Kind of like the 1/6/11 rules - 20/40 MHz channels in 2.4 is another one...
The secondary channel has always been opportunistic since 802.11n was released - it's only used if it passes the clear channel assessment process first, otherwise it's primary/control channel only...
There was one of the devs for Google Fiber that worked on their gateways that did a study using info from their gateways at a scale that only Google could do, and their findings were...
Better to enable 40MHz channels than to not - and the takeaway there was that the control channel is always present, the secondary channel is available to be used, mostly because not everyone is using the airtime _at the same time_ - so doing 20MHz only was leaving potential bandwidth on the table...
@drinkingbird - you can respond to me privately, or look at the sticky I've posted some time back...
I still feel the 40mhz rule applies - differing opinions don't have to be kept private.
Sometimes theoretical testing and real world are vastly different (more than sometimes in my experience).
I've got two examples.
During COVID, when everyone fled NYC, a friend stayed in a huge (several hundred unit) new apartment complex outside of the city. Router was an older 2.4ghz only N150 that they owned. No matter how much tweaking and tuning I did, could not even sustain 10M, often falling to 1 or below or completely timing out on a speed test. Looking at inSSIDer there were dozens of very strong 2.4ghz networks and probably half of them were showing 40mhz. Sure, a few of those people may have set it that way intentionally to try and get more speed, but many were just on the default auto setting from the ISP (most likely all or most had the ISP's router) and obviously the band was not "clear" for 40mhz use, but those routers were picking it anyway. Most people didn't notice as the ISP router was dual band, but a few there were complaining that their older chromecast or other IOT stuff was not working.
In my local neighborhood, have been helping neighbors try to get better signal to their Blink cameras. The cameras are outside of windows and the AP inside. Signal strength shows excellent on both ends but throughput often can't keep up. Their TP-Link AP happily goes into 40mhz mode if I don't lock it to 20. That one may be valid since indoors there are several neighboring networks but not ridiculous, and the signals on most are relatively weak. Personally from looking at them, I would still think it is too saturated for 40mhz, but the router doesn't. However outside, where the cameras are mounted, those 40mhz networks are quite strong, along with 15-20 20mhz ones. This obviously impacts the cameras and is part of the issue. It also impacts me, my outdoor AP is 2.4ghz only, and over the past 5 years or so I've gone from being able to push 75 no problem to 20-30. So obviously there is congestion, but plenty of 40mhz out there (and believe me, I know who most of them are based on their network name and they're running the ISP router and don't even know it has a GUI, much less logged in and changed anything).
Some of this is that the decision is based on some threshold/algorithm that each company programs in, whether they are using reasonable settings or not depends on the person who decided on the numbers. The other part is that while two APs indoors or far away from each other may believe that there is clear space to use 40, but a device in between them, or outside, etc will be negatively impacted.
I can't think of a time I've fired up a scanner and not seen several 40mhz networks even in densely populated areas with lots of networks. So I still believe the "neighborly" thing to do is stick to 20, and in reality it is probably going to help you get more stability/consistency too.
I couldn't resist a couple good deals on security cameras during Prime Day, just want one aimed at the front door to alert me when a package is out there, and one on my driveway due to a few incidents in the area of unlocked cars getting stuff stolen (I lock my truck, but sometimes you accidentally unlock it, or maybe someone gets desperate and decides to start going for locked ones). I'm sure I'm going to have an aggravating time getting them placed with good enough signal and cursing those 40mhz neighbors.
Maybe if I set up a bunch of APs with 40mhz and really high power for a day or two all their routers will dump to 20mhz mode. They probably aren't even using anything 2.4.
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