40MHz 802.11N setup saved but is 20MHz wide

Lantis1008 your link http://https://www.smallnetbuilder.com/wireless/wireless-features/31743-bye-bye-40-mhz-mode-in-24-ghz-part-1 tells us that most routers failed to comply with the "fallback to 20MHz".

This "fallback to 20MHz is there to prevent 802.11N from interfering with 802.11ABGs radios (legacy radios). I can't find an 802.11G in my operating area. Its a good thing I can force openWRT to 40MHz because I have neighbors using 40MHz 802.11N operating in my area. For two 802.11N radios operating on co-channel or adjacent channel it is useless to "fallback".

I found this document http://https://www.arubanetworks.com/pdf/technology/whitepapers/wp_Designed_Speed_802.11n.pdf a concise explanation on the matter written well.

My openWRT "Allow legacy 802.11b rates" is UNchecked and "Force 40MHz mode" is checked. I can tell you that DLink, TPLink, Netgear, Linksys, and Belkin routers all persist at 40MHz using 802.11N BUT I've always disabled legacy 802.11 radios in those router too. If you guys are still conserving bandwidth for legacy devices good luck.

I'm going to be running speed test over the next few days and see if the bandwidth makes a big difference.

It doesn't help when you read selectively

from the same document cached

According to 802.11-2012, APs and routers must default to 20 MHz bandwidth mode in the 2.4 GHz band . They may switch to 40 MHz bandwidth mode only after satisfying multiple criteria, including no "fat channel" intolerant bit set and no interfering APs. In addition, to meet spec, APs are not allowed to have a "40 MHz only" mode in the 2.4 GHz band .

Which probably means OpenWrt should remove the option to have "40 MHz only" so it doesn't get abused.
PS
It may also explain why AP manufactured prior to 2012 still have 40 MHz only modes

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Per the above quote from documentation...it appears a factor in switching to 40 MHz is that there be no neighboring interference...this lends to my previous statement:

@mbo2o also clearly stated why the force 40 MHz does legally exist on routers manufactured prior to ~2012...and even some manufactured after (which doesn't necessarily make it a problem, i.e. the miniPCIe card is manufactured before, etc.).

I think my illegal interference comment should be more serious to you now...

I :pray: you don't live in a heavily-congested area (i.e. a high rise apartment complex) and end up jamming your neighbors all week...

For anyone reading this at a later date, there are some misconceptions about the role of OFDM as used by 802.11. The "orthogonality" of the sub-carriers are not used so that each station/AP can "pick" a "clear" subcarrier, but to reduce the overall symbol rate. As such, there is no station-to-station orthogonality, no magic that keeps stations or APs from interfering with each other. By reducing the symbol rate, making each symbol (usually between a "bit" and a "byte" of data on the air) longer in time, the impact of multipath (signals arriving at the receiver "bounced" of things, at slightly different times) and other imperfections in the transmit-receive path on error rate can generally be reduced.

As pointed out a few times in this discussion, if there were such magic, then the 802.11 protocols wouldn't need a "one talker at a time" policy, nor would there be hundreds of articles on channel planning that describe how to deploy APs on different channels so as not to interfere with each other.

For those interested in a reasonably clear discussion of how OFDM works without getting into too much math and signal-processing theory

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