Thursday, May 3, 2007

Ripple Effect - Redux

This article is out of date.

Early in the year I posted an article about how the Cisco WLAN controller system may behave strangely in some conditions. I got some email from some folks that had major issues with it. One poster said that, "Before Cisco purchased the technology from Airspace, they had already put dampeners in the RRM so the hysteresis you describe wouldn't occur." This is just plain wrong. Cisco wants to sell more switches and routers and they found out if they purchased the Airespace system they would do just that but they did not make this significant change before releasing it with their name on it. And they are still changing the behavior of the WCS today because this problem still exists.

Did I lose you? As a refresher for those who did not see the original article it is posted HERE.

Since I published that comment back in early February I have spoken to quite a few people who have seen the same effect in their environments in recent months. One network engineer wrote, "I can vouch for having observed this recurrent DCA behavior, also in a hospital environment (12-24 channel changes per day across 10 floors of APs, as you depict in your example). The architecture is not alerting us to this being the result of interference or noise (no WLC or WCS events of either type), and the RSSI of rogue APs is above the threshold required for triggering DCA (neg 85dB)."

I was asked by the nay-sayers what Cisco told it's customers to do and here is what that same engineer said, "We have been told by Cisco that the 100mW AP neighbor beacons, used to determine the picture of the network, does not get input into DCA. Cisco claims these 100mW beacons are used only for dynamic power control, which we hold static -- do you think this voids the dynamic algorithms? Other docs say the RSSI of neighbor APs is the most important criterion in DCA behavior! In lieu of noise and interference alerts we can only surmise its the APs themselves that are the cause of their own DCA ripple effect."

This is just one example. I also have spoken to other folks who say that the Aruba system they are running does not do this. They say it is much more stable and after the original "learning" time it settles down and stays that way as long as the network is in use. I think this makes sense, why change the whole network because of one interferer? Better to be alerted to the fact and deal with it yourself.

I am collecting comments on this and would like to post more testimonials about this effect. If anyone wants to support this claim publicly, please feel free to drop me a line to or comment to this post. My goal here is not to raise hysteria but get things fixed and level the playing field. The infrastructure vendors tend to pitch the idea that they offer a panacea for all wifi woes and I feel that that is just a flavor of "Kool-Aid" I am unwilling to drink.

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