A fax is the transmission of one or several pages over the telephone network or by radio fax enterprise in the amateur radio. As transmitters and/or receivers thereby mostly similar fax devices, one serve can however also from a PC by means of fax servers and modem or fax over IP of faxes send away and receive.

Thereby the standard G2 or G3 is used (see CCITT Recommendation T.4 Standardization OF Group 3 facsimile Apparatus for to document transmission and T.30-Protokoll).

The word fax is a of fax, which is again a from Telefaksimile (facsimile compares), thus a remote picture copy, therefore also the German designation remote copy.

Technology and standardization

For ISDN existed with the standard G4 a standard for a digital fax, which works with a clearly higher speed (64 kbit/s instead of to 14,400 bit/s), is in practice however used only rarely.

The type of MIME reads image/g3fax, the file label .g3.

Programs, with which at the PC entered faxes can be regarded, are e.g. under X xli and kfax, under Mac OS X FAXstf (or the system program "Vorschau.app") and under Windows IrfanView. There is also fax software, which works with TIFF or SFF files.

Coding and compression of the data

In black/white faxes the picture information becomes loss-free with the algorithms MH (Modified Huffman, RLE been based), MR (Modified READ, also called "fax Group 3" or linear coding) or MMR (Modified Modified READ, also called "fax Group 4" or two-dimensional coding) compresses. The compression rates are dependent on the graphic data, generally supply with to MMR however the best compression, followed from MR. Those MH-coding which can be implemented relatively simply is today still used, in order to communicate with older fax devices or with fax software over the CAPI interface.

The picture information in color faxes is usually transferred in the lossy JPEG format. Loss-free JPEG compression is intended, however as well as is not used. The color information is coded thereby in the lab color area. Gray tone pictures contain only the L-component.

Paper formats and dissolutions

With the Negoziierung the paper format used by the transmitter is communicated to the receiver. This determines the length of the scanned line (Scanline). Together with the number of scanned pixels per line the dissolution results. After T.4 the following horizontal dissolutions are possible:

• For ISO A4, type character and legal
  • 1728 pixels on a Scanline of 215 mm 1% tolerance) (s/w and color)
  • 3456 pixels on a Scanline of 215 mm 1% tolerance) (s/w and color)
  • 2592 pixels on a Scanline of 215 mm 1% tolerance) (only for color fax)
• For ISO B4
  • 2048 pixels on a Scanline of 255 mm 1% tolerance) (s/w and color)
  • 4096 pixels on a Scanline of 255 mm 1% tolerance) (s/w and color)
  • 3648 pixels on a Scanline of 255 mm 1% tolerance) (only for color fax)
• For ISO A3
  • 2432 pixels on a Scanline of 215 mm 1% tolerance) (s/w and color)
  • 4864 pixels on a Scanline of 215 mm 1% tolerance) (s/w and color)
  • 3648 pixels on a Scanline of 215 mm 1% tolerance) (only for color fax)

Possible vertical dissolutions are 3.85 Zeilen/mm, 7.7 Zeilen/mm and 15.4 Zeilen/mm for black/white faxes as well as 7.7 Zeilen/mm and 15.4 Zeilen/mm for color faxes. In annex E to T.4 not only the dissolutions 200 become dpi and 400 dpi, but additionally 300 dpi, which corresponds to 11.55 Zeilen/mm, certified for color faxes. With color faxes the restriction applies that horizontal and vertical dissolution must agree.

The two most common black/white formats are:

• 1728 pixels with 203 or 204 dpi horizontal; 98 dpi vertically (low dissolution)
• 1728 pixels with 203 or 204 dpi horizontal; 196 dpi vertically (high resolution)

Color faxes are few common.

Simply post and converting show that the line lengths and dissolutions specified in T.4 are inaccurate or contradictory. So 1728 pixels a broad 200-dpi-Zeile must be about 219.5 mm long, which corresponds to the US formats type character and legal, but is not representable on A4. In order to represent a line with 1728 pixels on A4, a dissolution of 204 is needed dpi. In various file formats, which are suitable for storing fax documents (e.g. Tiff), for fax documents frequently horizontal dissolutions of 200, 203 are used dpi or 204 dpi as well as vertical dissolutions of 98/100 dpi and 196/200 dpi.

The paper format determines only the horizontal dimensions of the fax document. The length of a fax side is not fixed. It can amount to few lines or extend over several pages, so that for example a bilateral fax document on three or more pages must be printed out.

Most devices are not in a the position to print a side on over their entire width or in scanning. The fax standard meets this fact, as the transmission of some millimeters of the side edges is not guaranteed.

Fax minutes

The T.30-Protokoll is the basis of the G3-Faxstandards, which makes communication possible between fax devices, fax modems, fax maps and fax servers.

In this minutes becomes the connection establishment, which defines transmission of call number, date and time as well as the receipt acknowledgement. The transmission can take place thereby with a data transmission rate from 2400 bit/s to 33,600 bit/s.

During the transmission of a fax data are converted into tones. So that the devices understand themselves, fax minutes specify, like transmitters and receivers react one on the other (handshake).

CNG (Calling clay/tone)

To minutes in accordance with group 3 (S. o.) the transmitter should announce itself to the receiver with a signal of the frequency 1100 cycles per second, in each case from 0,5 s duration and 3 s is enough for tracing, in order to signal a fax transmission.

CED (Called terminal identification)

The call answer the receiving fax equipment with one approx. to 3 s are enough 2100 cycle per second for clay/tone (see clay/tone example, first signal).

DIS (digital identification signal)

Immediately after the CED clay/tone the called equipment lets a signal of 2s length follow and repeats it in the 3s distance, if the caller does not react. The signal contains data concerning the equipment in V.21 FSK modulating (300 Bit/s) and its receipt characteristics (see clay/tone example, second and third signal).

DCS (digital COMMAND signal)

The caller transfers for his part its capability characteristics. On these both devices specify the optimum transmission standard for mutual communication.

Training and TCF (training check function)

The caller sends test datas, which it terminates with a TCF signal.

CFR (Confirmation ton receive)

The called FAX acknowledges with CFR, if it could understand the data.

Training, data and eop (end to OF procedure)

The caller sends the data and locks the transmission with eop.

MCF (Message confirmation)

The receiver confirms the receipt with MCF.

DCN (Disconnect)

The caller terminates the data link.

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