Much of the 5 GHz band is shared with weather, military and airport radar, so an AP there must listen for radar before it transmits and leave the channel when radar appears. Play through a power-on and a radar event, and see what it costs your clients and which channels are left.
What DFS asks of an AP
On a DFS channel (52–64 and 100–144; 100–140 in Europe) an AP must first listen for radar for the channel availability check (CAC) before sending anything, beacons included. While it operates it keeps listening. If it detects radar, it must stop using the channel within the channel move time, telling its clients where it is going with a Channel Switch Announcement in its beacons (IEEE 802.11-2024 9.4.2.17), and it may not return to that channel for the non-occupancy period. The rules come from regulators; the 802.11 standard supplies the tools to follow them (IEEE 802.11-2024 11.8).
1 · The AP
Rules
Channel width
Click a channel to put the AP there.
No DFSDFSDFS, weather radar band (5600–5650 MHz)Not allowed for Wi-Fi hereThe AP's channelWhere radar is detectedWhere the AP moves
2 · The radar event
Where the AP goes after the radar, and whether it comes back, are chosen in the flowchart below: click a branch.
3 · What happens
Serving clientsChannel availability check: silent, no serviceMoving: no serviceChannel blocked (non-occupancy period)
4 · The rules side by side
Definitions
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.
Notes
Sources: United States: 47 CFR 15.407(h) (DFS in 5.25–5.35 and 5.47–5.725 GHz; 60 s channel availability check; stop within 10 s, with normal traffic for at most 200 ms; non-occupancy at least 30 minutes). Europe: ETSI EN 301 893 V2.1.1, Table D.1 (60 s, or 10 minutes for channels touching 5600–5650 MHz; off-channel CAC 6 minutes to 4 hours, or 1 to 24 hours near weather radar; channel move time 10 s; channel closing transmission time 1 s in total; non-occupancy 30 minutes). Thailand: NBTC TS 1035-2562 accepts testing to ETSI EN 301 893 V2.1.1, so the ETSI values are shown; the NBTC text itself has not been checked here, so confirm details with NBTC. The 802.11 side: IEEE 802.11-2024 11.8 (DFS procedures), IEEE 802.11-2024 9.4.2.17 and IEEE 802.11-2024 11.8.9 (Channel Switch Announcement), IEEE 802.11-2024 9.4.2.161 (Channel Switch Wrapper, whose Wide Bandwidth Channel Switch subelement announces a new width of 40 MHz or more) and IEEE 802.11-2024 9.4.2.159 (Wide Bandwidth Channel Switch).
Channels: the channel lists are simplified. In Europe channel 144 is outside 5470–5725 MHz and not allowed; in Thailand and the US it is allowed as a DFS channel. Channels 149–165 are open to Wi-Fi in Thailand and the US; in Europe that band is mostly for short-range devices and not used by normal Wi-Fi APs. National rules change: check the current ones before a design.
Simplified timing: a client that hears the announcement follows the AP in about 50 ms after the count reaches zero; one that misses it rescans and reconnects in about 3 s (much longer to a DFS channel, which it may only scan passively). Real APs and clients vary. Some APs pick a random channel after radar, as ETSI asks usable channels to be used with about equal probability.
2.4 and 6 GHz have no DFS. In 6 GHz, standard-power APs are controlled by an automated frequency coordination (AFC) system instead.