jitter

Imagine a friend mailing you one postcard every day. Even if delivery is slow, as long as one arrives each day you can read them on a steady rhythm. But if three pile up one day and none the next, your reading rhythm is wrecked — even though, on average, you still get one a day. Jitter is exactly this for packets: not how long they take, but how unevenly they arrive. It is the variation in end-to-end delay from one packet to the next. The sender may emit a voice packet smoothly every 20 milliseconds, but the network can deliver them clumped, spread out, or shuffled.

Why does delay vary? The main culprit is queuing delay in routers. A packet that finds a router's queue empty sails through; the next, arriving when the queue is full, waits behind many others. Because queue lengths fluctuate moment to moment, successive packets of the same stream experience different delays, so the gaps between arrivals no longer match the gaps at the sender. The receiver can measure jitter by comparing each packet's timestamp (carried in its RTP header) with its actual arrival time and watching how that difference wobbles. Steady arrivals mean low jitter; bursty, irregular arrivals mean high jitter.

Why it matters: jitter, more than raw delay, is what makes real-time audio and video stutter. Constant delay just shifts the whole conversation a bit later — annoying past a point, but smooth. Jitter, left untreated, makes playback speed up and slow down and gap out. The standard cure is the playout buffer, which deliberately holds arriving media for a short time so that even late packets are usually there when their turn to play comes — trading a little extra startup delay for smooth output. A common confusion: jitter is about variation, not magnitude; a link can have high average delay yet low jitter, or low average delay yet high jitter.