Fax terms explained

ITU-T standards with code-like names, signals known only by their initials, line settings: faxing has a jargon of its own. Here are fourteen terms you will come across in a device manual, on an error report or when talking to your phone provider, each explained in a few lines.

Group 3 (G3)

The family of standards behind modern fax since 1980: digital transmission of a page over an ordinary phone line in under a minute. Analogue Groups 1 and 2 have vanished; Group 4, designed for ISDN, never caught on.

T.30

ITU-T recommendation that runs the call: opening tones, capability exchange, line test, acceptance of each page and call clear-down. Think of it as the conversation protocol between two fax machines.

T.4 and T.6

The recommendations covering image coding. T.4 describes scanning and the MH (one line at a time) and MR (difference from the previous line) compression schemes; T.6 adds the more efficient MMR, which G3 machines use alongside ECM.

T.38

The fax-over-IP protocol: a gateway converts the modem signal into data, sends it in packets, then rebuilds a fax signal at the far end. Essential if a fax machine is to work reliably behind a broadband router.

ECM (Error Correction Mode)

The page is split into numbered blocks; the receiver reports which ones arrived damaged, and only those are resent. Slower on a bad line, but with no streaks or missing lines.

DIS and DCS

The two key negotiation frames. With DIS (Digital Identification Signal), the called machine lists what it can do; with DCS (Digital Command Signal), the caller announces the settings it has chosen for the rest of the call.

CNG and CED

The opening tones of a fax call. CNG (1,100 Hz, one beep every three seconds) identifies a calling device; CED (2,100 Hz, continuous) marks a fax machine answering. They are what lets a switchboard route a call to the fax line.

V.29, V.17, V.34

The modulations that carry the page: 9,600 bit/s for V.29, 14,400 for V.17, up to 33,600 for V.34 (so-called "Super G3" devices). The higher the speed, the cleaner the line must be.

Standard, fine and superfine resolution

Three vertical densities for the same width of 1,728 dots per line: about 98, 196 and 391 lines per inch. Fine mode is the sensible choice for small print; superfine is mostly for drawings and detailed signatures.

PSTN

The public switched telephone network, historically built on copper, where fax was born. European operators are dismantling it in favour of fibre and voice over IP, which forces fax machines to go through a gateway.

G.711

The uncompressed audio codec of digital telephony (64 kbit/s). When T.38 is not available, passing a fax over IP in G.711 often works, provided the network drops very few packets.

TSI and CSI

The identifiers two fax machines exchange at the start of a call: TSI for the sender, CSI for the receiver. They are usually the fax number, appear on the device's display and are printed on the transmission report.

Transmission report

The summary the sending side produces at the end of the call: number dialled, date, time, duration, page count and outcome. When every page has been accepted by the receiver, it confirms successful delivery.

Cover sheet

The page placed at the front of a fax: who is sending, to whom, about what, and how many pages follow. It lets the receiving office route the document without reading it in full.