Network and infrastructure
ADSL
Acronym: ADSL
ADSL is an internet access technology that carries high-speed data over the telephone copper pair, on frequencies the ear cannot hear.
Asymmetric means the download speed is far higher than the upload speed. Data uses frequencies above those of the voice, which let the analogue phone and the internet share the same line with a simple filter. The speed depends heavily on the length of the line to the exchange.
Why it matters for fax
As long as the subscription kept the analogue line, fax stayed in its own frequency band and worked as before, provided the machine sat behind a filter. With full unbundling, the phone goes through the router as voice over IP, and fax inherits every weakness of that mode.