Fax (short for
facsimile, from
Latin fac simile, "make similar",
i.e. "make a copy") is a
telecommunications technology used to
transfer copies (
facsimiles) of
documents, especially using affordable devices operating over the
telephone network. The word
telefax, short for
telefacsimile,
for "make a copy at a distance", is also used as a
synonym. Although
fax is not an
acronym (as it is abbreviated from
facsimile), it is often written as “FAX”. The
device is also known as a telecopier in certain industries. When
sending documents to people at large distances, faxes have a
distinct advantage over postal mail in that the delivery is nearly
instantaneous, yet its disadvantages in quality have relegated it
to a position beneath
email as the prevailing
form of electronic document transfer except where the legal status
of a sent fax and its accompanying sending report are
desired.
Overview
A "fax machine" usually consists of an
image scanner, a
modem,
and a
printer.
Although devices for transmitting printed documents electrically
have existed, in various forms, since the 19th century (see
"History" below), modern fax machines
became feasible only in the mid-1970s as the sophistication
increased and cost of the three underlying technologies dropped.
Digital
fax machines first became popular in Japan
, where they
had a clear advantage over competing technologies like the teleprinter, since at the time (before the
development of easy-to-use input
method editors) it was faster to handwrite kanji than to type the characters. Over time,
faxing gradually became affordable, and by the mid-1980s, fax
machines were very popular around the world.
Although businesses usually maintain some kind of fax capability,
the technology has faced increasing competition from
Internet-based systems. However, fax machines still
retain some advantages, particularly in the transmission of
sensitive material which, due to mandates like
Sarbanes-Oxley and
HIPAA,
cannot be sent over the Internet unencrypted . In some countries,
because
electronic signatures
on contracts are not recognized by law while faxed contracts with
copies of signatures are, fax machines enjoy continuing support in
business.
In many corporate environments, standalone fax machines have been
replaced by "
fax servers" and other
computerized systems capable of receiving and storing incoming
faxes electronically, and then routing them to users on paper or
via an
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.
Capabilities
There are several different indicators of fax capabilities: Group,
class, data transmission rate, and conformance with
ITU-T (formerly
CCITT)
recommendations.
Fax machines utilize standard
PSTN lines and
telephone numbers.
Group
Analogue
Group 1 and 2 faxes were sent in the same manner as a frame of
analogue television, with each
scanned line transmitted as a continuous analogue signal.
Horizontal resolution depended upon the quality of the scanner,
transmission line, and the printer. Analogue fax machines are
obsolete and no longer manufactured. ITU-T Recommendations T.2 and
T.3 were withdrawn as obsolete in July 1996.
- Group 1 faxes conform to the ITU-T Recommendation T.2. Group 1
faxes take six minutes to transmit a single page, with a vertical
resolution of 96 scan lines per inch.
Group 1 fax machines are obsolete and no longer manufactured.
- Group 2 faxes conform to the ITU-T Recommendations T.30 and
T.3. Group 2 faxes take three minutes to transmit a single page,
with a vertical resolution 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
Group 3 and 4 faxes are digital formats, and take advantage of
digital compression methods to greatly reduce transmission times.
- Group 3 faxes conform to the ITU-T Recommendations T.30 and
T.4. Group 3 faxes take between six and fifteen seconds to transmit
a single page (not including the initial time for the fax machines
to handshake and synchronize). The horizontal and vertical
resolutions are allowed by the T.4 standard to vary among a set of
fixed resolutions:
- Horizontal: 100 scan lines per inch
- Vertical: 100 scan lines per inch
- 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.72, T.411 to T.417. They are
designed to operate over 64 kbit/s digital ISDN
circuits. Their resolution is determined by the T.6 recommendation,
which is a superset of the T.4 recommendation.
Fax Over IP (FOIP) can transmit and receive pre-digitized documents
at near realtime speeds. Scanned documents are limited to the
amount of time the user takes to load the document in a scanner and
for the device to process a digital file. The resolution can vary
from as little as 150 DPI to 9600 DPI or more. This type of faxing
is not like the e-mail to fax service that still uses fax modems at
least one way.
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 fax devices do fax data transfer where 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.
- Class 2 fax devices perform T.30 session management themselves,
but the T.4/T.6 data compression is performed by software on a
controlling computer. The relevant ITU-T recommendation is
T.32.
- Class 2.0 is different from Class 2.
- Class 2.1 is an improvement of Class 2.0. Class 2.1 fax devices
are referred to as "super G3"; they seem to be a little faster than
Class 1/2/2.0.
- Class 3 fax devices are responsible for virtually the entire
fax session, given little more than a phone number and the text to
send (including rendering ASCII text as a raster image). These
devices are not common.
Data transmission rate
Several different telephone line modulation techniques are used by
fax machines. They are negotiated during the fax-
modem handshake, and the
fax devices will use the highest data rate that both fax devices
support, usually a minimum of 14.4 kbit/s for Group 3 fax.
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Note that 'Super Group 3' faxes use
V.34bismodulation 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 are:
Modified Huffman
Modified Huffman (MH) 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 (MR) 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. MR 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 two for 'Standard' resolution faxes, and four
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 for 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. In this case, there is no maximum number of
lines for which the differences are encoded.
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.
Typical characteristics
Group 3 fax machines transfer one or a few printed or handwritten
pages per minute in black-and-white (bitonal) at a
resolutionof 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(formerly CCITT) fax group 3 or 4.
The most basic fax mode transfers black and white only. The
original page is scanned in a resolution of 1728
pixels/line and 1145 lines/page (for
A4). The resulting raw data is
compressedusing a modified
Huffman codeoptimized 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.
Fax machines from the 1970s to the 1990s often used direct
thermal printersas their printing
technology, but since the mid-1990s there has been a transition
towards
thermal transfer
printers,
inkjet printersand
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-T30efor
faxing in color; unfortunately, it is not yet widely supported, so
many of the color fax machines can only fax in color to machines
from the same manufacturer.
Fax paper
As a security precaution, thermal fax paper is typically not
admissible as evidence in a court of law unless photocopied. This
is because the ink used on fax paper is eradicable and brittle, and
it tends to come off over long periods of storage.
Alternatives
One popular alternative is to subscribe to an
internet faxservice. Fax service providers
allow users to send and receive faxes from their personal computers
using an existing email account. No software, fax server or fax
machine is needed. Faxes are received as attached
.TIFor
.PDFfiles, 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 even offer secure
faxing to comply with stringent
HIPAAand
Gramm-Leach-Bliley
Actrequirements to keep medical information and financial
information private and secure. Utilizing a fax service provider
does not require paper, a dedicated fax line, or consumables.
Another alternative to a physical fax machine is to make use of
computer
softwarewhich allows people to
send and receive faxes using their own computers. See
Fax server,
Unified
messagingand
internet fax.
History
Wire transmission
Scottish inventor
Alexander
Bainworked on chemical mechanical facsimile type devices and in
1846 was able to reproduce graphic signs in lab experiments.
Frederick Bakewellmade several
improvements on Bain's design and demonstrated his device at the
1851
Great Exhibitionin London.
Bain and Bakewell's systems were inferior and could reproduce only
poor quality images. They lacked synchronization between the
transmitting mechanism and the receiving mechanism. In 1861, the
first practical operational electro-mechanical commercially
exploited telefax machine, the
Pantelegraph, was invented by the Italian
physicist
Giovanni Caselli. He
introduced the first commercial telefax service between Paris and
Lyon in 1865, some 11 years before the invention of workable
telephones.
In 1881, English inventor
Shelford
Bidwellconstructed the
scanning phototelegraphthat was
the first telefax machine to scan any two-dimensional original, not
requiring manual plotting or drawing anymore. Around 1900, German
physicist
Arthur Korninvented the
Bildtelegraph,
widespread in continental Europe especially since 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
Bélinografby
Édouard Belinfirst, then since the 1930s
the
Hellschreiber, invented
in 1929 by
Rudolf Hell, a pioneer in
mechanical image scanning and transmission.
Wireless transmission
As a designer for the
Radio
Corporation of America(RCA), in 1924,
Richard H.Rangerinvented the wireless
photoradiogram, or transoceanic
radio
facsimile, the forerunner of today’s "Fax" machines. A
photograph of President
Calvin
Coolidgesent 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. Radio fax is still in common use today for transmitting
weather charts and information. Also in 1924,
Herbert E.Ivesof
AT&Ttransmitted and reconstructed the first
color facsimile, using color separations.
Telephone transmission
Prior to the introduction of the ubiquitous fax machine, one of the
first being the
ExxonQwip 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
photocellso that the
current in a circuit would vary with the amount of light. This
current was used to control a tone generator (a
modulator), the current determining the frequency
of the tone produced. This audio tone was then transmitted using an
acoustic coupler(a speaker, in this
case) attached to the microphone of a common
telephone handset. At the receiving end, a handset’s
speaker was attached to an acoustic coupler (a microphone), and a
demodulatorconverted the varying tone
into a variable current which 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. A pair of these
expensive and bulky machines could only be afforded by companies
with a serious need to communicate drawings, design sketches or
signed documents between distant locations, such as an office and
factory.
Computer facsimile interface
In 1985, Dr.
Hank Magnuski, founder of
GammaLink, produced the first computer fax
board, called
GammaFax.
See also
References
- Istituto Tecnico Industriale, Rome, Italy. Italian
biography of Giovanni Caselli
- The Hebrew University of Jerusalem - Giovanni
Caselli biography
-
http://www.nytimes.com/1985/02/22/business/an-exxon-sale-to-harris-unit-the-exxon-corporation-said.html
- http://www.mahalo.com/qwip-machine
External links
| ITU Standard |
| Released Date |
| Data Rates (bit/s) |
| Modulation Method |
|
| V.27 |
| 1988 |
| 4800, 2400 |
| PSK |
|
| V.29 |
| 1988 |
| 9600, 7200, 4800 |
| QAM |
|
| V.17 |
| 1991 |
| 14400, 12000, 9600, 7200 |
| TCM |
|
| V.34 |
| 1994 |
| 28800 |
| QAM |
|
| V.34bis |
| 1998 |
| 33600 |
| QAM |
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