‘Is radar breaking my WiFi?’

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The question in the headline was posed yesterday by a reader of Reddit’s section that is devoted to networking.

“This just seems so ... far-fetched,” the questioner writes. “I wouldn't believe it if someone told me this story.”

Here’s his story:

“So, I'm still trying to wrap my head around what happened this morning. I have been chasing a problem with a few of our Aruba Ap105 access points for a few weeks now. They would intermittently lose network access (and generate Nagios alerts) only when school was in session, and only at one school...our highest wireless utilization point across the district. Cisco switches showed packet errors, so that's where I started.

“After hours of digging, cable checking, reading cisco logs, a day came where I decided to plot all of the APs having problems. The day I plotted them (based off of timestamps of events) they were almost in perfect order from North to South. What the hell?

“I then started looking at the Aruba logs, and started to put the pieces together. The first log from one of the APs said "Missed 8 heartbeats. Rebootstrapping"

“Aruba tells me this is due to network congestion, misconfiguration, cabling, etc. We upped the heartbeat to 20 before rebootstrapping. But the network problems continued. Now in the logs, corresponding with the outages, a new event appeared: ‘Radar detected on channel’

“After a few minutes of searching, I learned all about DFS channels and radar interference. Sure enough, our controller was enabled on all of these regulated channels. Removed them from the channels, saved changes, and all aforementioned problems have disappeared. Even the packet errors on the switch which I assumed were due to wireless drops during transmission.

Is my analysis correct? Is this what really happened?”

The networking experts on Reddit assured the fellow that he is correct.

And it’s not a bug it’s a feature, as Cisco explains here (.pdf):

DFS is the process of detecting radar signals that must be protected against interference from 5.0 GHz (802.11a/h) radios, and upon detection switching the operating frequency of the 5.0 GHz (802.11a/h) radio to one that is not interfering with the radar systems. TPC is used to adapt the transmission power of a radio based on regulatory requirements and range information.

Mystery solved.

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010815blog internet pipes Erik Solheim

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