topic AP behavior when detecting radar on DFS channels in Wireless

AP behavior when detecting radar on DFS channels

bakaholic39 — Tue, 11 Feb 2025 10:37:02 GMT

Hi all,

In the following process of DFS on Cisco Wireless Network (Either AireOS or IOS-XE WLC)

Could someone please confirm that I understand this correctly, particularly the 30-minute period of non-occupancy?

1.) DFS Non-Occupancy Period Rules
When an AP detects radar on a DFS channel (e.g., Channel 132), it must:

Immediately stop transmissions and vacate the channel within 1 second.
Avoid using the detected channel for at least 30 minutes (called the Non-Occupancy Period).
Select a new channel, either a non-DFS channel (preferred) or another DFS channel (which requires a Channel Availability Check).
Key Regulation (FCC, ETSI, etc.):

30-minute Non-Occupancy Period: The AP must not use the detected channel for at least 30 minutes after radar detection.

2.) AP Behavior After Radar Detection
Case 1: Non-DFS Channel Available
✅AP moves immediately to a non-DFS channel and resumes operation.

Case 2: Another DFS Channel is Selected
✅AP moves to another DFS channel but must perform a Channel Availability Check (CAC):
60 seconds of listening for radar before accepting clients.

Case 3: No Other Channels Available
⏳The AP remains offline until a valid channel is assigned.

3.) Can the AP Reuse Channel 132 Before 30 Minutes?
No, the AP must wait 30 minutes before reusing Channel 132.
Even if RRM selects Channel 132, the AP must reject it if it’s within the Non-Occupancy Period.

Re: AP behabior when detecting radar on DFS channels

Flavio Miranda — Tue, 11 Feb 2025 10:31:50 GMT

@bakaholic39

The only information I think is not right is this "Even if RRM selects Channel 132, the AP must reject it if it’s within the Non-Occupancy Period."

Re: AP behabior when detecting radar on DFS channels

bakaholic39 — Tue, 11 Feb 2025 10:36:12 GMT

Hello Flavio,

Thanks for sharing, this is just my understading, may be I'm miss. But for the AP could not reuse the same DFS channel again after detecting a radar for 30 mins, is it right on a Cisco wireless?, if this behavior is a standard, this can apply to every vendors?

Re: AP behavior when detecting radar on DFS channels

JPavonM — Tue, 11 Feb 2025 11:06:24 GMT

To add more, in Cisco, if the Cisco AP is configured to use a wide channel (40-, 80-MHz wide) it will turn to 20-MHz channel in whatever new channel it would be, and even when it turns to use the previous DFS impacted channel (132 in your example). Bonding channels won't be available until next RRM calculation.

Re: AP behabior when detecting radar on DFS channels

Flavio Miranda — Tue, 11 Feb 2025 11:19:02 GMT

@bakaholic39

Yes, this Will apply to all vendor because this is regulared for agencies around the world. What can vary is the Channels that Will be using.

Re: AP behabior when detecting radar on DFS channels

liviu.gheorghe — Tue, 11 Feb 2025 11:26:49 GMT

This is my understanding as well - if a radar signal is detected on a DFS channel, the AP and its clients must stop using it for 30 minutes and move to a different channel. This is standardised in 802.11h in order to avoid co-channel interference with systems that predated Wi-Fi like military or weather radars.

HTH