What is DFS?
Dynamic Frequency Selection (DFS) is a rule for 5 GHz Wi-Fi. Some 5 GHz channels share spectrum with radar systems, including military, airport and weather radar. An access point that uses one of those channels must listen for radar pulses and move to another channel when it detects one, so Wi-Fi doesn't interfere with the radar.
In the United States (FCC), DFS applies to these channels:
| Sub-band | Frequency range | Channels | DFS required |
|---|---|---|---|
| UNII-1 | 5.150 to 5.250 GHz | 36 to 48 | No |
| UNII-2A | 5.250 to 5.350 GHz | 52 to 64 | Yes |
| UNII-2C | 5.470 to 5.725 GHz | 100 to 144 | Yes |
| UNII-3 | 5.725 to 5.850 GHz | 149 to 165 | No |
How DFS works
- Channel availability check: before transmitting on a DFS channel, the access point listens for radar, commonly for 60 seconds. That's why a network on a DFS channel takes longer to appear after a reboot.
- In-service monitoring: while it runs, the access point keeps listening.
- Channel move: when it detects radar, it must stop using the channel within seconds and switch to another one, taking its clients with it.
- Non-occupancy period: it can't return to that channel for a set time, commonly 30 minutes.
Client devices don't detect radar themselves; they follow the access point and scan DFS channels passively, waiting to hear an access point before they transmit. Some older or low-cost clients don't support DFS channels at all.
DFS symptoms and fixes
- The access point changes channel by itself, and its log shows a radar detection event. Detections can also be false alarms caused by other interference.
- Wireless clients drop briefly and then reconnect, more often near airports or military bases.
- A device can't see the network at all because it doesn't support the DFS channel the access point picked.
To fix it, move the access point to a non-DFS channel (36 to 48 or 149 to 165), narrow the channel width so it no longer includes DFS channels, or exclude DFS channels from automatic channel selection. A Wi-Fi analyzer shows which channel each network is using.
The other DFS: Distributed File System
In Windows Server, DFS means Distributed File System, an unrelated topic. DFS Namespaces present shared folders from several servers under one folder path, and DFS Replication keeps copies of those folders in sync between servers. If you see DFS next to file shares and servers, that's the meaning. Next to Wi-Fi channels, it's Dynamic Frequency Selection.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.2 covers channel regulations and channel selection. DFS isn't named in the objectives or the acronym list, but it explains why 5 GHz channels 52 to 64 and 100 to 144 behave differently from 36 to 48 and 149 to 165. Distributed File System isn't in the Core 1 objectives either.