Bye Bye 40 MHz Mode in 2.4 GHz

[wpns]

Apple Airport Extreme Base Stations (and Time Capsules) at the latest firmware (V7.6.1) come OOB in "Use Wide Channels" mode.

You can't access the setting from "Airport Utility", you must use "Airport Utility 5.6", a separate download.

 

[thiggins]

Is that in 2.4 GHz and 5 GHz?

 

[wpns]

Dunno

Doesn't seem to distinguish between them. Just sez:

Use Wide Channels
Wide channels provide higher throughput in your network, but might interfere with nearby networks.

It's under:
<Pick an Airport>
Manual Setup
Airport
Wireless
Wireless Network Options

I always disable it, I've never been asked about throughput problems, and I'm always fighting connectivity and interference problems.

 

[thiggins]

I'll have to take a look at it for part 2. Thanks for the heads up.

 

[wpns]

I'll be extremely interested to know what it's actual behavior is in each mode. 8*)

Thanks!

 

[thiggins]

No joy. 40 MHz mode is still disabled in 2.4 GHz even with the "Use Wide Channels" option enabled.

 

[wpns]

So it's only applicable to 5GHz?

 

[thiggins]

So it's only applicable to 5GHz?

That is correct.

 

[somms]

Is it safe to assume this is only the case running certain stock firmwares and under third-party firmwares like tomato & dd-wrt the 40MHz bandwidth setting can be set and won't fall back to 20MHz?

 

[wpns]

Is it safe to assume this is only the case running certain stock firmwares and under third-party firmwares like tomato & dd-wrt the 40MHz bandwidth setting can be set and won't fall back to 20MHz?

I certainly hope none of the 3rd-party firmwares will allow "Trash Your Neighbor" mode!

 

[thiggins]

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.

 

[somms]

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...

 

[thiggins]

Thanks for the info. Unfortunately, that is what I am finding with many of the products I tested for Part 2 with stock firmware.

 

[somms]

Thanks for the info. Unfortunately, that is what I am finding with many of the products I tested for Part 2 with stock firmware.

Well, I would prefer not to be forced to drop to the slower 20MHz due to circumstances beyond my control like my neighbors AP...

 

[stevech]

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.

 

[thiggins]

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:

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.

 

[stevech]

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.

 

[beisser]

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.

 

[beisser]

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.

 

[thiggins]

As beisser reaffirmed, 20MHz in 2.4 GHz is in the 802.11-2012 spec. "40 MHz only" mode in 2.4 GHz is also against spec.