Bufferbloat test

This is the measurement that explains why your call falls apart the moment someone else starts a download.

Bufferbloat is what happens when a network device holds on to far more data than it should. Memory is cheap, so routers and modems shipped with large buffers on the theory that holding packets is better than dropping them. The result is the opposite of what was intended: when the link fills, that buffer fills too, and every packet behind it waits in a queue that can be seconds deep.

The symptom is unmistakable once you know it. Everything is fine until someone starts a large upload or download, and then calls stutter, pages hang and games become unplayable — while a speed test run at the same moment still reports the full advertised speed. The bandwidth was never the problem. The queue was.

How this test measures it

Idle latency is measured first, on an unloaded connection. Then latency is probed again during the real download, while the link is genuinely saturated by the test's own traffic. The difference between those two medians is your bufferbloat, and it is graded from A+ (under 5 ms of added latency) to F (over 200 ms). Measuring during the actual download rather than a synthetic load means the result reflects a condition your connection really experiences.

What the grade means for you

A+ or A means your router keeps queues short and your calls will survive other people's downloads. B or C means noticeable queueing that you will feel during video calls. D or F means your latency multiplies under load — a 20 ms connection becoming 500 ms whenever anyone downloads — and no speed upgrade will fix it, because the constraint is the queue, not the capacity.

How to fix it

The fix is Smart Queue Management: an algorithm such as fq_codel or CAKE that keeps queues short and shares the link fairly between connections. Many modern routers include it under names like SQM, Smart Queues, Bufferbloat control or Adaptive QoS. OpenWrt supports it fully. Setting it up typically means enabling SQM and telling it your line's real speed, slightly under-declared so the queue forms in your router — where the smart algorithm controls it — rather than in your provider's equipment, where nothing does.

Frequently asked questions

Related tests

• Ping testLatency decides how responsive your connection feels. Speed does not.
• Jitter testJitter is why a call breaks up on a connection whose speed test looks perfect.
• WiFi speed testMeasure what your wireless connection actually delivers right now — not what the plan promises.

Run the full test — every metric in one pass →