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Does anyone know the technical details behind Intel's 5100 AGN client device setting called "Fat Channel Intolerant" ?

Their website is poor on technical details. In brief, it says "it communicates to surrounding networks that the Wi-Fi adapter is not tolerant of 40 Mhz channels in the 2.4 GHz band". Elsewhere it mentions being incompatible with 802.11n - duh.

Is this merely the way of telling the adapter to only make 20 MHz connections, or is it some broadcast that tells nearby neighbors not to use it also ?

My guess is the former - but the opposite would be good to know about.

My understanding is setting that flag does both - tells the card not to use 40 MHz channels and also begs the neighbours not to use 40 MHz channels as well.

I say beg because I did some poking around on some Aruba gear at my place and it looks like ArubaOS is set to 'Honor 40 MHz Intolerance' by default but it is just a check box [url=http://www.flickr.com/photos/simplywifi/5798295274](see screenshot here)[/url] so anyone could simply decide not to honor that setting at any time. I can only assume other enterprise vendors have a similar toggle. Not sure about home gear though.

Seems like a good option for 2.4 GHz in theory, but if people can just turn it off I'm wondering how effective it really is.

In case anyone is interested, here is a screenshot showing the 40 MHz intolerance bit in Wireshark - [url=http://www.flickr.com/photos/simplywifi/5797819731](Intolerance bit not set)[/url]

Cool thread.

WLANMAN, why wouldn't an agn card be n capable? I just did a quick search on 5100AGN ... looks like an old intel card that was designed during pre-n era...is that the one you're referring to? Looks like 5300 is the newer card, post n ratification.

I agree with you both that it does seem like a smart option, to be able to request no 40 MHZ channels for 2.4. (Of course, why not just run 5GHZ and be done with it.) But "not allowing" doubled channels on the device itself at 2.4 almost seems useless to me; who would want to do that anyhow, and is INTEL just proecting people from themselves? =) But the idea of it being able to tell neighbors seems it might be useful...of course if I were running a 40 MHZ network and some yahoo card was able to bump me down to 20 I might be irritated. I am curious about exactly how this feature works now, what it specifically does. Thanks for the screen capture btw SimplyWiFi. Interesting that the bit's set by default. I wonder if any of the SOHO devices address this option. Something more *sigh* to research! =) Good stuff!

Here's the intel whitepaper I saw for INTEL's cards: http://www.intel.com/products/wireless/adapters/5000/index.htm?wapkw=(5100)

SeaLass,

Setting the "Fat Channel Intolerant" and at the same time allowing your own device to allow 40MHZ is what would be incompatible. Which Intel seems to agree with.

It would definitely not be "playing nice" to run 40MHz yourself, and not let your neighbors to do the same.

If you are running 40 MHZ channels in a sea of 2.4 GHZ 20 MHz channels you're definitely being a bandwidth "road hog" - that is, if you are not following the rules.

I live in an area where several people are running "pre-n" AP's. In this case Belkin. They defineitely don't "play nice", and stomp all over everyone elses 20MHz wide comms. They don't seem to care what is going on around them.

Aha! THAT makes sense, got it. Thanks!!

At the current time the WFA says "Wi-Fi CERTIFIED n products are configured to operate using 20 MHz channels by default, [u]and must employ coexistence mechanisms[/u] to help ensure that the device defaults to 20 MHz operation when sharing the frequency with other Wi-Fi networks."

It seems many people (of the type who automatically set output power to MAX) also like to set 40MHz channels when they don't really need it.

Like you said - if you want 40 MHz, please go to 5 GHz.

....well, but still....if one's running in 2.4 GHz, it just seems like it would be shooting oneself in the foot to run 40 MHZ channel width, unless using Meru controller or similar, because of limitations on AP channel separation. I think you'd be down to only one channel option then, or possibly two...

...but I guess it really comes down to whether we're talking Enterprise or SOHO...in the problem you are describing it would be SOHO. Right? I guess if you only have one AP you don't much care about managing your spectrum.

...and if enterprise, it comes back to the thought that I wouldn't want to be running in 2.4 anyhow so I guess the point would be moot.

Yes?

I didn't see your last post before I posted mine... we're in synch. =)

Yes