I would not assume anything. The fallback mechanisms are part of the 802.11n spec and are supposed to be implemented, whether the product is Wi-Fi Certified or not.
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Tim Higgins
Managing Editor,SmallNetBuilder.com
I would not assume anything. The fallback mechanisms are part of the 802.11n spec and are supposed to be implemented, whether the product is Wi-Fi Certified or not.
Just confirmed that neither tomato or dd-wrt will fallback to slower 20MHz bandwidth in the presence of 2.4GHz interference at least in my environment! Tested on ASUS RT-N66U w/tomato, WRT-600N w/dd-wrt and E2000 w/tomato...
there was, some time back, an agreement but not an IEEE requirement, but maybe a WiFi alliance recommendation, that automatic use of 40MHz would be disabled if any 20MHz networks were in range. I suppose a manual choice of 40MHz would override this "courtesy" mode.
there was, some time back, an agreement but not an IEEE requirement, but maybe a WiFi alliance recommendation, that automatic use of 40MHz would be disabled if any 20MHz networks were in range. I suppose a manual choice of 40MHz would override this "courtesy" mode.
I downloaded a copy of 802.11-2012 (only $5). Section 10.15 and 10.17 are very helpful in understanding what is really supposed to happen.
Section 10.15.3.2 is quite clear:
Quote:
An FC HT AP 2G4 shall maintain a local boolean variable 20/40 Operation Permitted that can have either the value true or false. The initial value of 20/40 Operation Permitted shall be false.
This means that a 2.4 GHz AP must start in 20 MHz mode and can only switch to 40 MHz if numerous conditions are met. Conditions include no reception of the 40 MHz Intolerant bit and a channel scan that finds no interfering networks.
I cannot find anything in 802.11n-2012 that allows a "40 MHz only" mode in the 2.4 GHz band.
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Tim Higgins
Managing Editor,SmallNetBuilder.com
Perhaps the 20MHz by default or as a courtesy to nearby 20MHz networks is a recommendation or requirement of the WiFi alliance rather than IEEE. The FCC regulations don't dictate this - just power limits and out-of-band emissions.
WiFi supposedly has criteria to be met, such as transmitted waveform quality (Rho) and packet error rate vs. SNR and so on, else you don't get certified and cannot use the WiFi logo. I have my doubts as to how stringent the alliance is in such verifications - the alliance is funded by meager dues from member manufacturers and has few resources.
tim said "I downloaded a copy of 802.11-2012 (only $5)." this is ieee, not wifi alliance. wifi alliance only tests interoperability, not defines standards. ieee is for the standards and 802.11-2012 is a standard.
so any device not falling back is indeed no longer 802.11 compliant.
Well, I would prefer not to be forced to drop to the slower 20MHz due to circumstances beyond my control like my neighbors AP...
well if there is a neighbors ap, your 40mhz mode wont work very well anyway as that ap WILL cause interference and framedrops and whatnot. what you will see is a rapid decline in performance and increasing packetloss. so sticking to 40 mhz mode wont do you any good in that scenario.
Tested on ASUS RT-N66U w/tomato, WRT-600N w/dd-wrt and E2000 w/tomato...
Linear Mode
