Network and infrastructure
Jitter
Jitter is the variation in packet delivery delay on an IP network: two packets sent at a regular interval do not arrive with the same delay.
Packets from the same call may take different paths and wait more or less long in the equipment they cross. To play back continuous sound, the receiver holds them in a buffer, the jitter buffer, which absorbs the differences at the cost of a slight delay. An adaptive buffer adjusts its size during the call.
Why it matters for fax
An ear does not notice an adaptive buffer imperceptibly speeding up or slowing down a voice. A modem does: it loses synchronisation and the transmission drops. That is why equipment designed for fax uses a fixed jitter buffer for the whole call.