What does DFS detection look for specifically?

SOLVED
combobulated
Getting noticed

What does DFS detection look for specifically?

We've suddenly seen a cluster of DFS detections that seem to be limited to a specific area of our campus.  We are pretty close to an airport and Coast Guard station but there is no fixed radar licensed on this band anywhere in our state.  It's possible that we're seeing glints of radar from military aircraft, however it seems that this would show up in a larger area of our footprint and it's not.  I'm wondering about the specifics of the DFS detection mechanism in our A/Ps.   Does the scanning radio look for any energy in this band that doesn't have OFDM or QAM modulation on it and assume that it's radar?  I'm wondering if we're seeing a babbling wi-fi device.

1 ACCEPTED SOLUTION
kNeil
Meraki Employee

So here are the factors for detecting a DFS event:
  • Each country’s RF regulatory domain has slightly different radar definitions and requirements for detection of radar
  • Most of the radar types have a very narrow bandwidth
  • For an AP to detect some of the types, it has to see a minimum number of pulses in the burst
  • The radar detection threshold (a.k.a interference detection threshold) has to be -64 dBm. In other words, the AP is required to move away from the channel only if the radar signal power is greater than -64dBm at the antenna

Hope this perspective helps.

View solution in original post

4 REPLIES 4
kNeil
Meraki Employee

Thanks for initiating this discussion ! To address the first part of why you are suddenly noticing DFS detections, its because we added DFS detection events into the Alert Hub and also on the AP details page on our dashboard earlier this month. These same DFS detection events were being captured in the event log all along and to raise awareness of potential impact of DFS hits, we decided to add this additional visibility. You can find additional details on this and potential actions to consider captured here: https://documentation.meraki.com/MR/Radio_Settings/Dynamic_Frequency_Selection_(DFS) . Do feel free to reach out if you have additional questions or any feedback you may have on this new visibility.

combobulated
Getting noticed

Thank you for this kNeil.  Yes the additional visibility for DFS events is very helpful.  After searching the logs though, it does appear that this is a recent spurt of activity, which goes back to my original question regarding the criteria for declaring a DFS event.  Is the radio looking for a particular chirp pattern in radar?  Some radars use chirp, some don't, some are pulsed while others are continuous wave, and none of them have Automatic Transmitter ID data embedded in the signal (this would be bad for tactical reasons).  So how does the radio determine that it's radar and not some stray noise from a misbehaving 5ghz wi-fi client device? 

kNeil
Meraki Employee

So here are the factors for detecting a DFS event:
  • Each country’s RF regulatory domain has slightly different radar definitions and requirements for detection of radar
  • Most of the radar types have a very narrow bandwidth
  • For an AP to detect some of the types, it has to see a minimum number of pulses in the burst
  • The radar detection threshold (a.k.a interference detection threshold) has to be -64 dBm. In other words, the AP is required to move away from the channel only if the radar signal power is greater than -64dBm at the antenna

Hope this perspective helps.

combobulated
Getting noticed

Thank you again kNeil.  These are the kinds of specifics I was looking for.

Get notified when there are additional replies to this discussion.
Welcome to the Meraki Community!
To start contributing, simply sign in with your Cisco account. If you don't yet have a Cisco account, you can sign up.