Editing Fax (section)

=== Typical characteristics ===
Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a [[Optical resolution|resolution]] of 204×98 (normal) or 204×196 (fine) dots per square inch. The transfer rate is 14.4&nbsp;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. Group 3 faxes have the [[Filename extension|suffix]] <code>.g3</code> and the [[MIME type]] <code>image/g3fax</code>.

The most basic fax mode transfers in black and white only. The original page is scanned in a resolution of 1728 [[pixel]]s/line and 1145 lines/page (for [[A4 paper size|A4]]). The resulting raw data is [[data compression|compressed]] using a modified [[Huffman coding|Huffman code]] optimized for written text, achieving average compression factors of around 20. Typically a page needs 10&nbsp;s for transmission, instead of about three minutes for the same uncompressed raw data of 1728×1145 bits at a speed of 9600&nbsp;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.