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.