Queues, and why bigger is not better
Every router has a buffer: a queue where packets wait when they arrive faster than the next link can send them. A small buffer absorbs short bursts and is a good thing. The problem is that many modems and routers have buffers far bigger than they need, sized to avoid ever dropping a packet. When your line is full, that big buffer fills up, and every packet, including the tiny ones from your game or call, waits behind hundreds of milliseconds of queued download or upload data.
The result is the classic complaint: "my ping is 20 ms until someone in the house starts a backup, then it is 400 ms." The connection is not dropping packets; it is holding them far too long. That extra delay under load is bufferbloat.
How to test for bufferbloat
Run the latency test twice. First with nothing else using the connection, to get your idle ping. Then start a large download or upload on another device, such as a big file transfer or a speed test, and run it again while that is going. Compare the medians.
| Rise in ping under load | Bufferbloat |
|---|---|
| Under 5 ms | None to speak of |
| 5 to 30 ms | Mild |
| 30 to 100 ms | Noticeable in games and calls |
| Over 100 ms | Severe |
Test the upload direction especially. Home lines usually have much less upload capacity, so the upload queue fills first and bloats worst.
How to fix bufferbloat
- Smart queue management (SQM). Algorithms such as fq_codel and CAKE keep queues short and give small packets priority. OpenWrt routers support them, and several consumer and gaming routers include them, sometimes named "Smart QoS" or "Adaptive QoS".
- Set the shaper slightly below your line speed. SQM works by becoming the bottleneck itself. Set it to around 85 to 95% of your measured speed so the queue forms in your router, where SQM controls it, rather than in the modem.
- Limit the heavy uploader. If SQM is not available, a bandwidth limit on backup or sync software stops it from filling the line.
- Replace the ISP router. Provider-supplied routers rarely offer SQM; a router that does, placed behind the ISP one in bridge or modem mode, is a common fix.
Why a speed upgrade does not always help
A faster plan fills more slowly, so bufferbloat appears less often, but the moment the line is full the same oversized buffer fills and the same delay returns. It moves the problem rather than fixing it. SQM fixes it at any speed, which is why it is the recommended cure even on fast fibre.