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Error correction mode (ECM) is a fax setting that divides each page into small data blocks and asks the receiving machine to confirm every block. If line noise damages a block, only that block gets resent, not the whole page. It is on by default on most modern machines, and turning it off can sometimes push a fax through on a very noisy line.
Normal fax transmission is a one-way stream: the sender reads a page, converts it into data, and pushes it down the line. The receiver prints whatever arrives, garbage included. ECM changes that into a two-way conversation. The sending machine chops the page into frames of 256 octets (about 256 bytes each), sends them, and waits for the receiver to confirm each one.
If a frame arrives corrupted, the receiver answers with a partial-page request (a PPR signal), and the sender retransmits just those frames. Clean frames are never sent twice. The machines also keep training down the line speed when errors repeat: a fax that starts at the top speed of its class may settle at a slower, more reliable rate before the page finishes.
| ECM on | ECM off |
|---|---|
| Page split into 256-octet frames, each confirmed | Page sent as one continuous stream |
| Damaged frames are resent on their own | Damaged lines print as black streaks or drop out |
| Receiver sends PPR signals to request repeats | No feedback until the page is done |
| Best for documents where every line matters | Can finish faster on a bad line, with visible errors |
Both machines have to agree to use ECM during the handshake at the start of the call. If either side lacks it (some very old machines do), they silently fall back to non-ECM mode and the fax still goes through. On a clean analog line, ECM adds only a little overhead and guarantees the page arrives intact. For medical offices, law firms, and banks that fax documents where a garbled line could change a number or a signature, that guarantee is the whole point.
One detail most people miss: ECM is negotiated per call, not per machine. Your machine can use ECM with a doctor's office on Monday and skip it with a twenty-year-old machine at a warehouse on Tuesday, with no setting change from you. The handshake decides.
ECM fails gracefully, but it can fail slowly. On a noisy line, the sender and receiver can get stuck in a loop: send, corrupt, request, resend, corrupt again. The fax keeps retrying the same page and eventually times out or disconnects with an error. In that situation, turning ECM off often lets the fax complete. The page may show streaks or light bands, but the recipient gets something readable instead of nothing.
This is a common trick on VoIP phone lines, which compress voice in ways that mangle fax data. If a fax keeps failing at the same point with ECM on, the standard support-desk advice is: try once more with ECM off, then decide whether the result is good enough. Some machines label the setting "ECM" under Setup or Fax Settings; on Brother models it lives under Fax, then Miscellaneous.
A fax that never finishes is usually telling you the line is too dirty, not that the machines are broken. Run down this list: disable ECM for one attempt; drop the fax speed to 14,400 or 9,600 bps if your machine allows it; unplug other devices sharing the line (answering machines, splitters, DSL filters in the wrong spot); and try at a different hour, since line quality drifts through the day. If none of that works, the honest fix is a different route: an online fax service skips the analog line entirely.
LetterAgent's fax feature follows that same logic. You describe the fax to your AI assistant, approve the exact quoted price, and it is sent for you to US and Canadian fax numbers. Fax is currently available in testing, so send times are not guaranteed. There is no inbound fax.
Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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