fAx

Fax, short for facsimile, is a method of transmitting scanned printed material over a telephone network or other communications channels to another location, where it is printed or stored. A faxed document is created by scanning a page into a raster image, encoding it, and transmitting it as signals that travel over telephone lines or digital networks; at the receiving end the signal is decoded and printed or saved.

Most fax systems today rely on ITU-T standards such as T.30 for call signaling and Group 3 (G3) or Group 4 (G4) compression. G3 uses 2D image compression and is designed for slower dial-up lines, whereas G4 is optimized for high-speed digital networks. Fax over IP, commonly using the T.38 protocol, allows real-time fax transmission over IP networks. Many devices also support error correction mode to improve reliability on noisy connections.

History of fax traces back to the 19th century with the pantelegraph developed by Giovanni Caselli. Modern practical facsimile systems emerged in the mid-20th century, with plain-paper fax machines becoming widespread in the 1980s following the adoption of Group 3 standards. The advent of digital communication reduced dependence on fax, but the technology remains in use in certain sectors due to regulatory, legal, or workflow requirements.

In contemporary use, fax is implemented via standalone machines, multifunction devices, fax servers, and online or fax-over-IP services (often marketed as e-fax). Advantages include broad interoperability and legal acceptance in many jurisdictions; disadvantages include ongoing hardware costs, paper use, and security concerns, since many systems lack built-in encryption by default.