DFS channels: why 5GHz Wi-Fi goes quiet, and what Zero Wait DFS fixes

5GHz Wi-Fi shares its spectrum with radar. Ships, weather stations and the military were there first, and the terms of the sharing are set by Ofcom rather than by whoever made your access point. Those terms are called Dynamic Frequency Selection, or DFS.

In the UK the band falls into three parts.

Channels 36 to 48 carry no DFS obligation. Indoor use, modest power, and an access point may switch on and transmit.

Channels 52 to 64 and 100 to 140 are DFS channels. This is the bulk of the band – most of the room, and the only place to fit several wide channels without them treading on each other.

Channels 149 and above sit in the 5.8GHz band, where Ofcom has dropped the DFS requirement for indoor use. Not every access point offers them, and whether yours does depends on the model and its country setting.

Take the DFS channels away and four 20MHz channels remain. That is one 80MHz channel, or two 40MHz ones, for the whole site. An office with six access points cannot avoid re-using them, and re-used channels are why the Wi-Fi feels slow at eleven on a Monday while the internet line sits half idle.

Three obligations come with a DFS channel, and all three are worth knowing because each one is visible to your staff as something else.

It must check the channel before using it. The access point listens for sixty seconds and transmits nothing. If the channel is 120, 124 or 128 – the ones sitting next to weather radar – the check is ten minutes. Nobody can connect on that channel while it runs.

It must keep listening while it works. If radar appears, the access point has ten seconds to stop transmitting and leave.

It cannot go straight back. The channel is off limits for thirty minutes, whether the radar was a permanent installation or one sweep from a boat passing the bay.

None of this is a fault, and none of it is negotiable. The access point is doing exactly what it was certified to do. The cost lands on you as the gap between leaving one channel and being allowed to use the next.

Two of the services Wi-Fi shares this band with are on our doorstep, and one that everybody blames is not.

Ships. The range from 5470 to 5650MHz – channels 100 to 128 – is set aside internationally for shipborne navigation radar. Every vessel of any size crossing Poole Bay, running into Poole Harbour or working the Solent approaches is a transmitter moving about in the middle of the band, and it does not have to be near you to be heard by an access point sitting in a window or on a roof. A business on the seafront at Bournemouth, Poole or Christchurch lives inside that traffic. A unit inland on a business park largely does not.

Weather radar. The Met Office rain radars work at around 5.6GHz, which is precisely why channels 120, 124 and 128 carry the ten-minute check rather than the one-minute one. Dorset has sat inside that coverage for years – the county had its own radar at Wardon Hill near Dorchester, and the job has since passed to Dean Hill over towards Salisbury, which still covers this area.

Bournemouth Airport is not the culprit, and it is worth saying because it is the first thing people point at. Airport surveillance radar works at 2.7 to 2.9GHz, well below the Wi-Fi band, so the aircraft going into Hurn are not what pushed your access point off its channel. Military radar is another matter entirely – it does use these frequencies, and this stretch of coast has more military activity than most – but the airliners are innocent.

The practical upshot is that where your office sits changes the answer. Near the water, DFS events are a working assumption rather than an outside chance, and buying an access point that recovers from them without a pause is straightforwardly worth it. Ten miles inland, the case is weaker and you may be fine with the four non-DFS channels.

The wait exists because one radio cannot serve clients on one channel and silently listen to a different channel at the same time. Zero Wait DFS removes the conflict by moving the listening elsewhere. An access point with a radio to spare runs the availability check on a candidate channel in the background while the main radio carries on working. When the moment comes to move – at start-up, on a radar detection, or because the channel plan changed – it already holds a channel that has been checked and cleared, and it goes there immediately.

Be clear about what it does not do. It does not exempt anybody from the rules. The access point still stops within ten seconds of hearing radar, and it still cannot return to that channel for half an hour. What disappears is your wait, not the radar’s priority.

This is the part that costs people money. Zero Wait DFS needs hardware to do the checking, so it appears on the models built with it – the U7 Pro XGS, the E7 Campus and the E7 Audience among them – and not on cheaper units in the same family, which handle DFS the ordinary way. Every model has a specification page and the feature is listed there by name. That line is what to check. The brand on the box will not tell you, and neither will the Wi-Fi generation.

The wider argument for UniFi on a site like this is not the feature by itself. It is that the controller records which channel each access point is on and when it last changed, so an unexplained minute becomes an entry somebody can point at. Kit that cannot tell you why it moved leaves you guessing, and guessing about Wi-Fi is expensive.

Plenty of sites do not. Two access points in a small office, no radar for miles, and channels 36 to 48 will carry the load without ever touching DFS. Buying a feature for a problem you do not have is the usual way to waste money on network equipment.

The other answer is 6GHz. Wi-Fi 6E and Wi-Fi 7 access points can use a band with no radar in it, and so with no availability checks and no thirty-minute exclusions. The catch is that 6GHz does not travel as far or through walls as well, and only fairly recent devices can use it at all. For the next few years most of your traffic still lands on 5GHz, which is why what happens on the DFS channels is still your problem.