Dynamic Channel Assignment (DCA) handled by RF group leader for a particular RF group. DCA algorithm takes following information into account
1. Load Measurement (Every AP measure the % total time occupied by Tx or Rx 802.11 frames)
2. Noise (APs calculate noise value on all serviced channels)
3. Interference (APs report on % medium taken up by interfering 802.11 transmission)
4. Signal strength ( RSSI of neighbor messages are heard)
Based on these metrics, if the worst performing AP will benefit by at least 5dB or more a channel change (DCA) will take place. The decision of change channel is also weighted to prevent mass change within RF group. Less client AP will more likely to change channel.
When a AP first boot up (out of the box) it transmit on CH1 in 802.11b/g radio and CH36 on 802.11a radio. Even AP reboots afterwards it will remain on the same channel until DCA occurs.
There are two scenarios in DCA could impact wireless network negatively. “Pinning” & “Cascading” are those two.
Pinning occurs when DCA algorithm could find a better channel plan for some radios in RF group, but is prevented from pursuing such a channel plan because worst radio in the network does not have any better channel options. The worst radio in RF group potentially prevent other radios in the group from seeking better channel plan. The larger the network, the more likely pinning becomes.
Cascading occurs when one radio’s channel change results in successive channel changes to optimize remaining radios in the RF neighborhood.Optimizing these radio could lead to their neighbors and their neighbor’s neighbors having a suboptimal channel plan and triggering their channel optimization. This could leads to a network instability.
Since WLC code 6.0 DCA algorithm redesigned to address both pinning & cascading.
1. Multiple local seaches
DCA serach algorithm perform multiple local seraches initiated by different radios rather than performing a single global search driven by a single radio)
2. Multiple Channel Plan Change Initiators (CPCI)
Previously , the single worst radio was the sole initiator of a channle plan change. Now each radio within RF group is evaluated and prioratized as potential initiators. Intelligent randomization of the resulting list eliminates the potential for pinning.
For each CPCI radio, the DCA algorithm performs a local search for a better channel plan, but only CPCI radio itself and its one-hop neighboring AP are actually allowed to change the current Tx channels
4. Non RSSI based cost metric.
A cumulative cost metric measures how well an entire region, neighborhood or network perfrom with respect to given channel plan.
You can configure DCA in 3 ways, “Automatic” , “Freeze (on-demand)”, “Off”
Automatic is the default setting where DCA runs every 10 min for 10 times once controller boots up. Then you can anchor it to certain time of the day & repeat specified time interval.
In Freeze mode no RF changes will made until you click “Invoke Channel Update Now” button. You need to remember it will not run immediately & it will run next 10 min interval.
Here are the other settings in the menu.
1. Avoid Foreign AP interference: Enabled by default. When enabled the co-channel interference metric is included in DCA calculations. For 802.11a this is good to enable as there are multiple non overlapping channels to change. But in 802.11b/g there are only 3 non overlapping channels and always there will be co-channel interference. So you need to careful when to tick this box for b/g.
2. Avoid Cisco AP Load: Disabled by default. When enabled, the client load on an AP is taken into consideration before making a channel change. It is good idea to tick this in order to prevent channel change for large client associated AP.
3. Avoid non-802.11 noise: Enabled by default. When enabled non-802.11 interference is taken into consideration.
5. Channel Width: If you are using channel bonding in 5GHz select 40MHz. In 2.4 GHz it should be always 20MHz
6. Avoid check for non-DFS channel: Disabled by default. When enabled controller avoid checks for non-DFS channels. (dynamic frequency channels). Config guide (220.127.116.11) state this only required for outdoor mesh deployment. So we can leave this disabled
7. DCA Channel List: You can select/de-select channels you want to consider for DCA algorithm. If you want to include Extend UNII-2 channels tick that box as well.
Make sure you disable the radio band before make changes & enable it once you do that.