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Check your Spectrum!

October 3, 2011
at 10:28 am PST

Lately I had been spending a lot of time in the office rather than on the road.  Which isn’t all bad, as it gives me some semblance of a routine rather than living out of a suitcase.   It has also has given me some spare time to come up with another blog topic, which actually stems from some of the work I have been doing for customers lately.

Typically when a site survey is being done, we will do spectrum analysis work as well, part of my job entails creating and reviewing documents from this work, prior to delivering them to customers, which means I have been watching a lot of  spectrum analysis lately.   Most of the customers I have worked with recently have been with CleanAir APs, so they will be able to monitor their environment in real time, once the WLAN is up and running.  However it’s always a good idea to perform some spectral analysis while you are walking around doing a site survey.  And really why not?  If you are there and you have a few minutes, fire up the old spectrum card and get a capture of whats going on with your RF.   This helps make sure there aren’t any major layer 1 surprises when you go to install the new WLAN.  It doesn’t mean things won’t change, and they often will, due to the dynamic nature of RF.  It’s an ever changing environment, so what wasn’t there on Monday, might show up on Tuesday and be gone again by Wednesday.

Before jumping into particular types of interferes let’s talk about some of the data that Cisco Spectrum Expert can show you.  Two of the things I like to look at when looking at the RF in Cisco Spectrum Expert, are Real Time FFT and Duty Cycle plots, as pictured below.

The Real Time FFT is showing you is the RF energy in real time measured in dBm, so how loud or quiet the device is.  The next is the FFT Duty Cycle, which simply put it’s how utilized the RF is. Let’s say you have a device that is being captured as having a 1% duty cycle.  This means it’s using a very small amount of the available ‘air time’ to transmit its data.  Conversly if there is a device that is showing a 100% duty cycle it is using up all the ‘air time’ and not allowing other devices to use the RF medium to transmit.

Two other views I find helpful are the Spectrogram views.  These display the same info as the plots above, but are plotted out over time.  I use them in a few of the examples below.

So with all the captures I watched lately I found a few consistencies in the types of interferes across the different customers spectrum captures.  The 3 most common ones that I have seen are Microwaves, Bluetooth and DECT Phones.  Let’s take a look at them.

Bluetooth

This is EVERYWHERE, but generally doesn’t create a large problem because they tend to be low power and don’t generally send large amounts of data.   You can visually see a Bluetooth device the easiest in the Spectogram view.  Take a look at the image below and you should see little red dots scattered throughout the spectrum.  This is because Bluetooth is a frequency hopping device and will change channels within the 2.4Ghz spectrum when it transmits or receives data.

Microwave Oven

These appear quite often as well, since most places, warehouses, offices, stores,  have employees and that lends itself to having break rooms.  In a medical building you may also find these in labs.  These are very high power devices and depending on age and quality can produce low levels of  RF interference or very high levels of interference.  In most cases it is the latter.  If you look at the image below you can see a poorly shielded microwave that is disrupting almost the entire 2.4Ghz spectrum.  Notice that the interference isn’t persistence, but transient.  So while this won’t interrupt normal Wi-Fi communications forever, it is enough to cause a VoIP call to drop or your video to be choppy.

DECT Phone

If there are wireless DECT phones in the area they are on the 2.4Ghz, they will definitely show up in a spectrum capture.  I’ve seen a lot more phones that don’t use the 2.4ghz spectrum than do, but unless you check and look, you wont know.  Most of the time when I come across these devices they show up pretty low power(greater than -80dBm) which really isn’t much of a cause for concern.  However if you come across ones that are higher powered, like the image below(approx -65dBm) it is a cause for concern.  The reason being as they abide by the same rules of the road as Wi-FI devices.  They don’t stop and listen before transmitting, they just transmit over whatever is going on, which can cause headaches for you as the WLAN Engineer.  In the image below you can see these as the high spikes of RF energy in the real time FFT and red dots in the spectrogram, almost like Bluetooth.

This leads to the question of how to prevent or remove these interfering devices.  Unfortunately in my experience there is no easy answer.  Your organization could write a policy banning DECT Phones and Bluetooth, but who enforces it and how?  You could replace the leaky microwave, but what happens when the new becomes a problem?  It becomes a costly investment to replace them time after time.  Good luck trying to completely remove the microwave from the breakroom.  Ultimately the best bet is to be aware of these devices and monitor them over to time and adjust your RF channel plan around them if at all possible.  By being proactive about protecting your spectrum, you can have a good idea of where things were at and what they should look like when problems arise.

I only scratched the surface in this post of what is out there in terms of Wi-Fi interference, so I highly encourage you to do some Google searches, as there are lots of good resources out there to help you learn more.

-Travis

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2 Comments.


  1. Is there any timeframe on an USB (or something similar… I would take even FireWire, Thunderbolt, …) Version of Spectrum Expert? The CardBus Version is just too out-of-date…

       0 likes

    • Travis Schlafke

      I haven’t heard of anything, but that is also an entirely different team.

         0 likes