What is the difference between latency and Jitter

When a packet leaves the customer router and takes time to reach the far end, this time is called latency. A network path may sit at 18 ms all day.  Most users will never complain about that number while browsing the web. The packet flow feels normal because each packet arrives at roughly the same interval.

Things start to get ugly when those packets stop arriving in a steady rhythm. One packet shows up in 18 ms, the next shows up in 70 ms, and the one after that drops back to 20 ms. That swing is called jitter. A phone call notices jitter faster while a webpage may never be affected.

Latency is normally seen when the path is long or the route is bad. Wireless links tend to suffer from Jitter more than wired links.  A customer hitting a service across the country will see higher Round Trip times (RTT) than a customer hitting a cache in the same data center. A route that hairpins through transit can make a nearby service feel far away. The packet still arrives, but the round-trip takes longer.

A wireless AP can be a perfect example of where we see jitter. The user’s device still shows as connected, but the customer experiences poor VoIP during the call. The radio on the AP is too busy dealing with interference or a poor signal. The packets arrive at different types, causing Jitter to spike.

A speed test cannot catch the problem because it averages out the spikes and dips. Speedtests, like the one from Cloudflare, introduce jitter tests.  The download number may look fine, while the VOIP call still sounds bad. I like to look at ping spread, queue depth, wireless retransmits, and interface drops when someone says “the Internet is slow,” but the bandwidth test looks clean. Slow is often the wrong word but is what the customer sees.

The fix depends on which number is lying to you. High latency points to the path in some form, or another.. High jitter points you toward queues, RF noise, oversubscribed links, or bursty traffic. The path will tell the truth if you measure more than one number.

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