Should error correction (ECM) be turned off?

It is a setting offered on most fax machines. Without ECM, pages get through despite errors, at the cost of scrambled lines; with ECM, a corrupted page is retransmitted and can make the whole send fail. The choice depends on what matters: receiving an imperfect page or receiving none.

Is the problem the box or the operator?

Both, at different points. The box processes the sound at the start, the operator's network carries it afterwards. A local setting can be enough; when it is not, only switching to T.38 on the operator's side changes anything.

Fax and internet box

Fax and internet box: why the phone port is not a line

The socket is the same, so is the dial tone, and the fax machine cannot tell the difference. What changed sits behind the socket: the box digitises the sound before sending it.

Short answer

The phone port on an internet box is a gateway: it produces an analogue dial tone over a few centimetres, then converts the sound into packets. Three things matter: disabling the processing designed for speech where the interface allows it, forcing the fax machine's speed down to 9,600 bit/s, and enabling T.38 mode where it is offered.

Voice processing damages the tones

Echo cancellation, noise reduction, compression: these functions improve a conversation and degrade a fax, whose tones are not speech. Interfaces that expose them name them differently from one brand to the next, and not all expose them at all; the principle is the same everywhere, and it is the principle to look for rather than a particular label.

Speed and T.38 mode

Speed is set on the fax machine, not on the box: at 9,600 bit/s the modulation is more robust and copes better with lost packets and jitter. T.38 is set on the box or on the operator's side: it carries the fax data rather than its sound, and it is the only measure that addresses the cause. Not every box exposes it; where the option exists, it comes before the others.

Why the same machine worked the day before

On a switched line the fax machine had a channel that carried its signal without trying to improve it. Behind a box, the same signal is digitised by equipment tuned for the human voice: it removes what it takes for echo, cuts what it takes for silence, compresses what it takes for noise. The machine has not aged and the line is not down; what has disappeared is the assumption the fax rests on — a neutral channel. Orange stopped selling new PSTN lines in France in 2018 and has been closing existing lines in batches since.

Frequently asked questions

Should error correction (ECM) be turned off?: It is a setting offered on most fax machines. Without ECM, pages get through despite errors, at the cost of scrambled lines; with ECM, a corrupted page is retransmitted and can make the whole send fail. The choice depends on what matters: receiving an imperfect page or receiving none.
Is the problem the box or the operator?: Both, at different points. The box processes the sound at the start, the operator's network carries it afterwards. A local setting can be enough; when it is not, only switching to T.38 on the operator's side changes anything.

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