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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, a bitmap is a mapping from some domain (for example, a range of integers) to bits. It is also called a bit array or bitmap index. As a noun, the term "bitmap" is very often used to refer to a particular bitmapping application: t ...
, 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. Fax machines were ubiquitous in office environments in the 1980s and 1990s, but have gradually been rendered obsolete by Internet-based technologies such as
email Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mean ...
and the
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web ...
. They remain particularly popular in medical administration and law enforcement.


History


Wire transmission

Scottish inventor Alexander Bain 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 1 ...
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 de ...
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 (also known as the universal telegraph or all-purpose ...
. 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 telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
. In 1880, English inventor
Shelford Bidwell Shelford Bidwell FRS (6 March 1848 – 18 December 1909) was an English physicist and inventor. He is best known for his work with "telephotography", a precursor to the modern fax machine. Private life He was born in Thetford, Norfolk the elde ...
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 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 over telephone ...
first, then since the 1930s the ''
Hellschreiber The Hellschreiber, Feldhellschreiber or Typenbildfeldfernschreiber (also Hell-Schreiber named after its inventor Rudolf Hell) is a facsimile-based teleprinter invented by Rudolf Hell. Compared to contemporary teleprinters that were based on ty ...
'', invented in 1929 by German inventor
Rudolf Hell Rudolf Hell (19 December 1901 – 11 March 2002) was a German inventor and engineer. Career Hell was born in Eggmühl. From 1919 to 1923, he studied electrical engineering in Munich. He worked there from 1923 to 1929 as assistant of Prof. ...
, a pioneer in mechanical image scanning and transmission. The 1888 invention of the
telautograph The telautograph is an analog precursor to the modern fax machine. It transmits electrical impulses recorded by potentiometers at the sending station to servomechanisms attached to a pen at the receiving station, thus reproducing at the receivin ...
by 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 and before that fax and chart recorder printing which uses a special paper coated with a conductive layer over a contrasting backing, originally black carbon over white paper but later alu ...
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 (GE), Westinghouse, AT&T Corporation and United Fruit Com ...
(RCA), in 1924,
Richard H. Ranger Richard Howland Ranger (13 June 1889 – 10 January 1962) was an American electrical engineer, music engineer and inventor. He was born in Indianapolis, Indiana, the son of John Hilliard and Emily Anthen Gillet Ranger, He served in the U.S. Ar ...
invented the wireless photoradiogram, or transoceanic radio facsimile, the forerunner of today's "fax" machines. A photograph of President 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 Herbert Eugene Ives (July 31, 1882 – November 13, 1953) was a scientist and engineer who headed the development of facsimile and television systems at AT&T in the first half of the twentieth century. He is best known for the 1938 Ives–Stilwel ...
of
AT&T AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile te ...
transmitted and reconstructed the first color facsimile, a natural-color photograph of silent film star 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 company, headquartered in Denver, Colorado. Founded in 1851 as the New York and Mississippi Valley Printing Telegraph Company in Rochester, New York, the company cha ...
'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 ...
delivery vehicles. In the 1960s, the
United States Army The United States Army (USA) is the land warfare, land military branch, service branch of the United States Armed Forces. It is one of the eight Uniformed services of the United States, U.S. uniformed services, and is designated as the Army o ...
transmitted the first photograph via satellite facsimile 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 ...
from the Deal Test Site 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- Lockheed engineers, Daniel Hochman, President, and Don Weber, Vice President, building on their pioneering work on digital image compression invented for satellite communications. Their work resulted in ...
, which built on digital data compression technology originally developed at Lockheed for satellite communication.''The implementation of a personal computer-based digital facsimile information distribution system''
– Edward C. Chung,
Ohio University Ohio University is a public research university in Athens, Ohio. The first university chartered by an Act of Congress and the first to be chartered in Ohio, the university was chartered in 1787 by the Congress of the Confederation and subseq ...
, 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 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 of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or b ...
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 and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
), 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 interface device for coupling electrical signals by acoustical means—usually into and out of a telephone. The link is achieved through converting electric signals from the phone line to sound a ...
(a speaker, in this case) attached to the microphone of a common
telephone handset A handset is a component of a telephone that a user holds to the ear and mouth to receive audio through the receiver and speak to the remote party using the built-in transmitter. In earlier telephones, the transmitter was mounted directly on ...
. At the receiving end, a handset's speaker was attached to an acoustic coupler (a microphone), and a demodulator 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 GammaLink Inc. was founded in Sunnyvale, California by Hank Magnuski and Michael Lutz. The company was the first to invent PC-to-fax communications technology, GammaFax.''GammaLink fax board enshrined at Smithsonian'', Infoworld, July 20th, 1992, ...
, produced the first computer fax board, called
GammaFax The first personal computer fax board, GammaFax, was produced in 1985 by GammaLink GammaLink Inc. was founded in Sunnyvale, California by Hank Magnuski and Michael Lutz. The company was the first to invent PC-to-fax communications technology, Ga ...
. Such boards could provide voice telephony via
Analog Expansion Bus The Analog Expansion Bus is a hardware bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in p ...
.


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 '' network of networks'' that consists of private, pub ...
-based alternatives. In some countries, because
electronic signature An electronic signature, or e-signature, is data that is logically associated with other data and which is used by the signatory to sign the associated data. This type of signature has the same legal standing as a handwritten signature as long as i ...
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, 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, corner store or corner shop is a small retail business that stocks a range of everyday items such as coffee, groceries, snack foods, confectionery, soft drinks, ice creams, tobacco products, lottery ticke ...
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 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, including A4, the most commonly available paper size ...
size. Use of fax machines for reporting cases during the
COVID-19 pandemic The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing global pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The novel virus was first identi ...
has been criticised in Japan for introducing data errors and delays in reporting, slowing response efforts to contain the spread of infections and hindering the transition to remote work. 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 ...
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 Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mean ...
(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 transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
, 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 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 a department of His Majesty's Government responsible for government policy on health and adult social care matters in England, along with a few elements of the same matters which are not otherw ...
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 in Leeds, West Yorkshire, England. The Trust was formed in April 1998 after the merger of two previous smaller NHS trusts to form one citywide organisation. The former trusts were United ...
, 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 Internet fax, e-fax, or online fax is the use of the internet and internet protocols to send a fax (facsimile), rather than using a standard telephone connection and a fax machine. A distinguishing feature of Internet fax, compared to other Intern ...
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) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...
(formerly CCITT) recommendations. Since the 1968
Carterphone decision The Carterfone is a device invented by Thomas Carter. It manually connects a two-way radio system to the telephone system, allowing someone on the radio to talk to someone on the phone. This makes it a direct predecessor to today's autopatch. ...
, most fax machines have been designed to connect to standard
PSTN The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local teleph ...
lines and telephone numbers.


Group


Analog

Group 1 and 2 faxes are sent in the same manner as a frame of analog television, 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 scanning pattern, such as a line of video on a cathode ray tube (CRT) display of a television set or computer monitor. On CRT screens the horizontal scan lines are visually discernible ...
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- Lockheed engineers, Daniel Hochman, President, and Don Weber, Vice President, building on their pioneering work on digital image compression invented for satellite communications. Their work resulted in ...
Rapidfax first sold in late 1960s, which incorporated digital data compression technology developed by Lockheed 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 transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
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 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 across IP networks between existing Group ...
) 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 a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
using 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 sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
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 sma ...
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 modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...
handshake A handshake is a globally widespread, brief greeting or parting tradition in which two people grasp one of each other's like hands, in most cases accompanied by a brief up-and-down movement of the grasped hands. Customs surrounding handshakes a ...
, 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 Modified may refer to: * ''Modified'' (album), the second full-length album by Save Ferris *Modified racing, or "Modifieds", an American automobile racing genre See also * Modification (disambiguation) * Modifier (disambiguation) Modifier may re ...
(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 image compression standard from the Joint Bi-level Image Experts Group, standardized as ISO/IEC standard 11544 and as ITU-T recommendation T.82 in March 1993. It is widely implemented in fax machines. Now that the new ...
(T.82, T.85) for bi-level content, and JPEG (T.81), T.43,
MRC MRC may refer to Government * Medical Research Council (United Kingdom) * Medical Reserve Corps, a US network of volunteer organizations * Municipalité régionale de comté (regional county municipality), Quebec, Canada * Military Revolutionar ...
(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 transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work ...
. 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 image compression standard from the Joint Bi-level Image Experts Group, standardized as ISO/IEC standard 11544 and as ITU-T recommendation T.82 in March 1993. It is widely implemented in fax machines. Now that the new ...
(ITU-T T.82) as another
lossless Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistic ...
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 images.
JBIG JBIG is an early lossless image compression standard from the Joint Bi-level Image Experts Group, standardized as ISO/IEC standard 11544 and as ITU-T recommendation T.82 in March 1993. It is widely implemented in fax machines. Now that the new ...
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 Arithmetic coding (AC) is a form of entropy encoding used in lossless data compression. Normally, a string of characters is represented using a fixed number of bits per character, as in the ASCII code. When a string is converted to arithmetic e ...
, 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 mak ...
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) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Comm ...
(formerly CCITT) fax group 3 or 4. Group 3 faxes have the suffix .g3 and the MIME type 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, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the ...
s/line and 1145 lines/page (for A4). The resulting raw data is compressed 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 algori ...
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 process which produces a printed image by passing paper with a thermochromic coating, commonly known as thermal paper, over a print head consisting of tiny electrically heated ...
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 the ...
s,
inkjet printer Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensi ...
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 negatively-charged cylinder called a "drum" to ...
. One of the advantages of inkjet printing is that inkjets can affordably print in
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
; 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 and internet protocols to send a fax (facsimile), rather than using a standard telephone connection and a fax machine. A distinguishing feature of Internet fax, compared to other Intern ...
service, allowing users to send and receive faxes from their
personal computer A personal computer (PC) is a multi-purpose microcomputer 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 tec ...
s using an existing
email account Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic (digital) version of, or counterpart to, mail, at a time when "mail" meant ...
. 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 scanning, faxing, word process ...
or PDF 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 Act of Congress enacted by the 104th United States Congress and signed into law by President Bill Clinton on August 21, 1 ...
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 (1999–2001). It repealed part of the Glass–Steagall Act of 1933, removing barriers in ...
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 set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...
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 ...
s and
unified messaging Unified messaging (or UM) is a business term for the integration of different electronic messaging and communications media ( e-mail, SMS, fax, voicemail, video messaging, etc.) technologies into a single interface, accessible from a variety of ...
. 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 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. * V.21,
V.27ter ''V.'' is the debut novel of Thomas Pynchon, published in 1963. It describes the exploits of a discharged United States Navy, U.S. Navy sailor named Benny Profane, his reconnection in New York City, New York with a group of pseudo-bohemianism, b ...
, V.29, V.17, 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 The ITU standard for sending a fax-image file via e-mail to the intended recipient of a fax. *
T.38 T.38 is an 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 across IP networks between existing Group ...
The ITU standard for sending Fax over IP (FoIP). *
G.711 G.711 is a narrowband audio codec originally designed for use in telephony that provides toll-quality audio at 64 kbit/s. G.711 passes audio signals in the range of 300–3400 Hz and samples them at the rate of 8,000 samples per second ...
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 Packet loss occurs when one or more packets of data travelling across a computer network fail to reach their destination. Packet loss is either caused by errors in data transmission, typically across wireless networks, or network congestion.Kur ...
, jitter and clock synchronization. When using voice high-compression encoding techniques such as, but not limited to,
G.729 G.729 is a royalty-free narrow-band vocoder-based audio data compression algorithm using a frame length of 10 milliseconds. It is officially described as ''Coding of speech at 8 kbit/s using code-excited linear prediction'' speech coding (CS-ACEL ...
, 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 The term black fax refers to a prank fax transmission, consisting of one or more pages entirely filled with a uniform black tone. The sender's intention is generally to use up as much of the recipient's fax ink, toner, or thermal paper as possible ...
*
Called subscriber identification A called subscriber identification (abbreviated CSID) is a string that identifies a specific fax machine as the recipient of a fax. This string (typically less than 40 characters) is usually a combination of the fax machine's telephone number and i ...
(CSID) *
Error correction mode Error correction mode (ECM) is an optional transmission mode built into Class 1/2/2.0 fax machines or fax modems. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise. T ...
(ECM) *
Fax art Fax art is art specifically designed to be sent or transmitted by a facsimile machine, where the "fax art" is the received "fax". It is also called telecommunications art or telematic art. According to art historians Annmarie Chandler and Norie ...
*
Fax demodulator A fax demodulator is a device used to intercept fax messages by listening in on a telephone line or radio signal. A typical ( Group III) fax transmission requires a two-way conversation between two modem A modulator-demodulator or modem is a ...
*
Fax modem A fax modem enables a computer to transmit and receive documents as faxes on a telephone line. A fax modem is like a data modem but is designed to transmit and receive documents to and from a fax machine or another fax modem. Some, but not all, fa ...
*
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 ...
*
Faxlore Faxlore is a sort of folklore: humorous texts, folk poetry, folk art, and urban legends that are circulated, not by word of mouth, but by fax machine. Xeroxlore or photocopylore is similar material circulated by photocopying; compare samizdat i ...
* Fultograph *
Image Scanners An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
*
Internet fax Internet fax, e-fax, or online fax is the use of the internet and internet protocols to send a fax (facsimile), rather than using a standard telephone connection and a fax machine. A distinguishing feature of Internet fax, compared to other Intern ...
*
Junk fax Junk faxes are a form of telemarketing where unsolicited advertisements are sent via fax transmission. Junk faxes are the faxed equivalent of spam or junk mail. Proponents of this advertising medium often use the terms ''broadcast fax'' or ''fax a ...
*
Radiofax Radiofacsimile, radiofax or HF fax is an analogue mode for transmitting monochrome images via high frequency (HF) radio waves. It was the predecessor to slow-scan television (SSTV). It was the primary method of sending photographs from remote s ...
—image transmission over HF radio *
Slow-scan television Slow-scan television (SSTV) is a picture transmission method, used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color. A literal term for SSTV is narrowband television. Analog broadcast tel ...
*
T.38 T.38 is an 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 across IP networks between existing Group ...
Fax-over-IP *
Telautograph The telautograph is an analog precursor to the modern fax machine. It transmits electrical impulses recorded by potentiometers at the sending station to servomechanisms attached to a pen at the receiving station, thus reproducing at the receivin ...
*
Telex The telex network is a station-to-station switched network of teleprinters similar to a telephone network, using telegraph-grade connecting circuits for two-way text-based messages. Telex was a major method of sending written messages electroni ...
*
Teletex Teletex was ITU-T specification F.200 for a text and document communications service that could be provided over telephone lines. It was rapidly superseded by e-mail but the name ''Teletex'' lives on in several of the X.500 standard attributes u ...
* Transmitting Subscriber Identification (TSID) *
Wirephoto Wirephoto, telephotography or radiophoto is the sending of pictures by telegraph, telephone or radio. Édouard Belin's Bélinographe of 1913, which scanned using a photocell and transmitted over ordinary phone lines, formed the basis for the W ...
*
3D Fax 3D Fax is a computer program, developed for Microsoft Windows by InfoImaging Technologies in the mid-1990s, for file transfer via fax. The program encodes a file into an image, which the user would then print and send via a fax machine or transmit ...


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 ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it ...
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 American inventions Computer peripherals English inventions German inventions Italian inventions ITU-T recommendations Japanese inventions Office equipment Scottish inventions Telecommunications equipment