Glossary Call quality
What is Jitter?
Also called packet delay variation, jitter buffer
Jitter is variation in the delay between packets arriving. Voice is sent as a steady stream, so if packets arrive unevenly, some early and some late, the receiver cannot play them back smoothly. The symptom is audio that sounds choppy, robotic or clipped, rather than simply delayed.
The jitter buffer, and its cost
Receivers hold incoming packets briefly in a jitter buffer so late arrivals can be slotted into place before playback. This works well, but it is bought with latency: a bigger buffer absorbs more variation and adds more delay. Adaptive buffers resize themselves as conditions change, which is why a call can start clean and degrade as the network gets busier.
What causes it
Jitter is usually contention rather than distance. A congested uplink, a consumer router prioritising a large upload, or wifi interference will all produce it. It is one of the strongest arguments for QoS on the local network, because voice packets are small and frequent and suffer disproportionately when queued behind bulk traffic.
Measuring it properly
Jitter is reported in milliseconds and, unlike latency, a single reading tells you little because the whole point is variation over time. Under 20ms is generally fine; above 30ms most people can hear it. The useful measurement is continuous rather than a spot check, because the pattern is what identifies the cause: jitter that spikes every weekday at nine points at contention, jitter that correlates with one user's location points at wifi, and jitter that is constant points at a link that is simply too small.