Radar Detection Events anyone?

Unanswered Question

Hi all,

Wondering if anyone else is starting to see Radar Detection and 802.11h-driven Channel Change Events on their APs. We're seeing a sparse number of far-flung APs registering Radar in the 5GHz UNI-II band, where they should be more frequent and widespread given our proximity to a large metro airport in one of our dense deployment locations. Does anyone know of any bugs related to to 802.11h? I know there's a pending update to controller code to comply with the standard radar-detection dwell time.

Thanks for sharing your radar screens!

--Bruce Johnson

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (2 ratings)
didyap Wed, 03/04/2009 - 08:29

Dynamic Frequency Selection (DFS) is the process of detecting radar signals that must be protected against 802.11a interference, and upon detection switching the 802.11a operating frequency to one that is not interfering with the radar systems. Transmit Power Control (TPC) is used to adapt the transmission power based on regulatory requirements and range information.


Thanks for the definition. I'm wondering if anyone is actually seeing any radar events. They aren't occurring in a way that I expect (i.e. I should see many more based on some of our building's proximity to local metro airport). I am only seeing a handful - a few in disparate buildings, some not near any airport.

So, does anyone else echo that their APs are detecting radar echo? If so how often and does it appear to be a true positive?

Leo Laohoo Wed, 03/04/2009 - 14:45

I've never been in this scenario before so I'm approaching this in a curious/learning mode.

Does the 802.11H appear on the WLC/WCS as an Unclassified Rogue element?

dennischolmes Wed, 03/04/2009 - 18:49

Not all airport radars set off the alerts. Radars cover a broad spectrum of frequencies. For instance the radar range now known as the microwave oven produces signals in the 2.4ghz spectrum. DFS only occurs if the APs and controllers see radar on the actual channel being used. Then they discontinue useon that channel for 30 minutes.


We're talking about 802.11h here, 5GHz only. If it were radar, a lot more APs would detect it. Personally I think its a false positive, possibly due to 5.8GHz FH phones or the like. It begs the question however what the APs interpret as radar and whether it's in compliance with the 802.11h standard. I don't need another form of DCA ripple on false grounds.

dennischolmes Thu, 03/05/2009 - 04:25

I know Bob that we're only dealing with 11h but I was making a point that radars can exist on a variety of frequencies not just limited to 5ghz. The problem with radar detection is that you never really know when it is radar. Radar comes in so many flavors. Doppler, swept, stationary/directional, ground, and downlooking just to name a few. 11h was meant specifically not to interfere with some specific fixed directional aircraft radar similar to those you would find in the nose radomes of commercial and military aircraft for air contact avoidance. Usually the aircrafts' nose would be pointed up and away from your APs as they take off or land but when on taxi they could set off your APS. I agree that more than likely you are seeing false positives but it is possible you are right on the edge of a beam path when an aircraft makes a turn on the field. If that is the case then only one or two of your APs may actually see the electron beam of the directional radar in use. Kind of like falling into the headlights or a car at night.

Thanks Dennis,

This helps, and may explain the sparse nature of the events.

For those interested, this is what the radar event looks like:

WCS has detected one or more alarms of category AP and severity Major

for the following items:

{wlc1} Radar has been detected on channel '60' by AP base radio MAC '00:1e:bd:65:e3:70' on radio 802.11a.

E-mail will be suppressed up to 30 minutes for these alarms.

And the Channel Change event looks like this:

WCS has detected one or more alarms of category AP and severity Major

for the following items:

{wlc1} AP 'ap6', interface '802.11a' on Controller 'xxx.xxx.xxx.xxx'. Channel changed to '64' from '60' due to 'None'. Interference Energy before update is '0' and after update is '0'. Noise before update is '0' and after update is '0'. Interference before update is '0' and after update is '0'.

{wlc1} Radar has been detected on channel '60' by AP base radio MAC '00:1e:bd:65:e3:70' on radio 802.11a.

E-mail will be suppressed up to 30 minutes for these alarms.


kmcsweeg Sat, 03/14/2009 - 17:34


I'm also getting several Radar events, it's really causing alot of issues with my 1522's, 802.11h is not currently enabled, and it is still moving my backhaul links to random channels, see below.

200 Sat Mar 14 19:49:42 2009 Channel changed for Base Radio MAC: 00:22:be:43:04:00 on 802.11a radio. Old Channel: 149. New Channel: 64. Why: none. Energy before/after change: 0/0. Noise before/after change: 0/0. Interference before/after change: 0/0.

201 Sat Mar 14 19:49:42 2009 Radar signals have been cleared on channel 116 by 802.11a radio with MAC: 00:22:be:43:04:00 and slot 1

Any idea guys?

CSCse28941 Traffic can sometime cause the AP to think it is seeing radar

Cisco AP1510 Mesh AP's (MAPs) may become isolated when deployed in the

ETSI radio-frequency domain if the Dynanmic Frequency Spectrum (DFS) feature detects what it perceives to be radar signals, even if no actual radar is present. Because the rule is to cease using channels in which radar is thought to have been detected, a MAP may quickly run out of usuable channels and thus be rendered unable to communicate with its Rooftop AP (RAP). A MAP so affected becomes isolated and even the Lonely code cannot bring it back. Conversely it is not possible to reach or signal the MAP from either the Wireless LAN Controller (WLC) or from a RAP.

MAPs in the US domain cannot incur this condition because radar detection is not used, as US radar is not in the 5.8 GHz Radio Frequency (RF) band.

The workaround is to have previously performed a site survey to ensure that there is no interference detected in the 5.8 GHz band, so that there is no risk of falsely detected radar.

CSCsr01267 Radar detected across all channels


For RAPs/MAPs:

AP may report radar across all channels under certain circumstances.


For RAPs/MAPs:

Suspecting additional RF signal, that together with real radar limited to some channels, triggers detection across all outdoor 11a band



1000 Series Access Points and Radar Detection

The 1000 series access points perform radar detection on channels that do not require it (such as channel 36). If the access points detect radar on these channels, the controller captures it in log messages.


Leo Laohoo Thu, 04/02/2009 - 16:16

Greeeeeeeeeeat! An AP who think it's a Radar Warning Receiver. :P He he he ...


This Discussion



Trending Topics - Security & Network