Friday, February 10, 2012

Hey Window! You Make a Better Door Than You Do a Window.

Over the past two months, I have been battling with a Cisco Aironet 1310 wireless bridge between our main building and a physician's office that we rent across the street.  This physician's office is one building out of a cluster of 15 in a "Doctor's Park".

A few years ago when we put in this link, we were the only wireless in the area.  Within the last 6 months several of the physician's offices and a neighboring nursing home have added wireless infrastructure.

When our problems started, I noticed that our signal strength had gone from an average of -69dBm to -82dBm and our signal to noise ratio was around 10 dBm.  My first thought was obstruction having recently had a different link knocked out by a new HVAC project on the roof of one of buildings interfering too much with the Fresnel zone of another link.  I went up to where the radio was on the main building and took a look.  Nothing new was in the line of site.

Having ruled out the obvious, I fired up the Cisco Spectrum Expert on my laptop and sat up near the radio.  Almost immediately I saw something that wasn't quite right.  All of my APs and bridges are set to use channels 1, 6, or 11.  On the laptop though I was seeing active APs on channels 3 and 8.  Of course since the 2.4Ghz channels overlap, this was causing cross channel interference on all three of the channels that my bridge was configured to be able to use.  As I looked more closely at the Cisco Spectrum Expert information I found the SSIDs associated with these interfering APs.  This is where I got a break, I recognized one of the SSIDs as being from another healthcare entity from another town.  I went back to looking outside and thinking where this might be coming from.  Eureka, one of the physician's offices had a new sign out front with the other entity's logo.  Thankfully the network engineers on their side were gracious enough to reconfigure their stand alone APs to use channels 1 and 11 leaving me 6 for my bridge.

You're probably asking, what does this have to do with a window being a door?  Well yesterday after having resolved our bridge issues for almost a month by my detective work, the problem came back.  This time though, I couldn't find any interference.   Instead, the screen showed something on channel 6 with a very high duty cycle.  I confirmed it was my bridge that was on channel 6.  This puzzled me so I drove over to the far end of the bridge to see if something was amiss there.  I walked up to the radio with my laptop and saw the same type of high duty cycle, but again no interference.

As I did a visual sweep of the room I saw that the window in front of the AP (I know this isn't ideal, but we rent the building.) was partially open such that the metal frame of the window was just about centered in front of the radio.  I asked the clinician whose office it was how long the window had been open.  They told me it had been open for about 2 hours... exactly the amount of time that the bridge had been having problems.  I promptly closed the window and presto the signal strength went back to normal and the duty cycle returned to normal as well.  Once again, the network was undone by the Human Network.


  1. Ben,
    I haven't ever worked with wireless bridges but it sounds like it doesn't take much to screw things up. Nice post!


  2. Nice find Ben! It is amazing how 'little things' like that can be a thorn in our side. And people wonder why some IT guys can be grumpy.