Group Standards

The widespread use of fax in business is a more recent phenomenon, and its growth parallels that of the computer for much the same underlying reason. Desktop computers did not take off until the industry found a standard to follow—the IBM computer. Similarly, the explosive growth of fax began only after the CCITT adopted standards for the transmission of facsimile data.

Analog

The original system, now termed Group 1, was based on analog technology and used frequency shift keying, much as 300 baud modems do, to transmit a page of information in six minutes. Group 2 improved upon that analog technology and doubled the speed of transmission, up to three minutes per page.

Group 3

The big break with the past was the CCITT's adoption in 1980 of the Group 3 fax protocol, which is entirely digitally based. Using data compression and modems that operate at up to 14,400 bits per second, full-page documents can be transmitted in 20 to 60 seconds using the Group 3 protocol. New transmission standards promise to pump up the basic Group 3 data rate to 28,800 bits per second.

Resolution

Under the original Group 3 standard, two degrees of resolution or on-paper sharpness are possible: standard, which allows 1728 dots horizontally across the page (about 200 dots per inch) and 100 dots per inch vertically, and fine, which doubles the vertical resolution to achieve 200 by 200 dpi and requires about twice the transmission time. Fine resolution also approximately doubles the time required to transmit a fax page because it doubles the data that must be moved.

Revisions to the Group 3 standard have added more possible resolutions. Two new resolutions compensate for the slight elongation that creeps into fax documents when generated and transmitted in purely electronic form. New fax products may optionally send and receive at resolutions of 204 by 98 pixels per inch in standard mode or 204 by 196 pixels per inch in fine mode. Two new high-resolution modes of 300 by 300 pixels per inch and 400 by 400 pixels per inch were also established. The 300 by 300 mode enables fax machines, laser printers, and scanners to share the same resolution levels for higher quality when transferring images between them. To take advantage of these resolutions, both sending and receiving fax equipment must support the new modes.

Data Rates

The basic speed of a Group 3 fax transmission depends on the underlying communications standard that the fax product follows. These standards are similar to data modem standards. With the exception of V.34, data and fax modems operate under different standards, even when using the same data rates. Consequently, data and fax modems are not interchangeable, and a modem that provides high-speed fax capabilities (say, 9600 bps) may operate more slowly in data mode (say, 2400 bps).

The Group 3 protocol does not define a single speed for fax transmissions but allows the use of any of a variety of transmission standards. At data rates of 2400 and 4800 bits per second, fax modems operate under the V.27 ter standard (note that ter stands for tertiary). At 7200 and 9600 bits per second, they follow V.29 (or V.17, which incorporates these V.29 modes). At 12,000 and 14,400 bits per second, fax modems follow V.17. The V.34 standard will take both fax and data modems up to 28,800 bits per second. New standards will allow the use of the Group 3 fax protocol over ISDN and other future digital telephone services.

Fax modems are typically described by the communications standards they support or by the maximum data rate at which they can operate. Most modern fax modems follow the V.17 standard, which incorporates the lower V.29 speeds. Most will also fall back to V.27 ter to accommodate older, slower fax products.

Compression

In a typical fax machine, you slide a page into the machine, place the call, and the machine calls a distant number. Once the connection is negotiated, the fax machine scans the page with a photodetector inside the machine, which detects the black and white patterns on the page one line at a time at a resolution of 200 dots per inch. The result is a series of bits with the digital ones (1) and zeros (0) corresponding to the black and white samples each 1/200th of an inch. The fax machine compresses this raw data stream to increase the apparent data rate and shorten transmission times.

Data compression makes the true speed of transmitting a page dependent on the amount of detail that each page contains. In operation, the data-compression algorithm reduces the amount of data that must be transferred by a factor of five to ten. On the other hand, a bad phone connection can slow fax transmissions, as fax modems automatically fall back to lower speeds to cope with poor line quality.

Group 3 fax products may use any of three levels of data compression, designated as MH, MR, and MMR. The typical Group 3 fax product includes only MH compression. The others are optional, and MMR is particularly rare. To be sure that a given fax product uses MR or MMR, you will need to check its specifications.

MH stands for Modified Huffman encoding, which is also known as one-dimensional encoding. MH was built in to the Group 3 standard in 1980 so that a fax machine could send a full page in less than one minute using a standard V.27 ter modem that operated at 4800 bits per second. With 9600 bps modems, that time is cut nearly in half.

MR, or Modified Read encoding, was added as an option shortly after MH encoding was adopted. MR starts with standard MH encoding for the first line of the transmission but then encodes the second line as differences from the first line. Because with fine images, line data changes little between adjacent lines, usually little change in information is required. To prevent errors from rippling through an entire document, at the third line, MR starts over with a plain MH scan. In other words, odd-numbered scan lines are MH and even lines contain only difference information from the previous line. If a full line is lost in transmission, MR limits the damage to, at most, two lines. Overall, the transmission time savings in advancing from MH to MR amounts to 15 to 20 percent, the exact figure depending on message contents.

MMR, or Modified Modified Read encoding, foregoes the safety of the MR technique and records the entire page as difference data. Using MMR, the first line serves as a reference and is all white. Every subsequent line is encoded as the difference from the preceding line until the end of a page. However, an error in any one line will repeat in every subsequent line, so losing one line can garble an entire page. To help prevent such problems, MMR can incorporate its own error-correction mode (ECM) through which the receiving fax system can request the retransmission of any lines received in error. Only the bad lines are updated, and the rest of the page is reconstructed from the new data. MMR with ECM is the most efficient scheme used for compressing fax transmissions and can cut the time needed for a page transmission with MH in half.

Instead of individual dots, under MH (and therefore MR and MMR) the bit-pattern of each scan line on the page is coded as short line segments, and the code indicates the number of dots in each segment. The fax machine sends this run-length coded data to the remote fax machine. Included in the transmitted signal is a rudimentary form of error protection, but missed bits are not reproduced when the receiving fax machine reconstructs the original page.

The exact code used by MH under Group 3 fax uses four code groups—two for sequences of white dots and two for sequences of black dots. Sequences from 0 to 63 dots long are coded using terminating codes, which express the exact number of dots of the given color in the segment. If the segment of like-color dots scanned from the paper is longer than 63 dots, MH codes it as two code groups—a terminating code and a make-up code. The make-up code value indicates the number of 64-dot blocks in the single-color segment.

Binary File Transfer

More than just following the same modem standard, the capabilities of fax service are merging with those of standard data communications. New fax modems, for example, incorporate Binary File Transfer (BFT) capabilities, which enable them to ship BFT files from one fax system to another as easily as document pages. You could, for example, send a file from your computer to a printer for a remote printout or to a computer where it could be received automatically. The receiving fax modem picks up the line, makes the connection, and records the file as dutifully as it would an ordinary fax page—without anyone standing around to control the modem.

Group 4

In 1984, the CCITT approved a super-performance facsimile standard called Group 4, which allows resolutions of up to 400 by 400 dpi as well as higher-speed transmissions of lower resolutions. Although not quite typeset quality (phototypesetters are capable of resolutions of about 1200 dpi), the best of Group 4 is about equal to the resolving capability of the human eye at normal reading distance. However, today's Group 4 fax machines require high-speed dedicated lines and do not operate as dial-up devices. Group 3 equipment using new, higher-resolution standards and coupled to digital services offers a lower-cost alternative to Group 4.