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It sounds like a stretch that Wi-Fi would have a problem with radar, but if your site is near an airport or a port, it can be a real one. In this blog, we'll explain how radar affects 5GHz Wi-Fi, how DFS (Dynamic Frequency Selection) works, what it means for Wi-Fi phones, and how to plan around it.
Wi-Fi Dropping Out Near an Airport or Port? IPTel's Wi-Fi Assurance Review finds the root cause with diagnostic surveys, client debugging and a configuration review.
Wi-Fi Interference: Why Radar Is a 5GHz Problem
Wi-Fi shares its spectrum with other users, and in parts of the 5GHz band one of those users is radar. Weather, airport and port radars can operate on the same frequencies as Wi-Fi channels. When they do, the radar takes priority, and your access points (APs) have to get out of the way.
Wi-Fi runs in licence-exempt spectrum (in Australia, under the ACMA's class licence). The rules differ from country to country, so you tell your controller and APs which regulatory domain they're in and the right channels are made available.
The 2.4GHz band has far more interferers than 5GHz: microwave ovens (which Cisco APs detect and report), Bluetooth headsets, some types of lighting and so on. That's a big part of why 5GHz is the preferred band. But 5GHz isn't immune. We've seen mobile phone repeaters using 5GHz as backhaul, channel hopping and wiping out several channels in the process.
The big one, though, is radar. If you've seen a radome (a tallish building with a giant golf ball on top) near an airport or a port, it may well be operating in part of the 5GHz spectrum.
DFS: How Dynamic Frequency Selection Works
DFS (Dynamic Frequency Selection) lets Wi-Fi use the same 5GHz channels as radar without interfering with it. An AP on a DFS channel listens for radar. If it detects a radar signal, it must stop transmitting on that channel, move to another one, and leave the original channel unused for a set period.
Before an AP can start transmitting on a DFS channel, it has to listen for radar first, normally for 60 seconds (the Channel Availability Check). Once it's in service, it keeps monitoring the channel.
When radar is detected, the AP leaves the channel and marks it as unavailable. Under the ETSI rules that Australian 5GHz equipment is tested against, that channel then stays off limits for 30 minutes. If the new channel is also a DFS channel, the AP has to complete another 60-second check before it can serve clients again.
It's a sensible system, but it isn't seamless. Radar can disrupt Wi-Fi just as Wi-Fi can disrupt radar, so moving off a channel with radar on it can also reduce congestion in your Wi-Fi network by removing a source of interference. The cost is that the radio goes quiet while it moves.
Wi-Fi Phones: What a DFS Event Means for Voice
A DFS event is most noticeable on Wi-Fi phones. When radar is detected, the AP drops off its channel and every client on it has to roam. For data that's usually a brief pause, but a call can go silent while the phone finds the next best AP and re-authenticates.
In our experience, that silence can be up to around five seconds, although it's often shorter. One of the first signs you have a DFS problem is devices dropping out, and Wi-Fi phone dropouts are where you'll notice it most.
There's a secondary effect too. The best AP is now changing channel and may be off air for a minute or so, so phones join a second-best AP. If that AP is too far away, you can get further dropouts.
Radar Debug: What a DFS Event Looks Like on a Cisco AP
On a Cisco AP, a DFS event shows up clearly in the logs. The AP reports that DFS was triggered on a frequency, its 5GHz radio interface goes down and resets, and then it starts a 60-second scan of a new frequency before the radio comes back up. Spotting these messages confirms radar is the cause of the dropouts.
Here's an excerpt from the log of an AP that was being affected by DFS. The first lines show DFS triggering on 5260MHz (channel 52) and the radio going down; the later lines show the AP scanning 5320MHz (channel 64) for 60 seconds before the radio comes back up.
*Aug 15 04:52:08.499: %DOT11-6-DFS_TRIGGERED: DFS: triggered on frequency 5260 MHz*Aug 15 04:52:09.236: %LINK-6-UPDOWN: Interface Dot11Radio1, changed state to down*Aug 15 04:52:09.292: %LINK-5-CHANGED: Interface Dot11Radio1, changed state to reset*Aug 18 19:00:13.940: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio1, changed state to down*Aug 18 19:00:13.978: %DOT11-6-DFS_SCAN_START: DFS: Scanning frequency 5320 MHz for 60 seconds.*Aug 18 19:00:13.981: %LINK-6-UPDOWN: Interface Dot11Radio1, changed state to up*Aug 18 19:00:14.981: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio1, changed state to up
Channel Planning: How to Avoid DFS Problems
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If your site is near an airport, port or weather radar, plan your channels around DFS. In Australia, the UNII-1 (channels 36–48) and UNII-3 (channels 149–165) channels don't need DFS. The UNII-2A and UNII-2C channels in between do, so they're the ones radar can knock out.
The UNII-2C (Extended) channels give you more room, but they are DFS channels too, so they don't solve a radar problem on their own. In Australia, channels 120–128 aren't available at all, because that part of the band is reserved for weather radar.
If you only run data (not voice), you can often use DFS channels and never notice the effect. For voice, it's safer to keep the APs near the radar off DFS channels. And on newer APs, the 6GHz band (Wi-Fi 6E and later) doesn't use DFS, which gives you clean channels away from the radar altogether.
DFS is only one source of Wi-Fi problems. If you're seeing issues, it's worth undertaking a review of your Wi-Fi network before you change anything.
How Many Access Points Do You Need?
DFS channels only help if you have enough access points in the right places to use them. Our free Wi-Fi Access Point Calculator gives you a quick estimate for your building: enter each floor’s area and how many people use it, and it works out the access points, PoE budget and switch ports you’ll need, then emails you a full report.
Wi-Fi and Radar: FAQs
What Is the Problem With Radar and Wi-Fi?
Some radars use the same 5GHz frequencies as Wi-Fi. When an AP on a DFS channel detects radar, it must leave that channel, which disconnects its clients and can take the radio off air for about a minute. Wi-Fi phones are affected most.
What Is DFS in Wi-Fi?
DFS (Dynamic Frequency Selection) is the mechanism that lets Wi-Fi share 5GHz channels with radar. APs listen for radar before and while using a DFS channel, and move to another channel if they detect it.
Which 5GHz Channels Use DFS in Australia?
The UNII-2A channels (52–64) and UNII-2C channels (100–140, with 120–128 excluded) require DFS. The UNII-1 (36–48) and UNII-3 (149–165) channels don't, which makes them the safer choice near a radar.
Does 6GHz Wi-Fi Use DFS?
No. The 6GHz band used by Wi-Fi 6E and Wi-Fi 7 doesn't require DFS, so it isn't affected by radar in the same way. You'll need 6GHz-capable APs and clients to use it.
Wi-Fi and Radar: Summary
Radar near your site can knock 5GHz Wi-Fi off DFS channels, and Wi-Fi phones feel it first. Knowing how DFS works lets you plan around it: keep APs near a radar on non-DFS channels, especially for voice, look at 6GHz where your devices support it, and check the AP logs if you suspect radar is the cause. For more background, George Stefanick's 30 Technical Wi-Fi Thoughts is well worth a read.
Need Help With Radar and DFS Interference?
Our Wi-Fi Assurance Review looks at your channel plan, DFS events and roaming behaviour, and our Wi-Fi survey and design team can redesign around a nearby radar. If that would be useful, get in touch with the IPTel team or email sales@iptel.com.au.