Bufferbloat

When your line is full, with a download, a backup or a video upload running, packets queue up somewhere: usually in your router or modem. Many of them keep a very long queue. Everything else, your call or your game included, then waits behind it, and the delay jumps from 20 ms to hundreds. That’s bufferbloat.

How Netutilus grades it

Speed Test measures the delay three times: with the line idle, while it downloads, and while it uploads. The grade is set by the largest rise:

• A+: under 5 ms added. A: under 30 ms.
• B: under 60 ms.
• C: under 200 ms. Calls stutter while the line is busy.
• D and F: more. The connection feels broken whenever anything big transfers.

Responsiveness expresses the same in round trips per minute (RPM): how many quick exchanges the line manages while loaded. Higher is better.

How to fix it

It’s rarely your provider’s fault, and a faster plan rarely helps. The fix is on the router:

• Turn on Smart Queue Management. Routers call it SQM, Smart Queue, Adaptive QoS, fq_codel or cake. It keeps the queue short, so small packets get through at once.
• Set its speed limits a little below your plan, around 90 to 95%. The queue then forms in the router, where SQM manages it, not in the modem.
• Upload is the usual culprit. It’s the smaller direction, so it fills first.

Run Speed Test again afterwards. Expect the grade to rise, even if the speeds drop slightly.

Related

• Video calls lag or freeze.
• Latency and Jitter.