Wireless Access

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,

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). 

Hi Howard,

How is the radar energy detected? Does AP 325 need to stop transmiting and receiving to detect radar singal?

Say we deploy DFS channel 50. We know for a fact that there is no radar energy on this channel. Would voice/vedio quality be affected due to radar energy detection?

Thanks,   

The AP has radar "signatures" that it uses to detect radar.  It does not need to stop transmitting to detect radar.

Radar can be random or constant, so sometimes you cannot know there is radar on a channel unless it shows up.  You can use the Aruba Utilities Android App to see if there is known radar on specific channels in your area.

Hi Joseph,

How about receiving? Would radar detection affect receving when there is no radar signal? Would AP pause receiving every once a while to detect radar, therefore, affect voice quality due to packet loss in uplink?

Thanks,

No.  Scanning for a DFS signature is a normal function that is done in the course of a client being served traffic.  The only disruption is when the AP has to vacate the channel immediately.

Hi guys,

I have read that DFS channels are not enabled by default in Aruba controllers:

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

What about in Instant APs? When I select the advanced options of RF on Instant, it appears like this:

It seems all 5 GHz channels are enabled by default (DFS and non-DFS), is this correct?

Hi,

Running the command "show ap allowed-channels" shows all 24 channels in the 5GHz band are allowed:

Then does this mean that in Instant APs DFS channels are enabled by default unlike controller based APs?

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.

Dear all,

Thanks for your response which help me a lot.

Best regards,