DFS channels: when to use them and when to stay away

Dynamic Frequency Selection exists because Wi-Fi is a guest in the 5 GHz band. Radar systems — military, aviation, maritime and weather — are the primary users. Wi-Fi must detect them and get out of the way.

What actually happens during a radar event

1. The AP's radar detector matches a pulse pattern on the operating channel.
2. Within 10 seconds, the AP must stop transmitting on that channel.
3. The channel is marked unavailable for 30 minutes (the non-occupancy period).
4. Clients are moved — with luck via 802.11h channel switch announcements, in practice often by simply losing the AP and re-associating.

The user experience ranges from a barely noticeable stutter to a dropped call and a thirty-second reconnect, depending entirely on client behaviour.

The channel map

Without DFS you have roughly 9 usable 20 MHz channels in most domains. With DFS you have around 25. At 40 MHz that is the difference between four channels and twelve — the difference between a design that works and one that is permanently co-channel.

A decision framework

Enable DFS when:

• You are more than roughly 30–50 km from an airport with terminal approach radar, and not on a coastline with weather radar.
• AP density is high enough that non-DFS channels would force co-channel reuse within earshot.
• The workload tolerates a rare short interruption — email, web, file sync, most video conferencing.

Disable DFS when:

• The site is near an airport, port, military base or naval facility.
• Real-time clinical, industrial control or broadcast traffic runs over Wi-Fi.
• You are supporting large numbers of cheap IoT devices with poor 802.11h handling — they frequently do not follow the channel switch and simply vanish.

Always exclude channels 120, 124 and 128 unless a long baseline measurement says otherwise.

Test before you commit

Do not decide from a map. Park a single AP on each candidate DFS channel for a fortnight and log radar events from the controller. Two weeks of real data beats any amount of theory, and it gives you a defensible answer when someone asks why half the spectrum is switched off.

Client-side reality

Some clients — particularly older IoT modules, some medical telemetry and a depressing number of budget Android devices — do not support DFS channels at all. They will never see an SSID on channel 100. If you have such devices, they need a non-DFS channel available somewhere in their coverage area, which usually means a dedicated SSID and band plan for them rather than an attempt to make everything universal.