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Glossary · Voice technology

What is Jitter?

Reviewed by Sujan ThapaliyaLast updated

Definition

Jitter is variation in the arrival time of voice packets. Steady delay is tolerable; uneven delay is what produces choppy, robotic audio. A jitter buffer smooths it out by holding packets briefly, at the cost of added latency.
The three STIR/SHAKEN attestation levels, which underpin how jitter behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, jitter included.

In practice

How jitter actually works

Keep jitter under about 30 ms and packet loss under 1% for calls that sound clean. One-way latency above roughly 150 ms starts causing people to talk over each other.

Jitter is usually a local network problem (Wi-Fi contention, an overloaded uplink, or missing QoS) long before it is the provider's.

Jitter is also the reason wifi is a bad default for anyone on calls all day. Wireless introduces variance that no amount of bandwidth removes, and the buffer compensates by adding delay until the conversation starts colliding. A cable fixes more calls than an upgrade to the connection ever does.

The jitter buffer hides variance by adding delay — over-correcting trades one problem for another.
JitterEffectFix
Under 30 msImperceptibleNothing
30–50 msOccasional clippingPrioritise voice traffic
50–100 msChoppy audioWired connections, check the uplink
Over 100 msUnusableNetwork problem, not a phone problem

Worked example

Packets sent 20 ms apart arriving 20, 45, 18 and 60 ms apart is 20 ms of jitter. The buffer smooths it by holding audio back, which is why fixing jitter with buffering always costs latency.

Jitter: common questions

What causes jitter on VoIP calls?
Variable delay between packets, usually from congestion, wifi interference, or a router prioritising other traffic. The packets arrive; they just arrive unevenly, and audio has to be played back at a constant rate.
What jitter level is acceptable?
Under about 30 milliseconds is generally imperceptible. Above that, the jitter buffer either stretches — adding delay — or discards late packets, which is the choppy audio people describe as the call breaking up.
How do you fix jitter?
Prioritise voice traffic on the network, use ethernet rather than wifi for fixed positions, check for a saturated uplink, and confirm the router is not running an ALG that mangles SIP. Adding bandwidth alone rarely fixes it.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

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