Seeing is believing. Be it by reading standards or by using tools and analyzers to get hands on experience on how wireless networks operate is what drives my professional interests and this blog. When it comes to the physical layer, i.e. the radio transmissions, tools are scarce, at least those that are affordable. Recently, however, I stumbled over a great tool called Wi-Spy from Metageek which has opened the door to Layer 1 of the 2.4GHz ISM band for me. This is the frequency band in which Wifi, Bluetooth and a couple of other wireless systems operate.
Metageek was nice enough to send me one of their advanced probes which sell for $399.-. Compared to other spectrum analyzers it’s almost a free ride. Since then I’ve used the probe day and night and have gathered hundreds of megabytes worth of data. I am absolutely fascinated and have learnt a great deal of how Wifi and Bluetooth behave, interact and interfere on the ISM band. Good to have a blog so I can share some of the results.
The first picture on the left shows two of the three graphs the Chanalyzer software creates in real time out of the data gathered by Wi-Spy. The upper diagram is a waterfall diagram that shows the frequency range on the x-axis and time on the y-axis. Activity on a certain frequency and intensity is drawn in different colors ranging from blue (low to nothing) to red (high signal strength). As can be seen on the y-axis, the graph shows the activity of the past 60 minutes. The lower diagram in the picture shows the amplitudes reached on the frequency band. The color indicates how often a signal was registered. Not much can be seen in the first picture except for the slight increase in activity between channel 3 and 4. As such this radio environment is a dream for deploying a new wireless LAN access point.
Things start to get much more interesting in picture 2 which uses the same scales and settings as in the first example. This trace, however, was taken at a place were 6 wireless LAN access points operate in parallel. Due to the long recording time of 60 minutes it becomes clear that three different wireless LAN devices operate on channel 6. They can be distinguished because each has a is received with a different signal strength by the probe which means that they are at different locations or have a different output power. My own access point operates on channel 11. During the recording time of 60 minutes all access points including mine were mostly in idle mode. The graph also shows that there is another access point on channel 1 and a further one on channel 9. Channel 9 is a most unfortunate choice since it overlaps and thus interferes with all access points on channel 6 and also with my access point on channel 11.
In the next picture I have zoomed on the topographic chart and have activated markers that show where the three possible non overlapping channels in the ISM band begin and end. I’d love to show this picture to the guy who owns the access point transmitting on channel 9 which tramples over the ones on the left and right of it. The impact such a partial and full overlapping has on performance will be discussed in a future blog entry.
The the last picture on the left shows the pretty congested radio environment in my Paris apartment. My own access point in this case is on channel 1 and I’ve done some file downloading over a 10 MBit/s ADSL2+ Internet connection at 40 minutes in the trace and a pretty long one between around 5 and 20 minutes in the trace. The traces shows my access point which is received at around -70dbm and the wifi transmissions of my notebook which are received at around -45 dbm (as the antenna is very close to the Wi-Spy probe). As I mostly downloaded information the Wifi signal of the access point is plotted in a lighter color (more activity) than the notebook. Also note the very active Wireless LAN on channel 11.
Since the Chanalyzer can be used to record and playback I saw that this network keeps transmitting 24h a day. The same applies for the wireless LAN access point on channel 3. Most likely these are two of the access points by French DSL provider Free. Their version 5 access point uses MIMO techniques to stream TV signals over Wifi to a set top box on the TV. This theory is supported by the SSIDs these networks broadcast. To make the partial overlaps complete there is another access point on channel 5. All signals by the way are strong enough to be easily received and decoded by my notebook so these signals are far more than faint background noise.
So much for this first part on Layer 1 Wifi tracing. In the next parts I will cover scenarios such as throughput measurements in partly and fully overlapping Wifi networks, how I detected a faulty Wifi card, how Bluetooth interferes with Wifi downloads and how it looks like when a microwave oven ruins your live TV signal streaming.
In the meantime if you want to check things out for yourself head over to the Metageek homepage where you can download the Chanalyzer software and some traces to start your own experiments. In case you think about buying and live in Europe, here’s a link to the list of national resellers.