The 30 seconds after a DFS event
"Clients dropping on channel 100. Weather radar 40 miles away passed through its scan arc." ·
A DFS event looks like an AP crash. The AP goes silent for 60 seconds, comes back on a different channel, and everyone assumes it rebooted. It did not reboot. It was doing exactly what the spec requires.
The exact sequence
T+0s → T+10s: AP sends CSA IE in remaining Beacons (Channel Switch Announcement).
T+10s: AP goes completely silent on ch 100.
T+10s → T+70s: AP silently performs CAC on target channel. No frames anywhere.
T+70s: CAC complete. AP begins Beacons on new channel.
Channel Switch Announcement IE. If present in the Beacon stream, a DFS event is in progress. Channel Switch Count field counts down Beacon intervals to the switch. Mode byte 1 = do not transmit before switch.
What the 60 seconds costs
No pre-scanned alternate channels = 60-second outage minimum. Enterprise APs with a dedicated monitor radio can pre-qualify DFS channels and switch immediately. If your vendor supports this, enable it. Alternatively, run client radios on UNII-1 (36/40/44/48) or UNII-3 (149-165) — no DFS required, no outage possible.
Recurring events at regular intervals
Weather radar rotates at 5-6 RPM. DFS events on the same channel every 10-12 seconds = a rotating radar beam. Move to a channel outside that radar frequency range.