Effect of DFS on 802.11ac

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I'd be interested to know how 802.11ac is affected by DFS.

If you have an 80MHz channel and one channel out of four is hit by a radar blast, does the whole bonded channel go down and then maybe re-form as a 40MHz channel (avoiding just the single channel that was hit) ?

Regards

Nigel.
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Nigel Bowden

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Posted 5 years ago

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Matthew Gast

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In 802.11ac, the channel map is well-defined. Unlike 11n, where you had to choose between 40-above and 40-below settings, the channels are always the same. 36 & 40 are always put together for a 40 MHz channel.

That's illustrated in this figure, where you can see 20/40/80/160 MHz channels on the map:


So, if you are using an 80 MHz channel, say 100 to 112, and you have a radar detection event on 108, then channel 108 is not usable. The best you can do is fall back to 40 MHz on 110 and 104, at least until you no longer detect radar on 108.

And this is the behavior you want! One of the main requirements for DFS was driven by the development of weather radar at airports. For more background, see this: http://blogs.aerohive.com/blog/the-wi... (one of my co-workers lived a few miles from one of the planes that crashed due to the lack of wind-shear detection!).
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Christopher

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you can come back to the same DFS channel after DFS wait time.has elapsed.
If bonded with 11ac client AP will still use 80MHz if not AP will drop to 40MHz bandwidth.
5600-5650 MHz weather channel is restricted for use in most countries.
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Mirin Lew

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Matthew, I have seen the figure that you referenced in various IEEE documents and presentations and it usually says it illustrates the non-overlapping channels available for 802.11ac. However, it's not clear to me that it means you cannot have different combinations of channels other than those shown. For example, it may be possible to have a 40 MHz channel being formed using channels 40 and 44 instead of 36 and 40. Have you seen some document that indicates these are the only allowed channels? It would be helpful to have a reference.
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EvaldasOu

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Hi Mirin,
I think this link points to the answer:
http://revolutionwifi.blogspot.com/20...
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Mirin Lew

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Thanks for the link. I think the blog contains an error though. When I compare the channel numbers there with those in Annex D of the 802.11ac draft standard, it looks like the move toward using the channel center frequency to refer to the channel only applies to the 80 and 160 MHz BW channels. For 40 MHz the primary 20 MHz channel number is still used to refer to the overall channel.
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Matthew Gast

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You'll notice that partway through the post, the image in use comes from the 802.11ac book we're giving away today, and I'd like you to think that I wrote the book with the spec on my lap. ;)
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Matthew Gast

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The doc that you're looking for is 802.11-13/0282r0 (https://mentor.ieee.org/802.11/dcn/13...), which is a tutorial of how the 5 GHz band is laid out. The author of the section with the channel map (slide 6) is one of the leading experts on regulatory matters in the industry.