Sunday, March 6, 2016

Using your autonomous AP as a Spectrum Analyzer


Most of us have all done an APoS (AP on a stick) survey, either active or passive, for a customer by now.  Many of us also take a snapshot of the spectrum while doing our WLAN surveys.  We either use an integrated Spectrum Analyzer, such as the DBx adapter coupled with Ekahau’s ESS software, or we use a spectrum analyzer adapter(hardware) and software to collect data for off-site analysis.


If you own the DBx adapter and use it with Ekahau, that doesn’t mean you own Chanalzyer, which is Metageeks Spectrum Analyzer software.  Which leads me to this post, as I do not own a copy of the software mentioned previously.


If you are on-site, using a Cisco 3602i series autonomous AP for your APoS active/passive survey, did you know that with just a little bit of effort you can use it to grab your Spectrum Analysis file?


I usually have my old Dell D630 workhorse with me, which has a PCM/CIA  slot, an old Cognio card, and Cisco Spectrum Expert loaded on it.  However, as most of you know, these machines weigh 4.6 metric tonnes after about three hours of carrying these monsters around, no matter what hand you carry it with.


After a brief conversation with @NoLANWiFi (giving credit where credit is due) I decided to lab it up.  I am assuming you have read @802Tophat’s blog on getting your site survey AP up and running – Thanks Richard for posting that for us!


I logged into my site survey AP running 15.3 code and changed both dot11 interfaces’ station roles to “station-role spectrum”.    Now you need the NSI key.   Type “show spectrum status” from the exec prompt and grab the NSI key.  It looks something like this:  NSI Key:  0FB30A960DA7F66952E30B59640563AC


There are two ways to connect to your “new” Spectrum Analyzer.  Connect to either one of the dot11 radio interfaces with your site survey laptop, like you normally do when active surveying, and use the other interface for spectrum collection.  Meaning, if you connect via the 2.4GHz radio, you will use the 5GHz radio for spectrum collection.  Or, connect via TCP/IP via the Ethernet interfaces.  This is how I am going to do it for this post.  I am going to use the Cisco POE injector, and one end is connected to my survey AP, the other to my laptop.  They’re both on the same /24, so I can simply connect via TCP/IP.


Next, you need to download Cisco Spectrum Expert.  If memory serves me, you need greater than 4.0 for remote spectrum analysis.   After downloading and installing it, go ahead and launch the app.


This is where you plug in that NSI key.



My laptop is, statically set on my Ethernet interface.  Notice how I am given a choice to go use either the 2.4GHz or 5GHz band.  This appears to be one of the downsides of using the AP for spectrum collection.  I cannot select both frequencies to collect data.


Here goes…  Let’s take a look.



As most of you probably already figured out, I have a known environment in my lab to test with, and sitting side by side is the old Dell D630 with the Cognio card.  After running some interference tests, it looks like the remote sensor and the Cognio are, for the most part, on the same page.


However…  something is missing!


When I look at the Channel Summary page, something looks awry.



I’m not seeing any Wi-Fi Present!  What’s the deal? (I know what the deal is, but wondering if you, the reader, can figure it out)


Now I am going to the Devices tab, and again, I don’t see anything!



What could be “wrong” with using the AP as a Spectrum Analyzer?   I left BIG hint for you in that last graphic…


All that said, I think I can use an AP for a spectrum analyzer, in a pinch, if I had to do it from a remote site.   Clearly, when using an AP as a remote spectrum analyzer, we don’t get all the functionality we would get out of our laptop/hardware based spectrum analyzers, such as Metageek’s Chanalyzer, Spectrum Expert (with Cognio card) or AirMagnet’s SpectrumXT.


Please use the comment section to chime in on why we are missing the data that we might want to see at a later date, if we were using the spectrum file as a baseline.










  1. Been there, buddy... but luckily got my managers to approve the acquisition of Ekahau SA.
    But as you rightly said, in a pitch that's the way to go, or as we say in Portugal "if you don't have a dog, hunt with a cat" :-)

  2. Spectrum Experts uses the local wireless card to gather the 802.11 information about networks etc...

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