The 1700, 2700, 3700,1570, 2800, 3800, 4800, and 1560 series APs can use CleanAir hardware to support additional DFS signal filtering to avoid false events.Use CleanAir APs with superior radar detection capabilities.Disable Smart DFS - enabled by default on WLC.Enable 802.11h Channel Announcement - enabled by default on WLC.In order to reduce the impact of DFS events.Then use DFS channels, to provide additional capacity.Consider channel assignments such that there is at least one UNII-1 channel available throughout the coverage area.For the ETSI domain: there are only four non-DFS channels (36-48 UNII-1).Except for very dense deployments, these are enough channels (if using 20MHz wide) to provide full coverage with tolerable co-channel interference at full (14-17dBm) power For the FCC domain: there are 9 non-DFS channels (36-48,149-165).Use 20MHz channel width, which also allows better reuse of non-DFS channels.In order to mitigate/reduce the impact of these events, we can: In general, if using DFS channels with dense client populations, one should be prepared to handle up to four false DFS events per AP radio, as well as, of course, real radar events. 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.Ĭisco 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. It is very difficult to determine whether or not radar detection events are "false". It sees an energy pattern that it believes is radar, even though it is not (it may be a signal from a nearby client radio). wcp/dfs :: RadarDetection: sending packet out to capwapd, slotId=1, msgLen=386, chanCnt=1 -100įor IOS APs Feb 10 17:15:55: %DOT11-6-DFS_TRIGGERED: DFS: triggered on frequency 5320 MHzįeb 10 17:15:55: %DOT11-6-FREQ_USED: Interface Dot11Radio1, frequency 5520 selectedįeb 10 17:15:55: %DOT11-5-EXPECTED_RADIO_RESET: Restarting Radio interface Dot11Radio1 due to channel change from 64 to 104 Living with DFS channelsĪ "false DFS event" is when a radio falsely detects radar. wcp/dfs :: RadarDetection: radar detected When we examine the AP logs, we can see messages similar to the following:įor COS APs Radar detected: cf=5496 bw=4 evt='DFS Radar Detection Chan = 100' If a radar signal is detected, the AP selects a different channelĭFS triggered channel changes impact client connectivity. If there are no radar signals on the new channel, the AP enables beacons and accepts client associations. If the AP selects a DFS-required channel, it scans the new channel for radar signals for 60 seconds. If the selected channel is not DFS then AP enables beacons and accepts client associations.Selects a different channel from the DCA (Dynamic Channel Assignment) list.Broadcasts an 802.11h channel-switch announcement.Stops transmission of data frames on that channel.When an access point (AP) uses a DFS channel and a radar signal is detected, the AP will behave as follow: Thus, if a radio should detect radar in its serving channel, then switch to another DFS channel, this will impose (at least) a one-minute outage. And, if an 802.11 radio should detect radar while using the DFS channel, it must vacate that channel quickly. (Consult the applicable Channels and Maximum Power spreadsheets to see the specific channels that require DFS for a given access point / domain.)Ĩ02.11 stations, before transmitting in a DFS channel, must validate (by first listening for 60 seconds) that there is no radar activity on it. In most regulatory domains, 802.11 stations are required to use Dynamic Frequency Selection (DFS) when using some or all of the channels in the 5GHz band. This document briefly explains how radar detection in Dynamic Frequency Selection (DFS) channels works, and how to mitigate its impacts on wireless networks.
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