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The Technology

Facsimile transmission typically involves a pair of standalone fax transmitter/receivers with a built-in modems, with transmission generally taking place across the analog Public Switched Telephone Network (PSTN). The transmitting fax scans the image document, looking for dots of black and white (some systems will also support 256 levels of grayscale). Through the modem, these variations are translated into modulations of analog sine waves, over an analog network. The receiving device reads the analog sine waves through a modem. The temperature of the output device of the receiving fax machine is varied to cause the image to be reproduced on chemically treated paper. In other words, fax transmission generally is an analog technology that is electrothermochemical in nature.

Group I and II (Table 5.1) fax machines, the first generations, employed obsolete electrochemical processes. As they were based on international standards developed by the CCITT (now ITU-T), compatibility was achieved within each Group. Transmission was on an analog basis, with Group I machines using Frequency Modulation (FM)—1500 Hz white frequency and 2400 Hz black frequency. Group II machines employed Amplitude Modulation (AM) at 2100 Hz.

Table 5.1 Facsimile generations and characteristics

Generation (ITU-T Group) Transmission Speed* (per page)Compatibility Resolution: Lines per Inch (LPI)

Group I 4–6 Minutes Group I 100
Group II 2–3 Minutes Group II 100
Group III [lte]1 Minute Groups I, II, and III 200
Group IV 3–4 Seconds Groups I, II, III, and IV 200–400


*Sensitive to density of image

Group III fax machines, which account for approximately 98% of all devices currently in use, conform to the above described electrothermochemical process, although plain-paper facsimile machines are essentially fax laser printers. Group III facsimile machines also are manufactured according ITU-T standards; thereby obviating issues of incompatibility. Alternatives to this standard description do exist, however. Group III devices transmit compressed digital data at 4800 to 9600 bps over analog lines through the use of an ITU-T standard modem protocol.

Group IV fax machines, which are highly specialized and relatively expensive fax computer systems, are designed to make use of digital circuits in order to speed the process and improve quality through compressed digital transmission at rates up to 64 Kbps. Group IV fax machines also are Group III-compatible and can be connected to analog circuits, as well. Table 5.1 provides a view of the characteristics of each ITU-T fax generation [5-1] [5-2].

Fax Boards and Fax Software (Group III/IV-compatible) are available for PCs and other computer systems. When sending a computer-based fax document, the fax software instructs the fax board to print the fax to a remote fax machine, rather than a printer. The computer fax board contains a fax modem, enabling any computer file to be transmitted to another similarly equipped computer or to a fax machine through fax emulation (imitation). The cost of such boards is quite reasonable, often less than $100. Fax software and fax boards often are included in a suite of application software packaged with a PC. WinFax and BitFax are included with Microsoft Windows, pre-packaged with most IBM-compatible PCs. Client/server versions include fax software resident on client workstations, with a relatively small number of fax boards residing on the fax server. The server may be a dedicated fax server or may be a partition of a file server. The fax software in the client workstations send the fax documents across a LAN to the fax server (see Figure 5.1), which queues them as required before transmission, thereby considerably reducing hardware and circuit costs [5-3], [5-4], and [5-5].


Figure 5.1  Facsimile transmission in a fax server environment.

There are however several drawbacks associated with these approaches. First, the transmitted documents either must exist as computer files or must be scanned and converted to computer files through the use of peripheral equipment. Second, the receiving fax computers must be left on and must be networked continuously in order to be accessible. Third, the process of fax receipt may interrupt other applications in progress at the receiving computer workstation. Finally, the received documents are memory-intensive image files, which must be converted to text files in order to be fully effective. Assuming a high-resolution fax at 400 lpi (lines per inch), a typical page of text requires approximately 50 KB. A single page of a graphic-intensive document can require many MBs of storage.


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