Wireless Access

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Occasional Contributor II
Posts: 24
Registered: ‎01-09-2015

What is the exact AP behavior when running on a DFS channel?

Dear all,

 

I read this really interesting topic about DFS:

http://community.arubanetworks.com/t5/Controller-Based-WLANs/What-is-Dynamic-Frequency-Selection-DFS-How-to-Enable-it-What/ta-p/179170

 

After reading, I'm just wondering what is the exact behavior of the AP when running on a DFS channel compared with a AP running on a no DFS channel.

 

I know the AP has to check the activity on the channel before starting working on it. But how many time the AP has to wait before starting his operations? Am I right by thinking that an AP running on a no DFS channel can start directly working on the channel.

At which interval does an AP running on a DFS channel stop serving the client to check that no Radar signal are there? How long take this regular scan?

 

Many thanks in advance for your help,

Best regards,

 

MVP
Posts: 1,302
Registered: ‎11-07-2008

Re: What is the exact AP behavior when running on a DFS channel?

For the most part there's no major delay when an AP comes up on a DFS channel. The thing to be aware of is if it detects Radar on the channel it's operating on, it will immediately vacate that channel, regardless of any ARM settings you may have made (or statically configured channels as well). So if the AP detects any radar pulses when operating on X channel, if will immediately drop and change channel (with ARM enabled) or it will shut off for some period of time and come back and check again (if statically assigned). 

Jerrod Howard
Sr. Techical Marketing Engineer
MVP
Posts: 1,110
Registered: ‎10-11-2011

Re: What is the exact AP behavior when running on a DFS channel?

I know the AP has to check the activity on the channel before starting working on it. But how many time the AP has to wait before starting his operations? Am I right by thinking that an AP running on a no DFS channel can start directly working on the channel.

 

When the AP moves to another DFS channel, the wait time is 60 seconds (Channel Availability Check Time) as it must listen for additional radar pulses.  I've researched this multiple times and haven't found anything that says if the AP moves to a non-DFS channel it may transmit immediately, but that just seems logical.  So if moving from channel 60, to 40, I would assume waiting is not required.  I just can't find anything that explicitely states this.

 

There is an option that can be enabled called "channel switch announcement".  This notifies clients on the channel that the AP will soon switch to another channel after radar is detected.  This allows the clients to move with the AP.

 

At which interval does an AP running on a DFS channel stop serving the client to check that no Radar signal are there? How long take this regular scan?

 

Radar detection is constant.  There is an additional knob that can be enabled that allows the AP to quiet the channel and detect radar on other channels, but it requires 60 seconds worth of passively listening.

 

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Occasional Contributor II
Posts: 24
Registered: ‎01-09-2015

Re: What is the exact AP behavior when running on a DFS channel?

Dear all,

 

Thanks for your response which help me a lot.

 

Best regards,

 

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