Jitter

Ping tells you how long a round trip takes; jitter tells you how steady that time is. A call app holds incoming audio briefly to even out arrival times, so the more the delay varies, the longer it must hold it or the more gaps you hear. A game has no such room, and a jumpy ping shows as players and objects snapping into place.

SystemCheck reports jitter the way RFC 3550, the RTP specification, builds it: the mean absolute difference between consecutive round trips. The RFC keeps a running estimate that moves a sixteenth of the way towards each new difference; the test takes the plain mean over its samples. The standard deviation is shown beside it as spread, a different number: a ping that drifts steadily has low jitter and a large spread, while one that jumps back and forth has high jitter. Our warn line for jitter is 50 ms.

Most jitter that starts at home comes from Wi-Fi or from a full router queue. Try a cable first; if jitter falls, the wireless link was the cause. Jitter that appears only while the line is busy is bufferbloat, and the fix is on the router. A browser cannot send an ICMP ping, so the test times small requests to this site, and it cannot separate upstream jitter from downstream.