Stop downloads from causing lag
When someone downloads or uploads, your router lets a queue build up and every game packet and call waits behind it (bufferbloat). Smart Queue keeps that queue short by limiting the line to just under its real speed.
Your Smart Queue numbers
Enter the speeds your Lag Check measured, wired if you can.
Pick cake (or fq_codel) where the router offers a choice. Some routers ask for kbit/s, others for Mbps.
Enter both speeds to see your limits.
Guides for your router
Where the setting is, what to enter and what to watch out for, brand by brand.
Don't know your speeds? Run the Lag Check first. Router by router steps are in the bufferbloat guide.
How it works
Enter the download and upload speeds your Lag Check measured, on a cable if you can. The calculator takes 90% of each and rounds down, giving the two limits to type into your router's Smart Queue (SQM) page, in Mbps and in kbit/s because routers differ in which they ask for. Set just below the line's real speed, the router decides which packets wait instead of the modem, and it lets small game and call packets go ahead of a big download. Then test again: if lag under load is still there, drop to 85%, then 80%.
Reading your result
- Grade A or A+ after the change
- Lag under load is under 30 ms. Smart Queue is doing its job: leave the limits where they are.
- Better, but still B or worse
- The limits are still a little above what the line really delivers at that moment. Lower them to the next step (85%, then 80%) and test again.
- No change at all
- Either Smart Queue isn't switched on for the internet (WAN) port, the router only offers priority QoS, or the queue is in the provider's modem. Check the guide for your router brand.
Questions
What should I set my SQM bandwidth limits to?
Start at 90% of the download and upload speeds you measured, not the speeds on your bill. If lag under load is still graded B or worse, try 85%, then 80%. Measure on a cable when nobody else is using the line.
Should I choose CAKE or fq_codel?
CAKE where the router offers it: it needs the least setting up. fq_codel is the next best and works well on routers with less processing power. Both keep the queue short. The router guides show which your brand has.
Do I need to set both download and upload?
Yes. Upload is usually the smaller side and fills first, for example during a photo backup or a video call, so a limit on download alone leaves half the problem. The bufferbloat guide explains why.