Fax glossary

Fax vocabulary mixes ITU standards, carrier jargon and product names. This glossary defines the fourteen terms that actually come up when you are trying to send a fax, receive one, or work out why yours no longer goes through.

T.30

The ITU-T standard governing the dialogue between two fax machines: recognition, speed negotiation, page transmission and confirmation. Published in 1980, it is still the one in force.

T.38 (FoIP)

The protocol that carries a fax over an IP network: the gateway demodulates the signal on the way in, moves the page data as packets, then remodulates it on the way out. Without it, fax over VoIP often fails.

PSTN (public switched telephone network)

The historical analogue telephone network, over copper pairs. It is the network fax was designed for, and the one European carriers are progressively closing.

VoIP

Telephony over IP: the conversation is cut into packets carried over the internet. Convenient for voice, hostile to fax, whose modem tolerates neither packet loss nor echo cancellation.

ATA (analogue telephone adapter)

The box that connects an analogue fax machine to an internet router. Whether it supports T.38 is the whole difference between a fax that goes through and one that fails.

Acknowledgment of receipt

The confirmation issued by the receiving equipment at the end of the transmission. It covers each page, which makes it stronger than an email read receipt.

Cover sheet

The first page transmitted: sender, recipient, subject and total page count. Many recipients set aside a fax that arrives without one.

Standard / fine resolution

The two standardised definitions: about 204 × 98 dots per inch, and 204 × 196. The second doubles vertical definition — so the legibility of small type — and doubles the sending time.

Fax-to-email

The service that receives a fax on your behalf and delivers it to you as a PDF by email. It is the only way to receive without a phone line or a fax machine.

Virtual fax number

A number assigned at a carrier, with no equipment at your end. It is rented — which is what makes receiving a paid service where sending can be free.

Fax gateway

The carrier equipment that translates between the analogue and the IP worlds. It demodulates an incoming fax to turn it into a PDF, or remodulates an outgoing one onto a classic line.

International dial code

The prefix identifying the destination country — +33 for France, +49 for Germany. It is dialled before the local number, whose leading zero is then dropped.

CNG and CED tones

The two whistles at the start of a call: CNG comes from the calling device, CED from the answering one. It is what you hear when picking up on a fax by mistake.

MH, MR, MMR compression

The compression modes of the transmitted image, from the oldest to the most efficient. They explain why a dense page takes longer than a nearly empty one: it is the black that costs, not the file format.