VoIP & SIP
JitterAlso known as Packet delay variation, Jitter buffer
Definition
Jitter is the variation in the time it takes voice packets to arrive across a network. Packets sent every 20 ms may arrive at 15, 32 or 21 ms intervals. VoIP receivers smooth this with a jitter buffer; if jitter exceeds what the buffer can absorb, packets are discarded and the caller hears choppy or robotic audio.
How jitter is measured
RFC 3550 defines “interarrival jitter” as a running average of the difference in transit time between consecutive RTP packets, reported in RTCP receiver reports. Most tools display it in milliseconds. It is distinct from latency (the absolute delay) and packet loss, though all three usually rise together on congested links.
| Jitter (typical) | Effect |
|---|---|
| < 20 ms | Good; absorbed by any jitter buffer |
| 20–30 ms | Acceptable; adaptive buffers add some delay |
| > 30–50 ms | Audible gaps or added delay likely |
Jitter buffers
A jitter buffer holds packets briefly and plays them out at a steady rate. A fixed buffer adds a constant delay (say 60 ms). An adaptive buffer grows and shrinks with measured jitter, trading delay for smoothness. Every millisecond of buffer adds to mouth-to-ear delay, and ITU-T G.114 advises keeping one-way delay under about 150 ms, so buffers should be no larger than needed.
Reducing jitter
- Prioritise voice with QoS (DSCP EF / 46) on LAN and WAN equipment.
- Avoid saturating uplinks; large uploads are a common cause.
- Prefer wired or dedicated links for gateways; shared Wi-Fi adds variable delay.
- Monitor per trunk and per device so you can see where jitter starts.
GSMCalls tracks latency, loss and jitter for trunks and devices on the platform, and the Smart Bridge connection keeps audio off the PC path entirely; see GSM gateway without a PC.
Frequently asked questions
What is an acceptable jitter for VoIP?
Under about 30 ms is generally acceptable, and under 20 ms is good. Consistency matters as much as the average.
Is jitter the same as latency?
No. Latency is how long packets take to arrive; jitter is how much that time varies from packet to packet.
Does a bigger jitter buffer always help?
It removes more gaps but adds delay to every word. Adaptive buffers balance the two automatically.