The Jitter Buffer — Smoothing Out Packet Arrival Variation
Learn about the jitter buffer — smoothing out packet arrival variation
Jitter is the disease; the jitter buffer is the treatment. It's the single most important mechanism for maintaining audio quality in VoIP, and understanding how it works is essential for interpreting call quality metrics and diagnosing choppy audio.
The Core Concept
Imagine packets arriving at these intervals (they should arrive every 20ms):
Packet 1: arrived at 0ms
Packet 2: arrived at 18ms (2ms early)
Packet 3: arrived at 42ms (2ms late)
Packet 4: arrived at 58ms (18ms early?... no, the network was fast)
Packet 5: arrived at 85ms (5ms late)
If you played each packet the instant it arrived, the audio would be a stuttering mess — gaps where packets are late, compression where they're early. Instead, the jitter buffer holds packets and plays them out at a fixed, regular interval.
With a 40ms jitter buffer, you'd wait 40ms before playing the first packet, then play one packet every 20ms regardless of when they actually arrived. As long as packets arrive within that 40ms window, audio is smooth.
Fixed vs. Adaptive Jitter Buffers
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