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What Interference Looks Like

Why Your Wi-Fi Sucks And How It Can Be Helped, Part 2
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In our 5 GHz interference testing, interference and adverse contention conditions were generated by the 60 Dell clients all connecting to an AP mounted to the ceiling roughly above the middle of the client cluster. In the corner of our office space, shown by the green dot on the previous environment map, we mounted the AP being tested to the ceiling. Thus we had two discrete wireless LANs, the small one (single client and AP under test) having to function in the face of 61 interfering Wi-Fi devices. In effect, this setup is like two people trying to have a normal conversation on a patio overlooking an adjacent open-air rock concert. We wanted two separate WLANs in order to isolate interference as our main variable, not interference and client load.

For our 2.4 GHz tests, we wanted a worst-case scenario, so we combined a 100-foot client-to-AP distance, plus obstructed line-of-sight, plus having a non-Wi-Fi RF noise generator placed right on the spot where our client sat for the 70-foot 5 GHz tests. This raises an interesting point from our part 1 discussion about the difference between types of interference and their impact on communication performance.

Using Metageek's Chanalyzer Pro, we took several measurements near our test access point. In this first image, you see the impact of running our non-Wi-Fi interference generator. In real life, this might be something like a microwave oven—some device spewing out gobs of noise smack on the same frequency used by channel 1 in the 2.4 GHz spectrum. As you can see in the duty cycle measurement, roughly 30% of the available bandwidth around our channel is blown out by the noise. Also notice how the amplitude of this noise registers just about the -80 dBm level.

Next, we add one client connecting to our target access point. The amplitude doesn't budge, but now we see the duty cycle spiking up over 80%.

If you’re curious, that bump in traffic around channel 11 is an unrelated WLAN running in a nearby building.

Finally, we add wireless traffic from all 60 of our Vostro clients into the mix. Amplitude jumps above -60 dBm and the duty cycle nearly redlines, peaking at 95%. You know how your PC performs when CPU utilization holds at or above 90%? Imagine something analogous with Wi-Fi contention. Refer back to our contention discussion in part 1 and consider how common it would be for packets to require resending over and over in such an environment. How the access point deals with this situation will be critical in determining the end-user’s experience.

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