Fax glossary

From ITU-T to codenames, from signals reduced to initials to line settings: fax speaks its own language. Here are fourteen terms you will meet in a machine manual, on an error log, or when talking to your operator. Each takes under a minute to read.

Group 3 (G3)

The set of standards governing fax since 1980: a digitised page, carried in under a minute over a plain telephone line. G3 eclipsed the analogue Groups 1 and 2, just as Group 4, designed for ISDN, stayed a niche.

T.30

The ITU-T recommendation that governs every phase of the call: handshake tones, capability negotiation, circuit check, page-by-page confirmation, and call teardown. Put another way, it's the “dialogue” between two fax machines.

T.4 and T.6

The recommendations dedicated to image coding. Under T.4 come scanning and two compressions, MH (line by line) and MR (compared with the previous line); T.6 adds MMR, more efficient, which G3 machines use in ECM mode.

T.38

T.38, also called the “fax over IP” protocol: a gateway turns the modem signal into data packets, transports them, and rebuilds a fax signal at the receiving side. Without T.38, a fax machine sitting behind a broadband router remains a gamble.

ECM (error correction)

Every page gets divided into numbered blocks. The receiving end flags any that arrive corrupted, and only those blocks are re-sent. This makes it slower on a poor line, but you get no streaks or missing lines.

DIS and DCS

The central frames of the handshake: the machine called lists its capabilities in the DIS (Digital Identification Signal); the caller then sets the parameters in the DCS (Digital Command Signal).

CNG and CED

Two tones characterise the opening: CNG (1,100 Hz, beeps repeated every three seconds) on the caller's side, CED (2,100 Hz, continuous) on the receiver's. An answering machine uses it to route the call to the fax.

V.29, V.17, V.34

V.29 tops out at 9,600 bit/s, V.17 at 14,400, V.34 at 33,600 — the “Super G3” club. Faster speeds demand a cleaner line.

Standard, fine, superfine resolutions

Three vertical densities at a constant width of 1,728 dots: roughly 98, 196 or 391 lines per inch. Fine mode balances legibility and speed; superfine is for plans and tightly set signatures.

RTC

The switched network — the historical copper — that gave birth to fax. Across Europe, carriers are taking it apart, replacing it with fibre and VoIP, which pushes fax machines onto the gateway.

G.711

The codec of digital telephony, raw audio with no compression (64 kbit/s). When T.38 is missing, a fax carried in G.711 over IP often gets through, provided the network loses almost no packets.

TSI and CSI

The identities that both machines swap when a call begins — TSI from the sender, CSI from the receiver. In practice, this fax number shows on the screen and appears in the transmission report.

Transmission report

That final summary, issued by the calling machine, records the dialled number, date, time, duration, pages processed, and outcome. Once the receiver has acknowledged every page, it serves as proof of delivery.

Transmission slip

The sheet that opens the fax: who is writing, to whom, why, and how many sheets accompany it. The receiving service routes the document without fully reading it.