FaxUp · Reference

Fax Glossary: T.30, T.38, ECM, Resolution and More

Fax terms in one or two sentences each. Most come from the ITU-T recommendations that define how fax works (T.30 for the call, T.4 for the pages, T.38 for fax over the internet); each entry links to its source.

Last updated: November 12, 2026

Fax (facsimile): Sending a copy of a document's pages over a phone line: the sending side scans each page into black and white dots, and the receiving machine prints or stores the same dots. A fax is a phone call between two machines.
Group 3 (G3) fax: The fax standard almost every machine and online fax service uses today, defined by ITU-T T.4 (how pages are encoded) and T.30 (how the two machines talk). Group 4 was designed for digital ISDN lines and is rare. Source: ITU-T T.4
T.30: The ITU-T procedure for a fax call: the machines identify themselves, agree on a speed and resolution, send the pages, and confirm each page. A "delivered" status means this procedure finished with every page confirmed. Source: ITU-T T.30 · See also: Fax confirmation
T.4: The ITU-T recommendation that defines Group 3 pages: the page width in dots, the vertical resolutions, and the compression (MH, MR) used to send them. Source: ITU-T T.4
T.38: The ITU-T protocol for carrying a Group 3 fax call in real time over an IP network (VoIP), instead of as audio. Online fax carriers use it so a fax survives the internet part of its route; the receiving machine still sees a normal fax call. Source: ITU-T T.38
T.37 (internet fax by email): The ITU-T method for store-and-forward fax over the internet, where the pages travel as an email attachment rather than as a live call. Most "email to fax" services work on this idea. Source: ITU-T T.37
ECM (Error Correction Mode): An optional part of T.30 where pages are sent in numbered frames and the receiver asks again for any frame that arrived damaged, so the page prints without streaks. It makes a fax more reliable on noisy lines, at the cost of some time. Source: ITU-T T.30, Annex A
Standard resolution: 3.85 scan lines per millimetre vertically (about 98 lines per inch), with about 8 dots per millimetre across the page (1,728 dots on a 215 mm line). Quick to send; small text can blur. FaxUp's later retries switch to standard resolution so a weak line has less to carry. Source: ITU-T T.4 · See also: Why faxes fail
Fine resolution: 7.7 scan lines per millimetre vertically (about 196 lines per inch), twice standard, with the same width. Sharper text and signatures, roughly twice as long to send. Source: ITU-T T.4 · See also: Fax preview tool
MH, MR and MMR: The compression schemes a fax uses for its black and white dots: Modified Huffman and Modified READ (T.4) and Modified Modified READ (T.6). Large dark areas compress badly, which is why a dark photo takes far longer to fax than text. Source: ITU-T T.6
V.17 and V.34 (fax speed): Modem standards fax machines use to send page data: V.17 at up to 14,400 bit/s, and V.34 ("Super G3") at up to 33,600 bit/s. During the handshake both machines settle on the fastest speed the line allows. Source: ITU-T V.17
CNG and CED tones: The beeps of a fax call. CNG is the calling machine's 1,100 Hz tone that says "I'm a fax"; CED is the answering machine's 2,100 Hz tone. If a person or voicemail answers instead of CED, the call can't become a fax. Source: ITU-T T.30
CSID and TSI: The identifiers machines exchange in the T.30 handshake: the receiving machine sends its Called Subscriber Identification (CSID), usually its fax number, and the sender its Transmitting Subscriber Identification (TSI). They are what a fax header and a transmission report show as the "to" and "from". Source: ITU-T T.30
MCF (message confirmation): The T.30 signal a receiving machine sends after a page that arrived correctly. If it sends RTN (retrain negative) instead, the page wasn't good enough and the machines may slow down and try again. Source: ITU-T T.30
Transmission report (fax confirmation): A record of a completed fax: the number dialled, time, pages and result. FaxUp emails a one-page PDF report when a fax is delivered, if email notifications are on in the app. See also: Fax confirmation guide
Cover sheet: A first page that says who the fax is for and from, how many pages follow, and how to reply. Not required by the fax standard, but many offices route incoming faxes by it. See also: Cover sheet maker
Busy: The fax line was already on a call. It is the failure most likely to fix itself: in FaxUp's records, 71% of faxes that hit a busy line were delivered on a later retry. See also: Fax delivery statistics
No answer and no fax tone: No answer: nothing picked up. No fax tone: something answered (a person, voicemail, a switched-off machine forwarding to voice) but never sent the CED tone. Both usually mean the number is wrong or the machine is off. See also: Why faxes fail
Online fax: Sending or receiving faxes through a service on the internet instead of a fax machine and phone line. The service places the actual fax call (often over T.38) and gives you the result in an app, a website or by email.
Fax number (receiving): A phone number that a fax service answers as a fax machine and turns into a PDF for you. With FaxUp, receiving needs the FaxUp Unlimited plan, which comes with a dedicated number. See also: Pricing
BAA (Business Associate Agreement): Under the US HIPAA rules, the contract a healthcare provider signs with a service that handles patient information for it. Faxing patient records through a service generally calls for one. FaxUp doesn't sign BAAs. Source: HHS: business associate contracts

Questions

What is the difference between standard and fine fax resolution?

Fine resolution has twice the vertical detail: 7.7 lines per millimetre against 3.85 (about 196 against 98 lines per inch), with the same width. It makes small text and signatures sharper and takes about twice as long to send.

What does T.38 mean for an online fax?

T.38 is the ITU-T standard for carrying a fax call over the internet in real time. It lets an online fax service reach an ordinary fax machine reliably; the machine at the other end sees a normal fax call.

What is ECM on a fax machine?

Error Correction Mode, an optional part of T.30. Damaged parts of a page are sent again, so the page prints cleanly. It can make a fax slower on a poor line, but more reliable.

Sources

ITU-T recommendation pages checked November 12, 2026.

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