What Is Dynamic Frequency Selection?
If you’ve never heard of Dynamic Frequency Selection before, don’t be too hard on yourself. If you’re not inclined to read radio standard whitepapers or dig through router menus with a magnifying glass, you’re unlikely to come across it.
And even if you are the sort to look closely at every menu, Dynamic Frequency Selection didn’t start routinely appearing in the menus of consumer routers until the widespread rollout of 5GHz Wi-Fi routers—DFS wasn’t necessary back when everybody had 2.4GHz only routers. Even today, not all routers have DFS because the inclusion of the channel allocation standard requires certification and extra hoops for manufacturers to jump through.
So what exactly is it and what does it have to do with Wi-Fi? The 5Ghz Wi-Fi found around your home and anywhere else modern Wi-Fi hardware is deployed uses a portion of the radio spectrum that is part of a much bigger communication band known as the “C Band.”
The C Band is a significant swath of the microwave radio frequency band ranging from 4Ghz up to 8Ghz. Various different portions of that range are used for all manner of things including satellite communication, military and civilian radar, weather radar, cellular communication, and more.
Among the wide variety of things in the C Band, the most common thing that overlaps with the same 5Ghz range used by Wi-Fi devices is weather radar. If you’ve ever heard your local meteorologist on the news talking about the results from the “Doppler radar”, they are talking about information collected using the Terminal Doppler Weather Radar (TDWR) system used widely in the United States and around the world to detect storms and hazardous weather conditions.
Noise from networking hardware interfering with critical things like weather radar is, naturally, not ideal. That’s where Dynamic Frequency Selection comes in.
DFS is a channel allocation scheme that was first introduced back in 2003, as part of IEEE 802.11h amendment to the 802.11 wireless standards, specifically to deal with this particular problem—Wi-Fi communication overlapping with and potentially interfering with more critical radio communications.
In theory, this is a win-win for everybody involved. Consumers can use part of the C Band for their home Wi-Fi when the airwaves are clear and the portion of the C Band is handed back over when more important services are needed. In practice, it’s not exactly a free lunch situation for consumers and real-world deployment of DFS can either be a boon or a hassle. Let’s take a look at when you should and shouldn’t use it.
What Are Pitfalls of Dynamic Frequency Switching?
Access to a portion of the Wi-Fi spectrum that would otherwise be inaccessible seems like an overall win for consumers, right? So why wouldn’t you want to use DFS to get more out of your router?
Typically speaking, when DFS is available on a router is disabled by default and must be enabled. This, right out of the gate, should tell you that the majority of major router companies like Linksys, ASUS, and so on, think that it’s a setting best used in an educated and as-needed fashion, and not turned on everywhere.
So before you immediately log in to the control panel and start searching for the setting, let’s take a closer look at the benefits and tradeoffs. Before we dig in, we want to preface this section by saying that we are purposely pointing out the potentially frustrating things about using DFS so that you are well prepared to troubleshoot it and consider if you even want to use it before just jumping right into using it.
DFS isn’t inherently bad, but if you don’t the know all the little nuances of how it works and what it looks like while active on your network, you might end up pulling your hair out trying to figure out why your Wi-Fi network just isn’t working the way you expect.
Your Router Remains Inaccessible for Longer After Restarting
Aside from the obvious potential issue with DFS—that it can interfere, however briefly, with more important radio traffic—the real reason router companies usually default to the offsetting is because of the headaches it can cause consumers.
Yes, when DFS is active the router can use the portion of the 5GHz band that overlaps with other services. And yes, that can significantly improve the Wi-Fi experience for the end-user.
But DFS is required to run a check, called a Channel Availability Check, before using the specific channels in that range. This check can take anywhere from 1-10 minutes depending on which channel within the restricted DFS range is being checked.
During this check period the 5Ghz network for the access point running the check is not available and it will either fall back, if possible, to the 2.4GHz network or, simply remain unavailable until the check is complete.
If the airspace is clear and the particular channel passes the Channel Availability Check, after the 1-10 minutes Wi-Fi the access point will be available for us. Every single time your router and access points are restarted you’ll have X minutes of downtime while the check is run.
Your Router Switches Channels When It Detects Interference
To comply with DFS rules, whenever your router or access points detect anything that appears to be interference from a higher-priority source on the portion of the DFS-allocated spectrum a rapid sequence of events unfolds.
The router blocks quickly dumps all clients on that channel, stops use of the channel, broadcasts new information for the local clients to guide them to the new channel, and traffic resumes. Further, the router complies with a Non-Occupancy Period (NOP) of at least 30 minutes (though most consumer routers will often leave the channel unused for longer, potentially until the next day).
When the Non-Occupancy Period has passed, the router will repeat the Channel Availability Check, albeit in a shortened form, before potentially switching to the new channel. Just like with the original longer boot-time scan, the router will be inaccessible to wireless clients on the 5Ghz band during this time.
Because DFS is designed specifically with the goal of making part of the wireless spectrum accessible to consumers while simultaneously protecting critical airspace for other tasks, it has a hair trigger when it comes to detecting interference.
The tiniest bit of traffic on the restricted frequencies will lead to your router immediately dumping it. There’s no back and forth, no confirmation if the traffic is actually a weather radar or such, the DFS system immediately defers to the other traffic and switches.
Not All Clients Are Compatible with Dynamic Frequency Selection
There’s no requirement that a client support Dynamic Frequency Selection nor, for that matter, that a router or access point support it either.
To use the DFS-spectrum channels a manufacturer needs to comply with the regulations regarding such use and get certified, but simply not offering the functionality is, in and of itself, a form of compliance with the restrictions for that portion of the radio spectrum.
This, of course, can introduce problems with connectivity on your network. You might find when you switch DFS on some clients have improved performance while others suddenly have connection problems.
Before we leave this section on the pitfalls of Dynamic Frequency Selection, however, there is one positive thing worth highlighting. With each generation of hardware, both on the router and the client-side, DFS handling is improving. While you still sometimes notice weird dropouts or network behavior, increasingly you don’t notice anything odd happening at all.
What Are the Benefits of Dynamic Frequency Selection?
You might be thinking “Wow, that section about the pitfalls of DFS was pretty long…” and wondering if there’s any reason to use it. But again, we’d much rather have you know what something is and if it’s a right fit for you than to just recommend it without giving you the run down.
Dynamic Frequency Selection Opens Up Significant Extra Space
We don’t want to totally put you off using DFS with your home router and access points. If you live somewhere with minimal-to-non-existent interference from those restricted channels, it’s completely worth playing around with accessing them.
In the United States, the 5GHz channel space includes 25 non-overlapping channels and 16 of them are in the DFS restricted space at the 20Mhz channel width. When you step up into the broader channel widths like the 40Mhz and 80Mhz widths the channels become wider and the number of available channels decreases. At 40Mhz there are only 4 non-DFS channels and at 80Mhz there are only 2 non-DFS channels.
So if you’re not in an area where radar or other interference is routinely sending your router into the DFS-dodging routines we described above, then enabling it opens up roughly two-thirds of the available 5GHz Wi-Fi spectrum for use.
That’s certainly nothing to sneeze at and worth investigating if you are at all unsatisfied with your current router performance and/or if you have a lot of Wi-Fi devices on your network.
Though we’ll warn you, if you’re really unhappy with your router there’s a good chance the issues are more numerous and bigger than simply limited channels. Do check out some of our tips to improve your Wi-Fi signal and be open to upgrading your router altogether.
Dynamic Frequency Selection Shines in Dense Environments
If you happen to live in an area packed with other Wi-Fi signals from adjacent condos or apartments, it’s absolutely worth playing around with DFS on your router for more than one reason.
Not only is having more channels to play around with in a very Wi-Fi dense area ideal simply because the more airspace for all the different competing networks the better, there’s an even “sneakier,” if you will, reason to turn on DFS.
Assuming you’re not really close to an airport or weather station that will immediately muck up your DFS expansion plans, switching DFS on when you’re in a Wi-Fi dense apartment complex is practically free real estate.
Remember how we talked earlier bout the majority of manufacturers shipping their hardware with DFS turned off? If you turn it on, there’s a good chance you’ll be the only person in your entire apartment building with DFS enabled. This means all the other routers are screaming over each other in a fairly limited portion of the 5GHz Wi-Fi range while your router will be free to stretch out in the almost entirely empty DFS portion.
And hey, even if you don’t live in a dense apartment or you live relatively close to an airport, feel free to experiment. There is zero potential that turning on DFS is going to break your router, screw up your Wi-Fi devices, or cause anything more than a temporary inconvenience if you don’t like the potential downtime or performance hits.
If DFS ends up not making your 5GHz network run any better or you experience irritating issues like your computer or console dropping the connection while gaming, just pop back into the router control panel and turn it off.
