AirMagnet Active vs. Passive for Interference

[jasonwirz]

When surveying with Airmagnet Survey, I've noticed my interference levels are extremely high when doing a passive survey, and very low during an active survey. I can assume this is because the card can only listen on the associated channel during an active survey. I guess this seems like a more accurate assesment of the airwaves since the client only cares about the channel its currently using. Is this an accurate assumption? We just put in a high density install of Cisco 3502e's and I was floored at the difference between the two results.

Thanks for any input.

[George Stefanick]

What was the numbers?

[jasonwirz]

Some screen shots to show the difference

Active

Passive

The red, blue, and green dots are the access points. The color refernces the level. The surveyed area is about 1200' long. So far client performance has been outstanding, its just the extreme difference between the two that had me wondering what I'm supposed to be using to accurately assess a wireless network.

[George Stefanick]

I see exactly what you're saying. Have you opened a ticket with AirMagnet ?

[Scott Fella]

Well I'm not a big fan of the Active survey:)  Back in the days when you have to do an active survey, you are suppose to walk one direction and save the data, walk the opposite direction and save the data and then do a passive survey and merge all three.  Roaming doing an active survey didn't work so well.  If you didn't look to verify you are still connected and recieving/sending a signal, your heatmap would look broken up. There are times when AirMagnet would not reconnect to another AP or with high density, you would not see all your AP's.   Just my 2 cents.

[jasonwirz]

I'm pretty sure the survey data is accurate. If you tune the settings on the active survey, the client roams just fine. We set ours to scan at -65 dbm for our higher density installs and the roaming points are very predictable. As long as the airwaves are clean from the client perspective (active survey) I would assume everything is working as intended. Its almost impossible to install an interference free environment according to the passive survey. We've tried our best in the autonomous AP world. I was just hoping someone might have some experience to solidify my thoughts.

[sarslanxx]

The interference heatmap is calculated based on the signal strength of APs seen at a given location.   When performing a passive survey, depending on the SSID and or channels being scanned, you may see several APs on the same channel and or adjacent channels that affect the interference heatmap calculations. When performing an active survey, depending on roaming parameters, you may or may not roam to APs on the same channel or adjacent channel which affect the interference heatmap calculations.  If you have more questions or would like to discuss this further, pelase send me a private message.