Glossary Call quality
What is Latency?
Also called delay, lag, round trip time, mouth to ear delay
Latency in a voice call is the time between a word being spoken and it being heard at the other end, sometimes called mouth-to-ear delay. Unlike jitter it does not distort the audio; it just arrives late, and the damage it does is to the rhythm of conversation rather than to sound quality.
How much is too much
Below roughly 150 milliseconds one way, most people notice nothing. Between 150 and 300 the conversation starts to feel slightly off and people begin talking over each other. Beyond 300 it is obviously awkward and participants start unconsciously waiting for gaps, which is why long-distance satellite calls feel so stilted.
Where the delay comes from
Some latency is unavoidable physics: distance takes time. The rest is processing, and it is additive. Codec encoding, jitter buffering, transcoding between codecs at a gateway and any media processing on the path all contribute. Transcoding is often the largest avoidable component, which is one reason keeping a single codec end to end is worth arranging.
How to see where it is going
A traceroute shows you the path and where the delay accumulates, which is usually more informative than a single round-trip figure. What you are looking for is a hop where the time jumps sharply and stays high for every hop after it, since that is the point where the delay is being introduced. Be aware that a hop that responds slowly to the trace but passes traffic on quickly is a red herring: routers deprioritise responding to traceroutes, so a slow single hop with normal times afterwards is not your problem.