronic systems are often unable to communicate with each other.


Capabilities

There are several indicators of fax capabilities: group, class, data transmission rate, and conformance with
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin ...
(formerly
CCITT The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other E ...
) recommendations. Since the 1968 Carterphone decision, most fax machines have been designed to connect to standard
PSTN The public switched telephone network (PSTN) provides infrastructure Infrastructure is the set of fundamental facilities and systems that support the sustainable functionality of households and firms. Serving a country, city, or other area, i ...
lines and telephone numbers.


Group


Analog

Group 1 and 2 faxes are sent in the same manner as a frame of
analog television Analog or analogue may refer to: Computing and electronics * Analog signal An analog signal is any continuous signal In mathematical dynamics, discrete time and continuous time are two alternative frameworks within which to model variab ...
, with each scanned line transmitted as a continuous analog signal. Horizontal resolution depended upon the quality of the scanner, transmission line, and the printer. Analog 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 ITU-T Recommendation T.2. Group 1 faxes take six minutes to transmit a single page, with a vertical resolution of 96
scan line A scan line (also scanline) is one line, or row, in a raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics file:Rgb-raste ...
s per inch. Group 1 fax machines are obsolete and no longer manufactured. * Group 2 faxes conform to the ITU-T Recommendations T.3 and T.30. Group 2 faxes take three minutes to transmit a single page, with a vertical resolution of 96 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

A major breakthrough in the development of the modern facsimile system was the result of digital technology, where the analog signal from scanners was digitized and then compressed, resulting in the ability to transmit high rates of data across standard phone lines. The first digital fax machine was the
Dacom Dacom, Inc. was founded in 1966 by two ex-LockheedLockheed (originally spelled Loughead) may refer to: Brands and enterprises * Lockheed Corporation, a former American aircraft manufacturer * Lockheed Martin, formed in 1995 by the merger of Lo ...
Rapidfax first sold in late 1960s, which incorporated digital data compression technology developed by
LockheedLockheed (originally spelled Loughead) may refer to: Brands and enterprises * Lockheed Corporation, a former American aircraft manufacturer * Lockheed Martin, formed in 1995 by the merger of Lockheed Corporation and Martin Marietta ** Lockheed Mart ...
for transmission of images from satellites. 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 6 and 15 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: ** Horizontal: 100 scan lines per inch *** Vertical: 100 scan lines per inch ("Basic") ** Horizontal: 200 or 204 scan lines per inch *** Vertical: 100 or 98 scan lines per inch ("Standard") *** Vertical: 200 or 196 scan lines per inch ("Fine") *** Vertical: 400 or 391 (note not 392) scan lines per inch ("Superfine") ** Horizontal: 300 scan lines per inch *** Vertical: 300 scan lines per inch ** Horizontal: 400 or 408 scan lines per inch *** 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.411 to T.417. They are designed to operate over 64 kbit/s digital
ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous Digital data, digital Transmission (telecommunications), transmission of voice, video, data, and other network services over the digitalised circui ...
circuits. The allowed resolutions, a superset of those in the T.4 recommendation, are specified in the T.6 recommendation. Fax Over IP (
FoIP T.38 is an International Telecommunication Union, ITU recommendation for allowing transmission of fax over IP networks (FoIP) in real time. History The T.38 fax relay standard was devised in 1998 as a way to permit faxes to be transported acro ...
) 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 The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A com ...
using
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
compression. T.38 is designed to work with
VoIP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia Multimedia is a form of communication that ...
services and often supported by
analog telephone adapter An analog telephone adapter (ATA) is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a voice over IP telephony network. An ATA is often built into a small ...
s 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.


Class

Computer modems are often designated by a particular fax class, which indicates how much processing is offloaded from the computer's CPU to the fax modem. * Class 1 (also known as Class 1.0) fax devices do fax data transfer, while the T.4/T.6 data compression and T.30 session management are performed by software on a controlling computer. This is described in ITU-T recommendation T.31. * What is commonly known as "Class 2" is an unofficial class of fax devices that perform T.30 session management themselves, but the T.4/T.6 data compression is performed by software on a controlling computer. Implementations of this "class" are based on draft versions of the standard that eventually significantly evolved to become Class 2.0. All implementations of "Class 2" are manufacturer-specific. * Class 2.0 is the official ITU-T version of Class 2 and is commonly known as Class 2.0 to differentiate it from many manufacturer-specific implementations of what is commonly known as "Class 2". It uses a different but standardized command set than the various manufacturer-specific implementations of "Class 2". The relevant ITU-T recommendation is T.32. * Class 2.1 is an improvement of Class 2.0 that implements faxing over V.34 (33.6 kbit/s), which boosts faxing speed from fax classes "2" and 2.0, which are limited to 14.4 kbit/s. The relevant ITU-T recommendation is T.32 Amendment 1. Class 2.1 fax devices are referred to as "super G3".


Data transmission rate

Several different telephone-line modulation techniques are used by fax machines. They are negotiated during the fax-
modem A modulator-demodulator, or simply a modem, is a hardware device that converts data from a digital format, intended for communication directly between devices with specialized wiring, into one suitable for a transmission medium such as telep ...

modem
handshake A handshake is a globally widespread, brief greeting Greeting is an act of communication Communication (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured ...
, and the fax devices will use the highest data rate that both fax devices support, usually a minimum of 14.4 kbit/s for Group 3 fax. : Note that "Super Group 3" faxes use V.34bis modulation that allows a data rate of up to 33.6 kbit/s.


Compression

As well as specifying the resolution (and allowable physical size) of the image being faxed, the ITU-T T.4 recommendation specifies two compression methods for decreasing the amount of data that needs to be transmitted between the fax machines to transfer the image. The two methods defined in T.4 are: * Modified Huffman (MH). * Modified READ (MR) (''Relative Element Address Designate''), optional. An additional method is specified in T.6: * Modified Modified READ (MMR). Later, other compression techniques were added as options to ITU-T recommendation T.30, such as the more efficient
JBIG JBIG is an early lossless Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio sig ...
(T.82, T.85) for bi-level content, and
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
(T.81), T.43,
MRC MRC may refer to: Government * Medical Research Council (United Kingdom) The Medical Research Council (MRC) is responsible for co-coordinating and funding medical research in the United Kingdom. It is part of United Kingdom Research and Inn ...
(T.44), and T.45 for grayscale, palette, and colour content. Fax machines can negotiate at the start of the T.30 session to use the best technique implemented on both sides.


Modified Huffman

Modified Huffman (MH), specified in T.4 as the one-dimensional coding scheme, is a codebook-based run-length encoding scheme optimised to efficiently compress whitespace. As most faxes consist mostly of white space, this minimises the transmission time of most faxes. Each line scanned is compressed independently of its predecessor and successor.


Modified READ

Modified READ, specified as an optional two-dimensional coding scheme in T.4, encodes the first scanned line using MH. The next line is compared to the first, the differences determined, and then the differences are encoded and transmitted. This is effective, as most lines differ little from their predecessor. This is not continued to the end of the fax transmission, but only for a limited number of lines until the process is reset, and a new "first line" encoded with MH is produced. This limited number of lines is to prevent errors propagating throughout the whole fax, as the standard does not provide for error correction. This is an optional facility, and some fax machines do not use MR in order to minimise the amount of computation required by the machine. The limited number of lines is 2 for "Standard"-resolution faxes, and 4 for "Fine"-resolution faxes.


Modified Modified READ

The ITU-T T.6 recommendation adds a further compression type of Modified Modified READ (MMR), which simply allows a greater number of lines to be coded by MR than in T.4. This is because T.6 makes the assumption that the transmission is over a circuit with a low number of line errors, such as digital
ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous Digital data, digital Transmission (telecommunications), transmission of voice, video, data, and other network services over the digitalised circui ...
. In this case, the number of lines for which the differences are encoded is not limited.


JBIG

In 1999, ITU-T recommendation T.30 added
JBIG JBIG is an early lossless Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio sig ...
(ITU-T T.82) as another
lossless Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image ...
bi-level compression algorithm, or more precisely a "fax profile" subset of JBIG (ITU-T T.85). JBIG-compressed pages result in 20% to 50% faster transmission than MMR-compressed pages, and up to 30 times faster transmission if the page includes
halftone Halftone is the reprographicReprography is the reproduction of graphics through mechanical or electrical means, such as photography or xerography. Reprography is commonly used in catalogs and archives, as well as in the architectural reprograph ...
images.
JBIG JBIG is an early lossless Lossless compression is a class of data compression In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio sig ...
performs adaptive compression, that is, both the encoder and decoder collect statistical information about the transmitted image from the pixels transmitted so far, in order to predict the probability for each next pixel being either black or white. For each new pixel, JBIG looks at ten nearby, previously transmitted pixels. It counts, how often in the past the next pixel has been black or white in the same neighborhood, and estimates from that the probability distribution of the next pixel. This is fed into an arithmetic coder, which adds only a small fraction of a bit to the output sequence if the more probable pixel is then encountered. The ITU-T T.85 "fax profile" constrains some optional features of the full JBIG standard, such that codecs do not have to keep data about more than the last three pixel rows of an image in memory at any time. This allows the streaming of "endless" images, where the height of the image may not be known until the last row is transmitted. ITU-T T.30 allows fax machines to negotiate one of two options of the T.85 "fax profile": * In "basic mode", the JBIG encoder must split the image into horizontal stripes of 128 lines (parameter L0 = 128) and restart the arithmetic encoder for each stripe. * In "option mode", there is no such constraint.


Matsushita Whiteline Skip

A proprietary compression scheme employed on Panasonic fax machines is Matsushita Whiteline Skip (MWS). It can be overlaid on the other compression schemes, but is operative only when two Panasonic machines are communicating with one another. This system detects the blank scanned areas between lines of text, and then compresses several blank scan lines into the data space of a single character. (JBIG implements a similar technique called "typical prediction", if header flag TPBON is set to 1.)


Typical characteristics

Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual make ...
of 204×98 (normal) or 204×196 (fine) dots per square inch. The transfer rate is 14.4 kbit/s or higher for modems and some fax machines, but fax machines support speeds beginning with 2400 bit/s and typically operate at 9600 bit/s. The transferred image formats are called
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications Telecommunication is the transmission of information by various types of technologies over , radio, , or other systems. It has its origin ...
(formerly CCITT) fax group 3 or 4. Group 3 faxes have the
suffix In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most languag ...
.g3 and the
MIME type A media type (formerly known as MIME type) is a two-part identifier for file formats and format contents transmitted on the Internet. The Internet Assigned Numbers Authority, Internet Assigned Numbers Authority (IANA) is the official authority for ...
image/g3fax. The most basic fax mode transfers in black and white only. The original page is scanned in a resolution of 1728
pixel In digital imaging Digital imaging or digital image acquisition is the creation of a representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imp ...

pixel
s/line and 1145 lines/page (for A4). The resulting raw data is
compressed Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
using a modified
Huffman code In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code proceeds by means of Huffman coding, an algorit ...
optimized for written text, achieving average compression factors of around 20. Typically a page needs 10 s for transmission, instead of about 3 minutes for the same uncompressed raw data of 1728×1145 bits at a speed of 9600 bit/s. The compression method uses a Huffman codebook for run lengths of black and white runs in a single scanned line, and it can also use the fact that two adjacent scanlines are usually quite similar, saving bandwidth by encoding only the differences. Fax classes denote the way fax programs interact with fax hardware. Available classes include Class 1, Class 2, Class 2.0 and 2.1, and Intel CAS. Many modems support at least class 1 and often either Class 2 or Class 2.0. Which is preferable to use depends on factors such as hardware, software, modem firmware, and expected use.


Printing process

Fax machines from the 1970s to the 1990s often used direct
thermal printer Thermal printing (or direct thermal printing) is a digital printing Digital printing is a method of printing from a Digital data, digital-based image directly to a variety of media. It usually refers to professional printing where small-run ...
s with rolls of thermal paper as their printing technology, but since the mid-1990s there has been a transition towards plain-paper faxes:
thermal transfer printer Thermal-transfer printing is a digital printing method in which material is applied to paper (or some other material) by melting a coating of ribbon so that it stays glued to the material on which the print is applied. It contrasts with direct ther ...
s,
inkjet printer Inkjet printing is a type of computer printing File:Printer.ogv, A video showing an Inkjet printing, inkjet printer while printing a page. In computing, a printer is a peripheral machine which makes a persistent representation of graphic ...
s and
laser printers Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a Electric charge, negatively-charged cylinder call ...
. One of the advantages of inkjet printing is that inkjets can affordably print in
color Color (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Engli ...

color
; therefore, many of the inkjet-based fax machines claim to have color fax capability. There is a standard called ITU-T30e (formally ITU-T Recommendation T.30 Annex E ) for faxing in color; however, it is not widely supported, so many of the color fax machines can only fax in color to machines from the same manufacturer.


Stroke speed

Stroke speed in facsimile systems is the rate at which a fixed line perpendicular to the direction of scanning is crossed in one direction by a scanning or recording spot. Stroke speed is usually expressed as a number of strokes per minute. When the fax system scans in both directions, the stroke speed is twice this number. In most conventional 20th century mechanical systems, the stroke speed is equivalent to drum speed.


Fax paper

As a precaution, thermal fax paper is typically not accepted in archives or as documentary evidence in some courts of law unless photocopied. This is because the image-forming coating is eradicable and brittle, and it tends to detach from the medium after a long time in storage.


Internet fax

One popular alternative is to subscribe to an
Internet fax Internet fax, e-fax, or online fax is the use of the internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and device ...
service, allowing users to send and receive faxes from their
personal computer A personal computer (PC) is a multi-purpose computer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or technician ...
s using an existing email account. No software, fax server or fax machine is needed. Faxes are received as attached
TIFF Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by image scanner, scanning, FAX, faxi ...
or
PDF Portable Document Format (PDF), standardized as ISO 32000, is a file format A file format is a standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification ...
files, or in proprietary formats that require the use of the service provider's software. Faxes can be sent or retrieved from anywhere at any time that a user can get Internet access. Some services offer secure faxing to comply with stringent
HIPAA The Health Insurance Portability and Accountability Act of 1996 (HIPAA or the Kennedy–Kassebaum Act) is a United States federal statute enacted by the 104th United States Congress The 104th United States Congress was a meeting of the legis ...
and
Gramm–Leach–Bliley Act The Gramm–Leach–Bliley Act (GLBA), also known as the Financial Services Modernization Act of 1999, () is an act of the 106th United States Congress The 106th United States Congress was a meeting of the legislative branch of the United Sta ...
requirements to keep medical information and financial information private and secure. Utilizing a fax service provider does not require paper, a dedicated fax line, or consumable resources. Another alternative to a physical fax machine is to make use of computer
software Software is a collection of instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Computer program, a collection of instructions Music * I ...

software
which allows people to send and receive faxes using their own computers, utilizing
fax server A fax server is a system installed in a local area network (LAN) server that allows computer users whose computers are attached to the LAN to send and receive fax messages. Alternatively the term ''fax server'' is sometimes used to describe a pr ...

fax server
s and unified messaging. A virtual (email) fax can be printed out and then signed and scanned back to computer before being emailed. Also the sender can attach a digital signature to the document file. With the surging popularity of mobile phones, virtual fax machines can now be downloaded as applications for Android and iOS. These applications make use of the phone's internal camera to scan fax documents for upload or they can import from various cloud services.


Related standards

* T.4 is the umbrella specification for fax. It specifies the standard image sizes, two forms of image-data compression (encoding), the image-data format, and references, T.30 and the various modem standards. * T.6 specifies a compression scheme that reduces the time required to transmit an image by roughly 50-percent. * T.30 (ITU-T recommendation), T.30 specifies the procedures that a sending and receiving terminal use to set up a fax call, determine the image size, encoding, and transfer speed, the demarcation between pages, and the termination of the call. T.30 also references the various modem standards. * ITU V.21, V.21, V.27ter, V.29 (recommendation), V.29, V.17 ITU recommendation, V.17, V.34 (recommendation), V.34: ITU modem standards used in facsimile. The first three were ratified prior to 1980, and were specified in the original T.4 and T.30 standards. V.34 was published for fax in 1994. * T.37 (ITU-T recommendation), T.37 The ITU standard for sending a fax-image file via e-mail to the intended recipient of a fax. * T.38 The ITU standard for sending Fax over IP (FoIP). * G.711 pass through - this is where the T.30 fax call is carried in a VoIP call encoded as audio. This is sensitive to network packet loss, jitter and clock synchronization. When using voice high-compression encoding techniques such as, but not limited to, G.729, some fax tonal signals may not be correctly transported across the packet network. * image/t38 MIME-type
SSL Fax
An emerging standard that allows a telephone based fax session to negotiate a fax transfer over the internet, but only if both sides support the standard. The standard is partially based on T.30 and is being developed by Hylafax+ developers.


See also

* Black fax * Called subscriber identification (CSID) * Error correction mode (ECM) * Fax art * Fax demodulator * Fax modem * Fax server * Faxlore * Fultograph * Image Scanners *
Internet fax Internet fax, e-fax, or online fax is the use of the internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and device ...
* Junk fax * Radiofax—image transmission over HF radio * Slow-scan television * T.38 Fax-over-IP * Telautograph * Telex * Teletex * Transmitting Subscriber Identification (TSID) * Wirephoto


References


Further reading

* Coopersmith, Jonathan, ''Faxed: The Rise and Fall of the Fax Machine'' (Johns Hopkins University Press, 2015) 308 pp.
Transmitting Photographs by Telegraph
, Scientific American article, 12 May 1877, p. 297


External links


Group 3 Facsimile Communication
a '97 essay with technical details on compression and error codes, and call establishment and release.
ITU T.30 Recommendation
{{Authority control Fax, 1843 introductions American inventions Computer peripherals English inventions German inventions Italian inventions ITU-T recommendations Japanese inventions Office equipment Scottish inventions Telecommunications equipment