Wifi Layer 1 Tracing with Wi-Spy – Part 3

For those interested in getting a feeling of how Wifi works on the physical layer, Metageek’s Wi-Spy is the ideal tool. I’ve already reported about my first experiences here and here. Wifi has become very popular in Paris due to DSL being quite cheap. So it should thus probably not be surprising that I can see 13 access points in my Paris apartment. As there are only three non-overlapping Wifi channels on the 2.4 GHz ISM band the question is what kind of impact such a high number of access points has on throughput.

Matters are made worse by the fact that some of the networks I can see are used for TV and video streaming. This is quite popular in Paris as this is offered by most DSL ISP’s and one even offers video streaming over Wifi to a remote set top box. The picture on the left shows a trace taken with Wi-Spy under these conditions. The lower graph in the figure shows the frequency range of the ISM band between 2400 and 2480 MHz. Instead of showing the frequency in MHz the x-axis labs show the 13 available Wifi channels. On the y-axis the amplitude of the signal received over the band is shown. The color of the peak depends on the intensity of the signal received during 60 minutes. Bright color indicates high activity. The graph shows five partially overlapping networks with their center frequency on channel 1 (only little traffic so the arch is not very well visible), channel 3, 5,6 and 11.The most activity can be observed in the wireless network that is centered around channel 11.

The upper graph shows a time graph over the frequency range. On the y-axis I’ve chosen a resolution of 60 minutes to show the activity in the ISM band in the course of one hour. The Wifi networks on channel 5 and 11 were most likely used from streaming as there is uninterrupted activity throughout the test period. The Wifi networks on 3 and 6 were also used for streaming. Streaming was stopped on the Wifi network on channel 6 after about 12 minutes while streaming was started on the Wifi network on channel 3 about 40 minutes into the trace.

To see what the impact of that streaming has on throughput in my network I used two notebooks, one connected via Ethernet, the other via Wifi and Iperf, a UDP and TCP throughput measurement tool. With a fully overlapping Wifi network which is used for TV streaming, capacity of the Wifi network under test was reduced to 72%. Partial overlapping entails an even bigger speed penalty and performance was reduced to 59%.

Here are the absolute values:

  • No interference: 22.5 MBit/s
  • Full Overlapping: 16.3 MBit/s
  • Partial Overlapping: 13.4 MBit/s

Looks like it is time equipment manufacturers are taking the 5 GHz band a bit more seriously…

For more traces take a look at my previous traces or head over to Metageek where you can download the software and check out some sample traces yourself.