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 192.168.0.5, 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.

 

 

 

 

 

 

 

 

WLAN Surveying and Validating with Ekahau's integrated Spectrum Analzyer

 

When doing any WLAN Assessment or Remediation, we ALWAYS look at the spectrum.   In about 60% of the WLANs we assess and remediate, we find interference from a device the customer didn’t know they had, or knew they had but didn’t know it was sharing the same spectrum as their Wi-F.

 

The complaints vary from Customers that have interference issues.  We hear “I only have two bars”, “my wireless is slow” and “when I stand right here, my Wi-Fi doesn’t work”.

 

We’ll first start out with stating the obvious.  The 5GHz UNII bands are license free, which means it is a free-for-all when it comes to who is doing what.  Most of us all understand that.  A company, or their neighbors, can pretty much deploy anything they want, as long as they abide by the rules.

 

When we first start out on an Assessment, we do what we normally call a WLAN Validation.  We walk the entire facility with Ekahau’s ESS – or Site Survey Software.  Ekahau recently updated their software to include Spectrum Integration, so here is our first look using it in the real world. 

 

In this case, we have an area that we know we have WLAN Interference.  We have looked at it with other tools, however since Ekahau is our tool of choice at the moment, we want to compare what we see with our new spectrum integration to what we are used to seeing in our legacy equipment.  Here is a view of the area known to have an interferer on channel 40.  We looked at our Survey Inspector, and the Spectrum Channel Power view.  We clearly see something there, and can compare it with a quick glance to the other channels. 

 

 

 

And now we are going to take a look at our Spectrum Utilization view.  We scrolled up to another area of the survey/walkabout where we know we have another source of interference.  Again, we can clearly see that there is an issue in another part of the building.

 

 

 

During our Spectrum Integration analysis, we notice another feature called RTFM.  Forget what you know about this acronym, because it stands for Real Time Frequency Monitor.   This is the kind of tool I would use if I had my survey rig in my backpack, and someone told me of an area that was having Wi-Fi issues.  I don’t even need to build a project – I open ESS and hit the RTFM button, select the frequency I want to look at and give it a glance.  Here’s what I see below.  I must say, that’s a nice feature!  Thank you Metageek (for the SpecAn) and Ekahau!

 

 

Now that I have seen the interference, I, for whatever reason, want to see it in my Spectrum Analyzer software.  This is available from Metageek – where you would most likely purchase your SpecAn hardware.  This view is with other known interference devices, all by the same manufacturer, turned on – for our testing purposes.  As you can see, there is a lot of interference here, and some remediation and spectrum management needs to take place.

 

 

 

Now for a look from some of our legacy tools.  As you can see, the view is not of the same exact slice in time, however I promise you that what you are looking at is all caused by the same equipment.   This is a view of four devices energized, and one of them is changing channels.  No wonder these folks are complaining about their Wi-Fi not working well for them!

 

 

 

 

Here’s a view of channel 40 from another Spectrum Analzyer.  AS you can see, the numbers vary from tool to tool, but in each you can tell there is an issue.

 

 

Af first, it took us a while to track down the equipment, as we didn’t want to go into an operating room while they were doing their thing.  After several days of intermittent troubleshooting, we finally came to the conclusion that our source was mobile.  We tracked it down to Operating Room “towers”, which were mobile Endoscopy equipment.  The gear has a wireless transmitter and a remote monitor or two, and those remote monitors were connected via the 5GHz spectrum.  This equipment was moved around to whichever Operating Room need it.

 

 

 

 

Then we discovered something else.  When the equipment was turned on, it searched for a channel to use.  Not in a very Wi-Fi friendly way, though.  It seems as if it always starts on channel 36, then works its way around the first eight channels until it finds “home”.

 

If you are good with math and have a great imagination, multiply this one source of interference, and what is does to your WLAN, by four.  There were at least four of these devices being turned on and off during the course of any workday, each booting up and trying to find a channel to use.  Stomping on the Wi-Fi as it went.

 

 

 

Something about these tools is worth mentioning. They are somewhat complicated and expensive to own – unless you use them on a weekly basis.  They all do a great job of displaying to the operator what the electromagnetic spectrum looks like at a given frequency.  However, there is no “magic button” that you can click on that will tell you what is wrong with your network and how to fix it.  I highly encourage anyone interested in owning and operating these tools to first go to CWNP.com and purchase the CWNA curriculum and read it several times.  Of course you also have to read the manual of the spectrum analyzer you finally end up purchasing.  You can get the Spectrum Analyzer (and the software) that integrates with Ekahau ESS from http://www.metageek.com/