Ping is how long a round trip takes; jitter is how much that time changes from one round trip to the next. A call or a game copes with a steady delay far better than a jumpy one.
- SystemCheck reports jitter the RFC 3550 way, as the mean change between consecutive round trips, with the spread (the standard deviation) beside it.
- Our warn line for jitter is 50 ms. VoIP planning guides commonly put the limit for voice at 30 ms; on a cable, single digits are common.
- Jitter that appears only while the line is busy is bufferbloat, and the fix is on the router.
The short answer
Jitter is the unevenness of the delay. A call app holds incoming audio briefly to even out arrival times, and the more the delay varies, the longer it has to hold it or the more gaps you hear. A game has no such room, so a jumpy ping shows as players and objects snapping into place. Most jitter that starts at home comes from Wi-Fi or from a full router queue, and both can be fixed.
Jitter and spread are different numbers
Two lines with the same spread can have very different jitter. Round trips of 20, 60, 20 and 60 ms jump by 40 ms every time: jitter 40 ms, spread 20 ms. Round trips of 20, 30, 40, 50 and 60 ms drift upwards by 10 ms a step: jitter 10 ms, spread about 14 ms. The first is the one a call suffers from; the second is a ping that is drifting, which a buffer can follow.
That is why the test shows both. Jitter catches a ping that jumps from packet to packet; spread catches one that wanders.
How the test measures it
Twenty small round trips go to this site, and the first two are dropped while the connection opens. The median is the latency, jitter is the mean absolute change between consecutive round trips, and spread is their standard deviation. Round trips continue while the line downloads and uploads, so jitter that appears only when the line is busy is not missed.
A browser cannot send an ICMP ping, so the round trips are small requests to this site, and it times round trips rather than one direction, so upstream and downstream jitter are not separated.
What lowers it
Work through these in order, running the test after each.
- Use a cable instead of Wi-Fi. If jitter drops, the wireless link was the cause.
- On Wi-Fi, move closer to the router and use the 5 GHz band.
- Turn off any VPN and run again.
- If jitter rises only while the line is busy, turn on Smart Queue Management on the router and pause large uploads.
- If it stays high on a cable with nothing else running, run the test at another time of day before calling the provider.
Terms used here
Questions
- What is a good jitter for gaming and calls?
- As low as you can get it. Our warn line is 50 ms, and VoIP planning guides commonly use 30 ms for voice. Single-digit jitter on a cable is common.
- Can I have low ping and high jitter?
- Yes. A line can average a low ping while individual round trips swing widely, and the swings are what freeze video and make games stutter. That is why the test reports jitter beside the median.