Dynamic Frequency Selection (DFS)
DFS is the process of detecting radar signals that must be protected against interference from 5GHz (802.11a/n/ac/h) radios, by dynamically switching the operating frequency of the 5GHz radio to one that does not interfere with radar. Transmit Power Control (TPC) is similarly used to adapt the transmission power of a radio based on regulatory requirements and range information. Certified devices have the ability to enable DFS to take advantage of additional channel options, while listening for radar communication to dynamically update their radio settings if necessary.
DFS-enabled radios monitor the operating frequency for radar signals. If radar signals are detected on the channel, the wireless device takes these steps:
- Blocks new transmissions on the channel.
- Broadcasts an 802.11h channel-switch announcement.
- Disassociates remaining client devices.
- Access Point selects a different channel permitted within the regulatory domain.
- After the DFS non-occupancy period has been reached for the original DFS channel, if no clients are associated it will move back to the original DFS channel and scan for 60 seconds. If there are no radar signals on the new channel, the wireless device enables beacons and accepts client associations. If no other DFS events are detected and there are no wireless clients associated to the SSID on 5GHz, the AP will revert back to the original DFS channel, usually in 30 minutes. If clients are detected, it takes 12 hours to go back to the original DFS channel.
The Meraki Cloud assigns the appropriate regulatory domain information to each Meraki Access Point. Which bands and channels that are available for a particular Meraki Access Point depends on the model, indoor/outdoor operation, and the regulatory domain. Since the Meraki Cloud enforces the wireless regulations, the channel list on the Radio Settings Page will show the channels the access point is certified to use.
For more specific certification and band information, please refer to the install guides and the regulatory information pamphlet included within the AP's original packaging.
The Event Log that is available via the Meraki cloud will report DFS events as they occur. Below is a example report for the Access Point Named "Gateway" which detected a radar on channel 100:
In addition to the event log, DFS/Radar events will show up in the AP status page as well, showing the channels that are being hit the most for a specific AP. Below is the screenshot showing this in Dashboard on an AP status page:
To provide a more holistic network-wide view, we have combined all these DFS/Radar events into one list for easier understanding and consumption. The view exists in the Network Alert hub in Dashboard:
Expanding on the Insights tile will provide more information on a per AP level and DFS/Radar events detected by each and every AP.
Radios on newer generations of MR access points (e.g. WiFi-6) have a higher sensitivity compared to older generations (e.g. WiFi-5 Wave 2) which might result in a higher number of DFS events detected by newer APs.
DFS Prone Areas
The number of DFS events that are seen by an Access Point is generally related to the physical location. DFS events will be most common near waterways and airports but can occur anywhere. Either manually selecting non-DFS channels and/or excluding DFS channels from the Auto Channel assignment are two approaches to ensure the wireless network is operating reliably if there many DFS events. For more information on selecting and planning wireless with DFS channels, please refer to the document, Meraki Auto RF: Wi-Fi Channel and Power Management