What is RRM ?

If you are getting ready to move to Cisco’s WLC system, one of the terms you should have heard by now is RRM. This is short for Radio Resource Management. Part of the communication between the LWAP’s and the controller(s) on your network is a near constant checking for interference/channel congestion on the channel each LWAP is currently operating on and the current transmitter (TX) power level each LWAP is running at. One of features of RRM and WLC is that the AP’s are continually checked for optimal TX power and that they are operating on the clearest channel possible.

Depending on the amount of other Wifi traffic in the same general area as your network, you may see a periodic changing of the channels. Depending on the amount of signal overlap from all of the LWAP’s that you have and you may see a situation where RRM when it has to make a change can actually cause a problem that can be more disruptive than the worse than the one that it is trying to fix. You will probably notice this more in the 2.4Ghz range used by Mode B/G than the 5Ghz range used by Mode A/N.

The best way that I can describe this would be to use the term Domino affect. Assume for example that you have a 3 LWAP Configuration. LWAP A sees interference/noise on Channel 1 and moves to Channel 6 as the “clearest” channel that it can find (WLC by default will only use 1,6 or 11 in the US unless you add additional channels to the list). The problem you will now see is that LWAP B is already on Channel 6 and now because of the change in channel of LWAP A from 1 to 6 and now LWAP B will now have to move from 6 to 11. LWAP C had been operating on Channel 11 and because of the previous two changes now has to make a channel change from 11 to 1.

This example may seem benign enough but as you can see, it could be very disruptive if it occurs at the right time of the day or to the wrong people at the right time. If you run into this problem, one option you may have to consider is to disable RRM for the AP’s at a particular location by locking them down to a specific channel until you can get onsite with the appropriate site survey gear to identify what the cause of the problem is and to see if you can resolve the problem.

Troubleshooting a RRM problem is a little easier to do when you have a WCS Server working in conjunction with your WLC(s). For a single WLC, having WCS on the network may be a little harder to justify but after dealing with a problem such as one involving channel changing problem that would seem to occur for no apparent reason, having WCS might may the problem a little easier to find and fix.

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