Zero Wait DFS, how does it work?

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The AP250's we use in our office support Zero Wait DFS, which apparently allows for fast channel switches to DFS channels as the name suggests.

Could an engineer explain how this technically works? I tried searching for details, requirements, examples, but only found a couple sales quotes, nothing technical.

Regards,
-Menno
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Menno

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Posted 1 year ago

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Luke Harris

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Menno

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Hi Luke, thanks for your reply. The article you posted explains DFS, but nothing on 'Zero Wait DFS', which is what I'm interested in. 

It's a new feature in the AP550 and actually apparently not the AP250's. 

Anyone?

-Menno
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Luke Harris

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Hey Menno, 

Sorry, I seem to have misread the question! Without sounding patronising, I would assume that 'Zero Wait DFS' works in exactly the same way as DFS but with shorter/reduced negotiation times between channels using a revised algorithm. I've just spent some time looking myself and can't seem to find a 'definition' or 'how it works'. 
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Robert Nicholas, Employee

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Hi, Menno--

In case you never received a satisfactory answer since you last posted this questions, I hope this provides sufficient detail:

Zero Wait DFS (also called Zero Wait CAC or Dynamic In-Service Monitoring) allows devices to continue transmitting on a channel while simultaneously looking ahead to a quiet DFS channel to monitor it for the requisite 60 seconds. After the 60 seconds have elapsed, the device can switch to the target DFS channel without having to wait an additional 60 seconds. In this way, there is a virtually seamless transition and almost no pause in transmission.

If I find an authoritative FCC source for the functional requirements, I'll be sure to post them here as well.
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Metka Dragos

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Just to add to Robert's comment, Zerowait DFS can 'scan ahead' because one of the spatial streams on the 5 GHz radio is dedicated to the continuous scan. This way the mandatory 60 sec scan before taking the channel is satisfied and it prevent the holes in a coverage that we get with the 'normal' DFS channel scanning requirements. Scanning requirements are part of 802.11h standard. 
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Menno

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Thanks Robert & Metka for your answers! Searching for Dynamic In-Service Monitoring gives a lot more useful results ;) 
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Devin Akin

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Zero-wait DFS is useful for DFS-to-DFS channel switching as needed due to ACSP channel changes, but not useful in the case of radar events. The reason is that with ACSP, you have plenty of time to make and execute the channel change, but with radar events, you have a maximum of 10 seconds for the AP and clients to vacate the channel.

https://apps.fcc.gov/kdb/GetAttachmen...

One way that other vendors get around the FCC rule requiring a 60-second pre-scan of the target DFS channel is by "statistical scanning." This can be accomplished with coordinated background scans among multiple APs in an area scanning the same "DFS fallback channel" over time, a little at a time. That will allow you to experience a radar event and then fail over to a a specific DFS channel of your choosing (instead of a non-DFS channel).

For more info, see here: http://divdyn.com/dual-5ghz-radio-aps/

Hope this helps.

Devinator
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Robert Nicholas, Employee

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Great addenda, Devin! And thanks for attaching the authoritative reference.