Bufferbloat: why video calls freeze when someone starts a download
Bufferbloat is the reason a video call can freeze the moment someone in the house starts a big download, even on a fast line. Routers and the network hold on to too much data in queues, and every packet waits behind the queue. Ping that was 20 ms at rest jumps to 200 ms or more under load. Download speed looks fine the whole time, which is why most speed tests never show it.
What it means for you
- If calls freeze when someone else is downloading or gaming, run a Standard test and look at the busy-line grade. A or B is healthy; D or F is bufferbloat.
- The fix is not more speed. A router with smart queue management (SQM, also sold as "QoS" or "traffic prioritisation") keeps the queue short. Full fibre lines usually grade better than copper.
- On the same line, a cable test and a Wi-Fi test can grade differently. If only Wi-Fi fails, the router is the bottleneck.
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| Busy-line grade | Rise in ping while the line is full | What it feels like |
|---|---|---|
| A | Under 30 ms | Stays responsive when busy |
| B | Under 60 ms | Slight lag when busy |
| C | Under 120 ms | Noticeable lag when busy |
| D | Under 250 ms | Calls and games suffer when busy |
| F | 250 ms or more | Severe bufferbloat |
Questions people ask
What is bufferbloat?
It is the rise in latency caused by over-full queues in routers and network equipment when a connection is saturated. Pulse measures it as the difference between your ping at rest and your ping while downloading or uploading.
What is a good busy-line grade?
A means ping rose by under 30 ms while the line was full; B under 60 ms; C under 120 ms; D under 250 ms; F means 250 ms or more.
Does a faster package fix bufferbloat?
Not by itself. The queue problem exists at any speed. Smart queue management on the router or a full fibre line are the usual fixes.
Use this infographic
Free to reuse on your site, in a report or on social media under a Creative Commons Attribution 4.0 licence. Please credit UKSpeedTest.co.uk with a link. Paste this to embed it:
Cite as: UKSpeedTest.co.uk (2026). Bufferbloat: why video calls freeze when someone starts a download. BroadbandSwitch.uk. https://ukspeedtest.co.uk/infographics/bufferbloat-why-calls-freeze-when-someone-downloads/
Related
- Broadband latency test
- Watch bufferbloat live on the BroadbandSwitch.uk quality monitor
- When to reboot your router and when to escalate
Sources
- UKSpeedTest.co.uk. (2026). How Pulse measures your speed (busy-line grade). ukspeedtest.co.uk
- Bufferbloat.net. (n.d.). What can I do about bufferbloat? www.bufferbloat.net