Wi-Fi monitoring

15.04.2014 17:25

Ever since we setup the wireless testbed in Logatec I wanted to use VESNA spectrum sensors to monitor Wi-Fi and other unlicensed radio usage in the 2.4 GHz band.

Unfortunately, it turned out that the low-power mesh network is way too slow to do any kind of real-time transfers. It was designed for reading battery-powered temperature sensors every once in a while, not streaming radio spectrum data. The 2.4 GHz CC2500-based radio generates around 2 kB per second. That maybe doesn't sound like much these days, but in ideal circumstances a raw data stream from one radio alone more than saturates the mesh, much less 50 of them.

There is a wired Ethernet interface in the works that should take care of the limited bandwidth problem. Meanwhile, motivated by rants about unusable Wi-Fi in some parts of Ljubljana, I improvised and made a pair of stand-alone devices that simply record spectrum data to a SD card.

Stand-alone VESNA RF spectrum sensors.

Here are two particularly colorful visualizations of measurements I took from a balcony in a residential district. They show changes in a two-dimensional histogram of RF power samples over time. This display is similar to the persistence mode on expensive spectrum analyzers (it only becomes feasible on this hardware though when you have relatively long-term measurements).

Each individual sample recorded the total received power, averaged over 1 ms, in a 400 kHz wide channel. Samples were taken at 255 different central frequencies, continuously covering the band between 2.4 GHz and 2.5 GHz. Color on the picture shows how often a sample with that frequency (X axis) and power (Y axis) was encountered.

Following is a recording of a little more than one day, compressed down to 40 seconds:

A day in Wi-Fi

(Click to watch A day in Wi-Fi video)

You can see here several IEEE 802.11 networks. The nice thing about this kind of monitoring is that you can detect access points as well as devices that connect to them. Devices using 802.11b standard with direct sequence spread spectrum modulation have a slightly rounded footprint. Newer and faster 802.11g/n standards with OFDM leave a more sharply square shape.

Some narrow-band transmissions correspond to wireless keyboards and mice. Others I haven't identified yet. Regularly spaced bumps in the noise floor are due to internal interference from the receiver.

Similarly here is the complete week. Each video frame comprises of 5070 scans of the spectrum.

A week in Wi-Fi

(Click to watch A week in Wi-Fi video)

Posted by Tomaž | Categories: Life


Hi Tomaž,

nice videos, can I ask what have you used to produce them?

One thing, have you considered the log scale on the colorbar? It will get single sample to be visible and the noise will still be there nicely red. You can take a look at my work for wispy's and telos nodes https://github.com/mchwalisz/smutools and the crewcdf_toolbox that sits behind plots, you should be able to find some data files in CREW.

I'm not as far as to make videos out of it but basically have some longer period data collected with wispy. Have to think about in spare time.


Hi Mikolaj,

videos were made with numpy and matplotlib. I'll publish the code on GitHub shortly.

I think the colorbar is already in log scale - it is displayed in powers of 10. Black is 0, dark blue is 1 and red is around 100 or 1000, depending on the video. Thanks for the link to your tools though, I'll have a look.

Posted by Tomaž

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