Fax and fibre optic
Fax and fibre optic: why transmission breaks part-way through
On copper, the fax machine spoke to the line. On fibre it speaks to a codec designed for the human voice, which treats its tones as noise to be corrected.
Short answer
A fax is not a file being transferred: it is modulated sound, designed for a switched telephone line. Over a fibre connection that sound passes through a voice codec, often G.711, which is not transparent to tones. The T.38 protocol exists for exactly this reason: it relays the fax data instead of carrying its sound. Without T.38, the call connects normally and the transmission breaks afterwards.
What a voice codec does to a fax signal
A codec such as G.711 is built for speech: it digitises, frames and corrects. The tones of a fax modem are not speech, and what improves a conversation damages them. A suppressed echo, a lost packet or a few milliseconds of jitter are enough to shift the modulation until the receiver no longer recognises the carrier.
T.38, and what to do without it
T.38 demodulates the fax at the edge of the network, carries the pages as packets and remodulates them at the far end: what crosses the network is no longer sound. The option has to be requested from the operator or found in the gateway; it cannot be assumed. Failing that, lowering the fax machine's speed to 9,600 bit/s makes the modulation more robust. The last route is to stop sending from the line at all: an online service hands the PDF to the telephone network from its own point of access.
The symptom nobody names
A transmission that always fails after the connection, never before, does not indicate a failing line but a transport that is not transparent to the signal. Dialling succeeds, both machines answer each other, sending begins, then synchronisation is lost on the second or third page and the machine hangs up reporting a communication error. A faulty line would prevent the connection itself. This is why replacing the machine or the cable never changes anything.
Frequently asked questions
- Does fax work over fibre?
- It works when the transport is prepared for it, meaning the gateway switches to T.38 or the codec stays on G.711 with voice processing disabled. Otherwise the transmission starts and then degrades.
- Why did the same machine work before?
- Because it was sending over a switched line, which carried sound without interpreting it. Orange stopped selling new PSTN lines in France in 2018 and has been closing existing lines in batches since; sending now goes through a transport that was never designed for this signal.
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