We’ve recently started to see a rise in customers using DFS channels when operating their Wi-Fi networks, so thought we would write an article all about it in case it’s also helpful for your own network.
What is DFS?
DFS is Dynamic Frequency Selection and is a type of Wi-Fi function that allows WLANs to use 5GHz frequencies (which are reserved for radar, for example, the military, satellite or weather).
What is the benefit of using DFS channels for Wi-Fi?
We’ve written before about how you can improve your Wi-Fi and prevent interference by utilising different channels. The main benefit of using DFS channels taps into this. You can increase the number of available Wi-Fi channels by using DFS channels to use these less-used frequencies.
How can I utilise DFS channels on my Wi-Fi network?
The first thing you need to do if you are wanting to use DFS channels, is to check that your wireless access points and wireless clients support the necessary functionality.
The 5GHz spectrum in the UK is broken into 3 different bands and runs from 5150MHz (5.15GHz) to 5850MHz (5.85GHz). The bands are as follows:
Band A
- Channels 36 – 64
- This is used only for Indoor wireless
- Does not require a license
Band B
- Channels 100 – 140
- Can be used both inside and outside
- Does not require a license
- Hardware must conform to DFS standards and be DFS enabled
Band C
- Channels 149 – 161
- Can only be used outside
- Requires a license from Ofcom
- Hardware must conform to DFS standards and be DFS enabled
Checking DFS Channel Availability
When you enable DFS, the Wi-Fi access points will need to verify that any radar in the proximity is not using DFS frequencies. This is a process called Channel Availability Check and is carried out during the boot process of an AP as well as during normal operations.
Should an AP detect that a local radar is using a certain DFS channel, it will automatically exclude that channel for this list of available ones. This will last for 30 minutes after which time the AP will check again to see if the channel can be used for Wi-Fi transmissions. You’ll be pleased to know that this exclusion of unavailable channels has very little impact on Wi-Fi clients.
DFS channels are not immediately available when an AP boots. This is because the Channel Availability Check can take anywhere between 1 minute and 10 minutes during the boot process.
What happens if an AP detects radar use during normal operations?
We know what you’re thinking – What happens if the AP was to detect during normal operations that the channel you are using becomes in use by a proximal radar?
If this happens, then the AP may communicate to its Wi-Fi clients to stop transmitting on that particular channel. The AP will then switch to a different, available DFS channel within the Channel Move Time. This is usually about 10 seconds.
Unlike above, this could affect Wi-Fi clients. An AP won’t always announce that it is changing channels to connected devices. When it switches to the available channel, it will cause those Wi-Fi clients to disconnect from the network and then re-connect to the new channel.
Are DFS channels right for my Wi-Fi network?
If you are considering using DFS channels for your Wi-Fi, you need to think carefully about whether business critical operations rely on that connection. If the answer is yes, you might want to avoid enabling DFS and not risk the disconnections caused by DFS frequencies.
