While teaching a Wi-Fi class recently, I had a conversation with one attendee that I’ve had more than just a couple of times before. It went something like: “Hey MetaGeek…

While teaching a Wi-Fi class recently, I had a conversation with one attendee that I’ve had more than just a couple of times before. It went something like: “Hey MetaGeek guy. Why doesn’t inSSIDer on my brand new laptop see this 802.11n network on channel 116, but my beat up Android phone does. What gives?” Without surprise, I looked over his shoulder and sure enough the phone displayed the network in question, while the shiny new laptop did not.

So what’s going on here?

It all has to do with a combination of things including thunderstorms, jet airplanes, the FAA, the FCC and various types of airport weather radar.

Let me explain: radar is used at airports and by the military in the United States and around the world to detect things like wind shear, which is quite a threat to jet traffic and has caused a number of plane crashes. In response, this specific weather radar technology was invented to make navigating through storms safer for pilots. However, in North America, radar uses the frequencies that inhabit the 5 GHz Wi-Fi channels 100 to 144. And, to allow that the radar gets uninhibited priority, every Wi-Fi base station must make sure that if it hears a radar pulse on those channels, it must move off of those frequencies in order to not interfere with radar operation. This FCC-mandated feature is called DFS—Dynamic Frequency Selection.

Not all Wi-Fi client devices see all channels

Here’s where this gets interesting. Since correct operation of DFS is 1) a matter of public safety and 2) non-compliance results in big fines from the FCC, many Wi-Fi gear makers do not even offer the use of these channels to the end user. Also, some vendors go as far as to not offer any operation with their 5 GHz-capable devices in the entire UNII-2-Ext band where these channels reside (which, by the way covers a pretty large swath of channels), just to be safe.

Valid 5 GHz Wi-Fi Channels

DFS makes up large share of 5 GHz band (Image credit: revolutionwifi.net)

What does this mean?

For starters, the first obvious result is that if your client device does not support the DFS channels or the UNII-2-Ext band you will not be able to connect to a network deployed on these channels. Additionally, since this client device does not scan or listen to beacons on these channels, your Wi-Fi utility will not even see them. This is also true for inSSIDer (and Chanalyzer’s Wi-Fi features) as well as any application that relies on this data.
In short, you may not be seeing everything.

What to look for

If you’re interested in having visibility of all the Wi-Fi channels and the networks that may be on them, you’ll have to do some research on the client devices or adapters you buy. We’ve found that most vendors do have the channel capabilities listed in their sales material, although a lot of the time they’re found deep in spec tables at the bottom of datasheets.

Some NICs see DFS channels, some don't

Some NICs see DFS channels, some don’t.

After doing a random spot check on the Wi-Fi USB adapters and client devices that I have, I found that about half have the DFS channels available for inSSIDer, and half do not. If you’re interested in using DFS channels in your Wi-Fi deployment, or you need to look for ALL unauthorized wireless access points in your network, you will want to keep this in mind and make sure your Wi-Fi card sees what you think it sees.