Network and infrastructure
PSTN
The PSTN, or public switched telephone network, sets up a dedicated end-to-end circuit for each call over analogue copper lines.
Switching means connecting: for each call, the network's exchanges join line segments end to end, and those segments stay assigned to the two parties until they hang up. The connection is therefore constant, with no queues and no packets. It is the network telephony was built on for a century, and the one whose gradual shutdown Orange has begun in France.
Why it matters for fax
Fax was designed for this network and this network alone. A dedicated circuit guarantees a fixed delay and a signal delivered untouched, two conditions that fax negotiation and fax modems take for granted. Its disappearance is why so many machines now fail partway through a transmission.