Who said 5GHz was ‘clean’? :)

Here I am at home today being a good survey engineer and making sure all of my tools are in proper working order prior to going out and having to rely on them for the week when all of a sudden, I’m presented with the following anomaly when I’m exercising my trusty Spectrum Analyzer:

Those of you that are familiar with Spectrum Analysis in general usually expect to see something this bad (high duty cycle) in the 2.4GHz spectrum but not the mid-5GHz spectrum! Having just reloaded my laptop with Windows 7 and installed Service Pack 1, I was in the ‘let’s test it all’ mode to make sure nothing unexpected happens. At this point, I was pretty blindsided by the obnoxious noise happening and the ‘Generic – Fixed Frequency’ tag wasn’t helping me any. At a loss for what this could be since I live an acre away from my nearest neighbor and several miles from the nearest airport, I pinged a few of my friends. They suggested the usual suspects – MRI machine, TDWR, neighbors, etc all of which I explained away by location. Being that TDWR is in the 5470-5725 frequency, I changed my card over to 5.725 – 5.850 and after some time got this equally disturbing read:

At this point, I started to suspect my Spectrum Analyzer since I was using a non-Cisco branded Spectrum Analyzer card with the Cisco Spectrum Expert software (the card I was using had the Cognio components that Cisco purchased and re-branded as their own). So I grabbed a copy of the card manufacturers software to rule out in compatibility and I got the same results.

At the end of the day, I was able to swap in a Cisco branded SA card and my results normalized. Clearly I have a flakey (old) SA card that was giving me improper readings. Lessons learned:

  • Always test your tools and keep them in good working order
  • Don’t assume that your tools are telling you the truth. If you see something suspect, dig into it and validate against another source
Now I’m sure that I have a good card in hand I can go confidently into my week and knock this survey out of the park!
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3 Responses to Who said 5GHz was ‘clean’? :)

  1. CENEXIP says:

    Hi Samuel,

    Great wisdom ! Always test your tools. I got the original Cognio card and sometimes I find discrepancies in results between Cognio and another SpecAn brand (name disclosed), leading me to question which one tells the truth. But when VoIP over WLAN calls keeps dropping, I know there is something wrong, even though both SpecAn tools do not classify the culprit the same way. Thanks for sharing.

  2. Sam says:

    Thanks for the comment! I find it interesting that our tools give us only their perceived opinion. 🙂 At the end of the day, it's down to the engineer to accurately read (or interpret) what's given to them.

  3. Pingback: Internets of Interest:30 Mar 11

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