Network and infrastructure
Packet loss
Packet loss is the share of data packets that never reach their destination, dropped by a congested network or arriving too late to be used.
In voice over IP, each packet typically carries twenty milliseconds of sound. The receiver hides a missing packet by extrapolating the neighbouring sound, which is enough to make the loss inaudible in a conversation. A few per cent of loss is acceptable for voice; beyond that, speech becomes choppy.
Why it matters for fax
For a modem, no concealment is possible: twenty missing milliseconds are a stretch of signal replaced by a guess. Without error correction, a line of the image is lost; during negotiation, the whole call can be. Dedicated fax over IP transports therefore send each piece of information several times.