Should I disable 20/40 Coexistence on a home 802.11n network?
I have been struggling with network speeds on a Windows10 PC, and observed that disabling the 'Enable 20/40 MHz coexistence' setting on my router has dramatically improved throughput. Going from a speedtest number of 4-8 Mbps to 40 Mbps (on a 40 Mbps FTTC internet connection).
This is with a Netgear D7000 router, TL-WN881ND card in the PC. Other machines seem to have no problem with the old settings. There is also a Netgear AC1200 on the network, but not involved in this connection. The router is about 4m from the PC, free line of sight.
Should I worry that some of my 20 odd devices in the house can't communicate any more? I understand that there should be a ~50% performance drop, but I seem to have experienced much more than that - before making the change web pages would actually fail to load fairly often. Maybe it is some old device that has done bad which I have pushed off the network with this change - or maybe I shouldn't worry?
Since it was mentioned in an answer, some of my IoT devices are 2.5G only, e.g. Sonoff.
1 Answer
No, you shouldn't disable that setting if any of your client devices use Bluetooth and sometimes need to use 2.4GHz Wi-Fi.
What Netgear calls "20/40MHz coexistence" is probably just the required respect for the "40MHz intolerant" bit that some clients set. You see, using 40MHz-wide channels in the rather small 2.4GHz ISM band (which is only about 80MHz wide total) doesn't leave enough room for things like Bluetooth. So when you have a smartphone or laptop that supports both Wi-Fi and Bluetooth, when that client joins the 2.4GHz radio of your router, it tells your router to stop using 40MHz-wide transmissions, so that it doesn't break Bluetooth. For 802.11n that means cutting the top transmission rates in half (actually slightly worse than half) from 300Mbps to 144Mbps, resulting in throughputs (after 802.11 overhead) of probably not much better than 90Mbps. And that 90Mbps is when your PC is the only thing really moving any traffic on the 2.4GHz band.
The real flaw in your setup is that your TL-WN881ND card in your PC is 2.4GHz-only, so it's stuck fighting for airtime in the crowded 2.4GHz band with slow legacy Wi-Fi devices and Bluetooth and microwave ovens and cordless phones and baby monitors and wireless speakers and cheap IoT devices.
You should get a dual-band, 802.11ac-capable, 2x2:2 or better card (that is, the "AC1200" speed class or better) for your PC, so it can join the 5GHz 802.11ac radio of your D7000 and get throughputs like 500-600Mbps.
One last tip: When you select a new AC1200-or-better card for your PC, opt for something that puts its antennas at the end of a cable, so the antennas aren't stuck hidden behind your PC. Decent cards can be bought for around US$35 from Amazon.
