Network and infrastructure
Echo cancellation
Echo cancellation is the processing that removes from a call the return of the sender's own signal, reflected back by the line or the network.
Part of the signal sent comes back to its source, where the two-wire line meets the network, or through a microphone picking up the loudspeaker. With enough delay, that return becomes an audible echo. An echo canceller models it and subtracts it continuously; on an IP network, the longer delay makes it essential.
Why it matters for fax
Fast fax modems handle echo themselves and are disrupted by a network canceller that alters the signal behind their back. That is why the machines' answer tones also serve to disable it. Equipment that ignores this signal can make a high-speed transmission fail.