Editing Fax (section)

== Capabilities ==
There are several indicators of fax capabilities: group, class, data transmission rate, and conformance with [[ITU-T]] (formerly [[CCITT]]) recommendations. Since the 1968 [[Carterfone decision]], most fax machines have been designed to connect to standard [[PSTN]] 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  ITU-T Recommendations T.2 and T.3 were withdrawn as obsolete in July 1996.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]]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 ====
[[File:Dacom DFC-10.jpg|thumb|upright=1.2|The Dacom DFC-10—the first digital fax machine<ref name="etd.ohiolink.edu"/>]]
[[File:Faxchip.jpg|thumb|upright=1.2|The [[Charge-coupled_device|CCD]] single-row image sensor in a fax machine. Only about one quarter of the length is shown. The thin line in the middle consists of photosensitive [[pixel]]s. The read-out circuit is at left.]]
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]] Rapidfax first sold in late 1960s, which incorporated digital data compression technology developed by [[Lockheed Corporation|Lockheed]] for transmission of images from satellites.<ref name="etd.ohiolink.edu"/><ref name="Fax 1971, Pages 112-114"/>

[[Group 4 compression|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&nbsp;3 faxes take between 6 and 15&nbsp;seconds to transmit a single page (not including the initial time for the fax machines to handshake and synchronize). They require sufficient [[random access memory]] to store two lines of scanned bits at a time.<ref name="huurdeman">{{cite book |last1=Huurdeman |first1=Anton A. |title=The Worldwide History of Telecommunications |date=31 July 2003 |publisher=John Wiley & Sons |isbn=978-0-471-20505-0 |page=516 |url=https://books.google.com/books?id=SnjGRDVIUL4C&pg=PA516 |language=en}}</ref> 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 are designed to operate over 64&nbsp;kbit/s digital [[ISDN]] circuits. They conform to the ITU-T Recommendations 
** T.563 (Terminal characteristics for Group 4 facsimile apparatus), 
** T.503 (Document application profile for the interchange of Group 4 facsimile documents),
** T.521 (Communication application profile BT0 for document bulk transfer based on the session service),
** T.6 (Facsimile coding schemes and coding control functions for Group 4 facsimile apparatus) specifying resolutions, a superset of the resolutions from T.4,<ref name="T6"/>
** T.62 (Control procedures for teletex and Group 4 facsimile services),
** T.70 (Network-independent basic transport service for the telematic services), and
** T.411 to T.417 (concerned with aspects of the [[Open Document Architecture]]). 

Fax Over IP ([[FoIP]]) can transmit and receive pre-digitized documents at near-realtime{{vague|date=March 2022}} speeds using [[T.38 ITU-T recommendation|ITU-T recommendation T.38]] to send digitised images over an [[IP network]] using [[JPEG]] compression. T.38 is designed to work with [[VoIP]] services and often supported by [[analog telephone adapter]]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&nbsp;DPI to 9600&nbsp;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.<ref name=peterson2000>{{cite book| first=Kerstin Day | last=Peterson | year=2000| title=Business telecom systems: a guide to choosing the best technologies and services| pages=191–192| publisher=Focal Press| isbn=1578200415| url=https://books.google.com/books?id=W79R0niNU5wC&pg=PA191| access-date=2011-04-02 }}</ref>
* 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&nbsp;2.0.<ref>{{Cite web | url = https://legacy.hylafax.org/site1//Modems/Supra/class2.html | title = Supra Technical Support Bulletin: Class 2 Fax Commands For Supra Faxmodems | date = June 19, 1992 | access-date = March 23, 2019}}</ref> All implementations of "Class 2" are manufacturer-specific.<ref name="Multi-Tech">{{Cite web |date=2017 |title=Fax Developer's Guide: Classes 2 and 2.0/2.1 |url=https://www.multitech.com/documents/publications/manuals/s000239.pdf |url-status=dead |access-date=2019-03-23 |publisher=Multi-Tech Systems|archive-url=https://web.archive.org/web/20200321191129/https://www.multitech.com/documents/publications/manuals/s000239.pdf |archive-date=21 March 2020 }}</ref>
* Class 2.0 is the official ITU-T version of Class&nbsp;2 and is commonly known as Class&nbsp;2.0 to differentiate it from many manufacturer-specific implementations of what is commonly known as "Class&nbsp;2". It uses a different but standardized command set than the various manufacturer-specific implementations of "Class&nbsp;2". The relevant ITU-T recommendation is T.32.<ref name="Multi-Tech"/>
* Class 2.1 is an improvement of Class&nbsp;2.0 that implements faxing over V.34 (33.6&nbsp;kbit/s), which boosts faxing speed from fax classes "2" and 2.0, which are limited to 14.4&nbsp;kbit/s.<ref name="Multi-Tech"/> The relevant ITU-T recommendation is T.32 Amendment&nbsp;1.<ref name="Multi-Tech"/> Class&nbsp;2.1 fax devices are referred to as "super&nbsp;G3".

=== Data transmission rate ===
Several different telephone-line modulation techniques are used by fax machines. They are negotiated during the fax-[[modem]] [[Handshake (computing)|handshake]], and the fax devices will use the highest data rate that both fax devices support, usually a minimum of 14.4&nbsp;kbit/s for Group&nbsp;3 fax.

:{| class="wikitable"
!ITU standard
!Released date
!Data rates (bit/s)
!Modulation method
|-
|[[ITU-T V.27|V.27]]
|1988
|4800, 2400
|[[phase-shift keying|PSK]]
|-
|[[ITU-T V.29|V.29]]
|1988
|9600, 7200, 4800
|[[quadrature amplitude modulation|QAM]]
|-
|[[ITU-T V.17|V.17]]
|1991
|{{val|14,400}}, {{val|12,000}}, 9600, 7200
|[[trellis modulation|TCM]]
|-
|[[ITU-T V.34|V.34]]
|1994
|{{val|28,800}}
|[[quadrature amplitude modulation|QAM]]
|-
|[[ITU-T V.34bis|V.34bis]]
|1998
|{{val|33,600}}
|[[quadrature amplitude modulation|QAM]]
|-
|[[ISDN]]
|1986
|{{val|64,000}}
|4B3T / 2B1Q (line coding)
|}

"Super Group&nbsp;3" faxes use V.34bis modulation that allows a data rate of up to 33.6&nbsp;kbit/s.

=== Compression ===
As well as specifying the resolution (and allowable physical size) of the image being faxed, the ITU-T&nbsp;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:<ref name="T4">{{cite web |url=http://www.itu.int/rec/T-REC-T.4/en |title=T.4: Standardization of Group 3 facsimile terminals for document transmission |publisher=ITU-T |date=2011-03-14 |access-date=2013-12-28}}</ref>
* [[Modified Huffman coding|Modified Huffman]] (MH).
* [[Modified READ]] (MR) (''Relative Element Address Designate''<ref>{{cite journal | url=https://ieeexplore.ieee.org/document/1456020 | doi=10.1109/PROC.1980.11751 | title=International digital facsimile coding standards | year=1980 | last1=Hunter | first1=R. | last2=Robinson | first2=A.H. | journal=Proceedings of the IEEE | volume=68 | issue=7 | pages=854–867 | s2cid=46403372 | url-access=subscription }}</ref>), optional.
An additional method is specified in T.6:<ref name="T6">{{cite web| url=http://www.itu.int/rec/T-REC-T.6/en |title=T.6: Facsimile coding schemes and coding control functions for Group 4 facsimile apparatus |publisher=ITU-T |date=November 1988 |access-date=2013-12-28}}</ref>
* [[Group 4 compression|Modified Modified READ]] (MMR).
Later, other compression techniques were added as options to ITU-T recommendation T.30, such as the more efficient [[JBIG]] (T.82, T.85) for bi-level content, and [[JPEG]] (T.81), T.43, [[Mixed raster content|MRC]] (T.44), and T.45 for grayscale, palette, and colour content.<ref name="T30">{{cite web |url=http://www.itu.int/rec/T-REC-T.30 |title=T.30: Procedures for document facsimile transmission in the general switched telephone network |publisher=ITU-T |date=2014-05-15 |access-date=2013-12-28}}</ref> Fax machines can negotiate at the start of the T.30 session to use the best technique implemented on both sides.

==== Modified Huffman ====
{{Main|Modified Huffman coding}}
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.<ref name="T4"/> 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.<ref name="T4"/>

==== Modified READ ====
Modified READ, specified as an optional two-dimensional coding scheme in T.4, encodes the first scanned line using MH.<ref name="T4"/> The next line is compared to the first, the differences determined, and then the differences are encoded and transmitted.<ref name="T4"/> 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 ====
{{Main|Group 4 compression}}
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.<ref name="T6"/> 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 [[Integrated Services Digital Network|ISDN]]. 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]] (ITU-T T.82) as another [[lossless]] [[Binary 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]] images.

[[JBIG]] 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&nbsp;=&nbsp;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 [[Optical resolution|resolution]] of 204×98 (normal) or 204×196 (fine) dots per square inch. The transfer rate is 14.4&nbsp;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]] (formerly CCITT) fax group 3 or 4. Group 3 faxes have the [[Filename extension|suffix]] <code>.g3</code> and the [[MIME type]] <code>image/g3fax</code>.

The most basic fax mode transfers in black and white only. The original page is scanned in a resolution of 1728 [[pixel]]s/line and 1145 lines/page (for [[A4 paper size|A4]]). The resulting raw data is [[data compression|compressed]] using a modified [[Huffman coding|Huffman code]] optimized for written text, achieving average compression factors of around 20. Typically a page needs 10&nbsp;s for transmission, instead of about three minutes for the same uncompressed raw data of 1728×1145 bits at a speed of 9600&nbsp;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]]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]]s, [[inkjet printer]]s and [[Laser printing|laser printers]].

One of the advantages of inkjet printing is that inkjets can affordably print in [[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 <ref>{{cite web| author=tsbmail |url=http://www.itu.int/rec/T-REC-T.30-200509-I/en |title=T.30 : Procedures for document facsimile transmission in the general switched telephone network |publisher=Itu.int |access-date=2014-02-16}}</ref>) 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.<ref>{{Cite web |last=Kang |first=Henry R. |date=2000 |title=Color Conversion between sRGB and Internet FAX Standards - |url=https://library.imaging.org/admin/apis/public/api/ist/website/downloadArticle/print4fab/16/1/art00062_2 |publisher=IS&Ts NIP16: 2000 International Conference on Digital Printing Technologies |publication-place=Peerless Systems Corporation, El Segundo, California}}</ref>

=== Stroke speed ===
Stroke speed in facsimile systems is the rate at which a fixed line perpendicular to the direction of [[Image scanning|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.<ref>{{FS1037C MS188}}</ref>

=== Fax paper ===
[[File:ThermalPaperG3-TMG.JPG|thumb|[[Thermal paper|Paper]] roll for [[thermal printing|direct thermal]] fax machine]]
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.<ref>"4.12 Filing rules: 19.Newspaper extracts or thermal facsimile paper should not be preserved as archives. Such extracts should be photocopied and the copy preserved. The original can then be destroyed." Office of Corporate & Legal Affairs, University College Cork, Ireland</ref>

=== Fax tone ===

{{expand section|reason=there are others|date=October 2023}}

A CNG tone is an 1100&nbsp;Hz tone transmitted by a fax machine when it calls another fax machine. Fax tones can cause complications when implementing [[fax over IP]].