I have setup an AP as a full blown router and want to setup SQM on its wan interface. What is the correct overhead in this case?
Same as always, the configured per packet overhead should be equal (or greater) than the real overhead you expect to see on the bottleneck link. For wifi itself with it's variable rate and aggregation level dependent overhead, I would recommend to try to get a chipset that supports air-time fairness as that will already help a lot, keep in mind that WiFi is inherently bursty so you will encounter higher latencies than over acwiref link. IIRC is the latency target for the fq_codel involved in airtime fairness 20ms as compared to the wired default of 5ms.
Good I asked. I will match the SQM settings on the main router.
Not sure I am following. The AP is wired to the main router. I will look into airtime fairness still.
How do I enable that? I can seem to find anything.
I believe that on ath9K wifi chips it is enabled by default on recent enough OpenWrt versions. Not sure about ath10K or the mediatek wifi driver...