GNU Radio / WiFi

So much spectrum – so few networks. The 5GHz WiFi band is in many areas not really utilized. OK, it doesn’t penetrate walls as nicely as the 2.4GHz band, but that’s not always a drawback.

One reason why the 5GHz band is not as crowded as it should be, might be the fact that WiFi is only a secondary user on many channels. These channels are allocated primarily to radars. Maybe you already came across the term Dynamic Frequency Selection, the feature that allows a WiFi device to detect radars. Without DFS, a device is not allow to open an access point on those channels.

The WiFi standard, however, only defines routines to migrate a network to another channel, once a radar has been detected. The actual radar patterns are defined by a regional regulatory body.

On Linux, you can check your DFS region with:

$ iw reg get
country DE: DFS-ETSI
    (2400 - 2483 @ 40), (N/A, 20), (N/A)
    (5150 - 5250 @ 80), (N/A, 20), (N/A), NO-OUTDOOR
    (5250 - 5350 @ 80), (N/A, 20), (0 ms), NO-OUTDOOR, DFS
    (5470 - 5725 @ 160), (N/A, 26), (0 ms), DFS
    (57000 - 66000 @ 2160), (N/A, 40), (N/A)

For Europe, the ETSI EN 301 893 Draft defines the actual radar patterns (see Annex D for the definition). Once a radar is detected, an access point has to blacklist the corresponding channel for 30 minutes.

Of course, I wanted to emulate a radar and see whether the APs move away. “Implementing” these radar patterns in GNU Radio is as simple as it can be. A flow graph like this is already enough.

The vector source can be used to create the radar pulses with something like:

[1]*5 + [0]*10

I tried the above flow graph with a B210 and a 6dBi dipole in the hall way of our institute. Worked pretty well, i.e., I managed to push away many networks from DFS channels.

By the way, macOS comes with a pretty nice tool to scan for WiFi networks. You can access it by alt-clicking the WiFi symbol in the task bar. This will open an extended menu, where you can click on Open Wireless Diagnostics. Then, WindowScan will open the tool.

You can see the tool in a brief video, where I’m poking some University networks around.