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. This connection feels normal to the user because each packet arrives at roughly the same interval.
Things start to get ugly when those packets stop arriving in a steady pattern. One packet has a trip time of 18 ms, the next has one up in 70 ms, and the next after that drops back to 20 ms. That difference is called jitter. A phone call notices jitter faster, while a webpage may never be affected.
Latency is normally seen when the path has lots of hops 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 travels through transit can make a nearby service feel far away. The packet still makes it, but the round-trip takes longer.
A wireless AP is a good place for us to explain jitter. The user’s device still shows connected, but the customer has poor VoIP during a call. The radio may be fighting interference or a poor signal. The packets arrive at different times, casuing to 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 okay, but a 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.
High latency points toward the route, distance, or upstream path. High jitter points toward queues, RF noise, or an oversubscribed link. Measure more than one number before you trust the speed test.
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