This document describes radar detection in Dynamic Frequency Selection (DFS) channels theory, and how to mitigate its impacts on wireless networks.
In most regulatory domains, 802.11 stations are required to use Dynamic Frequency Selection (DFS) when used with some or all of the channels in the 5GHz band. (Consult the applicable Channels and Maximum Power spreadsheets to see the specific channels that require DFS for a given access point / domain.)
802.11 stations, before they transmit in a DFS channel, must validate (listen for 60 seconds) that there is no radar activity on it. And, if an 802.11 radio detects radar while the DFS channel is used, it must vacate that channel quickly. Thus, if a radio detects radar in its serving channel, then switch to another DFS channel, this then imposes (at least) a one-minute outage.
When an access point (AP) uses a DFS channel and a radar signal is detected, the AP then:
DFS triggered channel changes impact client connectivity. When we examine the AP logs, we can see messages similar to the following:
For COS APs
[*04/27/2017 17:45:59.1747] Radar detected: cf=5496 bw=4 evt='DFS Radar Detection Chan = 100' [*04/27/2017 17:45:59.1749] wcp/dfs :: RadarDetection: radar detected [*04/27/2017 17:45:59.1749] wcp/dfs :: RadarDetection: sending packet out to capwapd, slotId=1, msgLen=386, chanCnt=1 -100
For IOS APs
Feb 10 17:15:55: %DOT11-6-DFS_TRIGGERED: DFS: triggered on frequency 5320 MHz
Feb 10 17:15:55: %DOT11-6-FREQ_USED: Interface Dot11Radio1, frequency 5520 selected
Feb 10 17:15:55: %DOT11-5-EXPECTED_RADIO_RESET: Restarting Radio interface Dot11Radio1 due to channel change from 64 to 104
A "false DFS event" is when a radio falsely detects radar. It sees an energy pattern that it believes is radar, even though it is not (it is possibly a signal from a nearby client radio). It is very difficult to determine whether or not radar detection events are "false". If there are multiple AP radios on the same DFS channel in the same location, then we can assume, as a rule of thumb, that if a single AP detects radar at a given time, then it is probably false detection, while if multiple radios detect radar at the same time, it is likely "real" radar.
Cisco has numerous improvements to our access points' ability to distinguish between real and false radar signals; however, it is not possible entirely to eliminate all false radar detection.
In general, if DFS channels are used with dense client populations, one must prepare to handle up to four false DFS events per AP radio, as well as, of course, real radar events.
In order to mitigate/reduce the impact of these events, we can:
Understand Dynamic Frequency Selection - DFS Actions
Spectrum Sharing in the 5 GHz Band - DFS Best Practices (IEEE)