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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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Top Comments
  • 15 Hide
    winner4455 , July 14, 2011 4:11 AM
    Hey, I still haven't read this article but right away I notice the new format. Just thanking you for listening to your readers! :) 
Other Comments
  • 15 Hide
    winner4455 , July 14, 2011 4:11 AM
    Hey, I still haven't read this article but right away I notice the new format. Just thanking you for listening to your readers! :) 
  • 9 Hide
    cangelini , July 14, 2011 4:17 AM
    Very welcome Winner. We thought the picture story format would work for that last part and didn't realize the text would come out to be so terrible. From now on, we'll only use picture stories when the captions fit without requiring another click!
  • 4 Hide
    tacoslave , July 14, 2011 4:20 AM
    cangeliniVery welcome Winner. We thought the picture story format would work for that last part and didn't realize the text would come out to be so terrible. From now on, we'll only use picture stories when the captions fit without requiring another click!

    "Now thats what i like to hear!"
  • 2 Hide
    nekromobo , July 14, 2011 5:06 AM
    What if you add few thin-foil balls to room (the size of fist or 2)

    That should add few rf-reflections or paths, right?
    Just your 2cent amplifier.. :) 
  • -8 Hide
    dead_rabbit , July 14, 2011 6:31 AM
    Quote:
    However, when push came to shove,

    what does this clause mean???
  • 2 Hide
    wifiguy99 , July 14, 2011 8:28 AM
    When will Part 1 get a makeover like this?
  • 2 Hide
    Anonymous , July 14, 2011 9:37 AM
    I wonder why you didn't include Juniper products (formerly trapeze)to this test. It's quit a big player here in europe. Trapeze also produced the 3com wireless manager and accesspoints which was sold widely here.
  • 3 Hide
    Hupiscratch , July 14, 2011 1:27 PM
    In the page "Benchmark Results: Close Range, No Interference", the HP AP is missing on the downlink graph.
  • 2 Hide
    Onus , July 14, 2011 1:32 PM
    This was an outstanding article. Going just by this, Ruckus and Cisco are the only two I'd consider out of the box, but it would be very interesting to do a follow on that features even a minimal amount of tweaking to see what changes. A consumer expects a product to work well out of the box, but an enterprise network engineer almost certainly does not.
  • 2 Hide
    Anonymous , July 14, 2011 2:08 PM
    Very thorough. Lots of hard work went into this and it shows. But how did you select client devices? Did you try any other chipsets? We tried something like this with more diverse clients and got results that were too variable to reach conclusions. (Some clients just did better with some APs than others.)
  • 1 Hide
    ashserratt , July 14, 2011 2:14 PM
    Why not include Aerohive?
  • -1 Hide
    Brazilian Joe , July 14, 2011 2:21 PM
    I would like to know about the exact model of the Airport Extreme tested: is it the previous generation model, or the recently refreshed model capable of 450Mbps?

  • 1 Hide
    rebel1280 , July 14, 2011 3:08 PM
    jtt283This was an outstanding article. Going just by this, Ruckus and Cisco are the only two I'd consider out of the box, but it would be very interesting to do a follow on that features even a minimal amount of tweaking to see what changes. A consumer expects a product to work well out of the box, but an enterprise network engineer almost certainly does not.

    As much as i want to see a follow up on tweaked APs did you read the cost of the setup, $15,000! I don't expect a follow up any time soon haha. By the way Toms, great articles. I didn't mind the initial layout but I like this one better truth be told. Good info, good read. Looks like I'm getting me a Cisco for the office :) 
  • 8 Hide
    awtull , July 14, 2011 3:09 PM
    You have confirmed what 6 years of operating and managing a TROPOS wireless mesh network has shown. As a municipality that deployed the network initially for mobile workers and public safety we did sell access to the network for affordable internet to our citizens. When we looked for a wireless bridge device for the customer that would give good performance along with reliable connectivity the hands down winner was Ruckus. We have probably installed close a 1000 of their dual zone bridges and I can say that everything that your tests have shown is what we have seen in true world application. Your article did a great job of addressing all of the various RF issues of wireless network and I commend you on a job well done.

    Anthony Tull CGCIO
    IT Director
    City of Granbury, TX
  • 2 Hide
    wiinter , July 14, 2011 3:24 PM
    Will - this has got to be one of the best online articles I've read in the last 15 years. Kudos to you and your team!
  • 1 Hide
    Onus , July 14, 2011 3:32 PM
    Quote:
    As much as i want to see a follow up on tweaked APs did you read the cost of the setup, $15,000!...

    Oh yes, of course. If they could take just a worst case result, e.g. for that sorry Meraki unit, and see if a few simple tweaks made it viable, hopefully that wouldn't take the time or expense, but would clearly show the benefit from tweaking (i.e. from being a competent network engineer).

    Edit: And, perhaps the cost could be picked up by Meraki, or Aruba, since it seems to clearly be in their best interests, IF it showed their units could hang with the big boys. Based on this article alone, I probably wouldn't touch their products with a ten foot dipole.
  • 0 Hide
    Anonymous , July 14, 2011 4:27 PM
    Great read, interesting article. Have about 7 wifi devices in my house and currently getting pretty random performance. Think i now know why. If Ruckus ever releases a 3X3:3 for close range performance that would be very interesting!
  • 2 Hide
    spammit , July 14, 2011 5:01 PM
    Isn't it Cisco Aironet, not Aeronet?
  • 0 Hide
    thearm , July 14, 2011 5:15 PM
    spammitIsn't it Cisco Aironet, not Aeronet?


    Lord... Does it really matter?

    Anyway, it's so weird here at Toms now an add will pop up because you move you mouse over it and you have to click X to close it. But yet, the pull down at the end of each story (with the chapters in it) will go away of you move your mouse off of it. You have to be very careful with your mouse, when trying to select another chapter, or it will go away. It's been like that for years. Doesn't this annoy anyone else?
  • -2 Hide
    Anonymous , July 14, 2011 5:39 PM
    By the title of these 2 articles, I was anticipating some information regarding how I can diagnose and fix issues with my WiFi. Whereas now I have a greater understanding of what issues can arise and what router to use in an office setting, I still do not know how to diagnose my own crappy WiFi performance or how to fix it. While I applaud your efforts, I imagine most readers do not have 60 laptops and 5 ipads in their home...
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