20/40 mhz coexistence default value

jsmiddleton4 wrote:
Nice tweak in DD-WRT and Tomato variants is the ability to manually select 20/40, 20 or 40 mhz. Makes more sense as an configuration option.

The problem with that as an option is that many people do not understand what it does, and even fewer understand the consequences. Many will just enable 40MHz bandwidth because they simply believe it makes their router go faster.

You might not have an issue, but it would appear that you have taken some steps to check what wireless activity you have around you and have come to the conclusion that you will not have a problem. Even in that case I would suggest measuring actual throughput rather than just relying on what the Link Rate is telling you.

If you want the analogy relating to road vehicles clarified. Well consider a three lane motorway, all vehicles are one lane wide (20 Mhz) all three lanes can run freely. Change the vehicles to two lanes wide (40 MHz), and what happens, overtaking becomes impossible if there are too many of that type of vehicle using the motorway, no one can overtake, everyone runs slowly.

This goes some way to explain the problem in WiFi terms;

http://www.smallnetbuilder.com/wireless/wireless-features/31743-bye-bye-40-mhz-mode-in-24-ghz-part-1

http://www.smallnetbuilder.com/wireless/wireless-features/31744-bye-bye-40-mhz-mode-in-24-ghz-part-2

Basically there are three non-overlapping 2.4 GHz channels in common use, channels 1, 6, and 11 (the three motorway lanes). If a network is able to force use of 40MHz bandwidth it will occupy most of the 2.4GHz band e.g. all of channels 1-6 or all of channels 6-11. Now it only takes a couple of networks to be doing that in proximity to each other and there will be contention across the entire 2.4GHz band. If the three separate channels are used at 20MHz bandwidth then you could have three networks operating with no contention at all.

There’s also this from dd-wrt; http://www.dd-wrt.com/wiki/index.php/Wireless-N_Configuration

The illustrations are useful as they show how many channels a 40MHz network occupies, but they are a little too neat. In my area there are 26 other WiFi networks which thankfully operate on channels 1, 6, and 11 with a fairly even distribution. I operate on just one of those channels so contend with roughly a third of those 26 networks. If I was able to force 40MHz operation I would see an excellent Link rate, but would then be contending with over two thirds of those 26 networks.

That dd-wrt article says “Effectively to use this feature, you will block 7–9 of the 13 channels and, of course, ch12 and ch13 aren't available in North America. So yes, it's possible that your neighbors will hate you if you turn this option on.” I think that’s somewhat misleading since it implies that only your neighbours will suffer, a quick look at how CSMA/CA operates will tell you that your network also suffers in that case. http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidance

So in your situation things may be fine now, but what if new networks appear in your area all forcing 40MHz bandwidth, you have no way to stop them and they probably won’t care or even know that while their Link Rate looks good, their throughput is actually pretty poor.

Basically, if you give people the option to override the 20/40MHz rule, they will almost always set the router to 40MHz. It can work in less densely populated areas, but can be a disaster where there are more than just a few active networks operating in range of each other and people just blindly enable the option.