[Discuss-gnuradio] Wi-Fi Channel Monitoring

[Stephen Berger]

FiFi, I have been working on this or similar problems for a while.  I am very much still in the learning process and I think everyone is on this topic.  In fact the post I put up earlier today is related because I want to get video of my captures both for further analysis and so that I can show other what was captured so they can apply their skills.

 

Let me offer a non-GNURadio solution.  This can probably be replicated in GNURadio but I haven’t done it and so would need to look into how feasible it is to implement.  You can use Wireshark and any WiFi NIC to capture packets.  If you can put the NIC is promiscuous mode you can sequentially tune it to each channel and record any packets that occur.  I was dwelling on each channel for 3 seconds and scanning all the 2.4 and 5 GHz channels.

 

In each captured packet is the channel number being used and the RSSI of the signal at each device.  For some purposes this RSSI is useful.  If you can further measure the RSSI at your USRP you have 2 measures of the signal and that could potentially be very useful.

 

I also looked at things like the number of access points and attached devices.  This is useful because you can start to compare environments based on the network configuration of the access points and the number of people using their devices in the area.

 

What this method does not tell you is what is out there that is not WiFi.  In some recent measurements I am seeing an increasing number of Bluetooth and Bluetooth Low Energy.  Not a surprise.

 

Here is the process I am currently using and this has lots of room for further development:

 

1.     I capture using a 25 MBs sampling rate, centered on WiFi channels 1, 6, 11 and the top of the band (that would be center frequencies 2412, 2437, 2462 and 2478).

2.     I then play back the captured file with a throttle block and FFT size that suites my purpose.

a.     I plan on adding a channelizer to just display the center 20 MHz of the 25 MHz capture to get rid of the edge effects.

3.     At the moment I am then using RecordMyDesktop to create a video file of the playback for further analysis and to allow others who aren’t familiar with GNURadio to see what has been captured.

 

A next project I am thinking through will be to go through those captures on probably a 1 MHz spacing and record the signal strength.  This will then be used to develop histograms of probability of occurance vs signal strength.  I am choosing 1 MHz because it will get WiFi, BT, BT LE, microwave ovens and pretty much anything that is putting energy into the band.

 

I think the long range objective is to bring these together and have a mix of information from layers 1, 2 and 3 coming out.  It would be fantastic to capture a signal and have the output give you a range of measures of the RF but also decode any packets and give layer 3 statistics such as number of active devices, number of access points etc.

 

Let me know how this compares to what you are trying to do and I expect we can compare note on various aspects of the implementation.

 

Best Regards,

Stephen Berger