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Fax (short for facsimile - from Latin "fac simile", "make similar", i.e. "make a copy" - or telefacsimile) is a telecommunications technology used to transfer copies (facsimiles) of documents, especially using affordable devices operating over the telephone network. The word telefax is also used as a synonym.
A fax machine is essentially an image scanner, a modem, and a computer printer combined into a highly specialized package. The scanner converts the content of a physical document into a digital image, the modem sends the image data over a phone line, and the printer at the other end makes a duplicate of the original document.
Fax machines with additional electronic features can connect to computers, can be used to scan documents into a computer, and to print documents from the computer. Such high-end devices are called multifunction printers and cost more than fax machines.
Although fax machines of some sort or another have existed since the mid-late 19th century (see "History" below), modern fax technology became feasible only in the mid-1970s as the sophistication and cost of the three underlying technologies dropped to a reasonable level. Fax machines first became popular in Japan, where they had a clear advantage over competing technologies like the teleprinter; 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.
However, although most businesses still maintain some kind of fax capability, the technology appears increasingly dated in the world of the Internet.
- 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 98 scan lines per inch. Group 1 fax machines are obsolete and no longer manufactured.
- Group 2 faxes conform to the ITU-T Recommendations T.30 and T.3. Group 2 faxes take three minutes to transmit a single page, with a vertical resolution of 100 scan lines per inch. Group 2 fax machines are almost obsolete, and not manufactured. Group 2 fax machines can interoperate with Group 3 fax machines.
- 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 synchronise). 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
- Horizontal: 100 scan lines per inch
- 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.
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 2.1 fax devices are referred to as "super G3"; they seem to be a little faster than the other 2 classes.
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.
|ITU Standard||Released Date||Data Rates (bit/s)||Modulation Method|
|V.29||1988||9600, 7200, 4800||QAM|
|V.17||1991||14400, 12000, 9600, 7200||TCM|
Note that 'Super Group 3' faxes use V.34bis modulation that allows a data rate of up to 33.6 kbit/s.
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 (MH) is a codebook-based run-length encoding scheme optimised to efficiently compress whitespace. As most faxes consists mostly of white space, this minimises the transmission time of most faxes. Each scanned line is compressed independently of its predecessor and successor.
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.
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.
Matshushita Whiteline Skip
A proprietary compression scheme employed on Panasonic fax machines is Matshushita 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.
Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 100x200 or 200x200 dots per inch. The transfer rate is 14.4 kilobits per second (kbit/s) or higher (but fax machines support speeds beginning with 2400 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 compressed using a modified Huffman code 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.
There are different fax classes, including Class 1, Class 2 and Intel CAS.
Fax machines from the 1970s to the 1990s often used direct thermal printers as their printing technology, but since the mid-1990s there has been a transition towards thermal transfer printers, inkjet printers and 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 for 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.
Frederick Bakewell made several improvements on Bain's design and demonstrated the device at the 1851 World's Fair in London.
As a designer for the Radio Corporation of America (RCA), in 1924, Richard H. Ranger invented the wireless photoradiogram, or transoceanic radio facsimile, the forerunner of todayís "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 Rangerís product began two years later. Radio fax is still in common use today for transmitting weather charts and information.
- 3D Fax
- error correction mode (ECM)
- Fax server
- List of fax software
- Junk fax
- Black fax
- Call Subscriber ID (CSID)
- A Brief History of Facsimile
- What is Fax and History of Fax
- FAQ: How can I send a fax from the Internet?
- Group 3 Facsimile Communication a '97 essay with technical details on compression and error codes, and call establishment and release.
- World's First Color Fax Machine
- Scanning & faxing dpi question