Protocols and standards
ECM
Acronym: ECM
ECM is the error correction mode of the fax protocol: it splits each page into verified blocks, resent whenever they arrive corrupted.
Without it, a page damaged by line noise arrives damaged and the receiving machine confirms it anyway. With it, a confirmed page is an intact page. The difference is not cosmetic: it separates 'something arrived' from 'exactly this arrived', and that is what gives the transmission report its weight.