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.
4. DCA Channel Sensitivity: You can choose DCA channel sensitivity from 3 levels (Low/Medium/High). Values are per radio (2.4 GHz & 5 GHz have different values) see below
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 (18.104.22.168) 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.
1. RRM Basics
2. Configuring RRM
3. Configuring TPC
4. Configuring CHD
5. Configuring ClientLink
6. Override RRM
8. Rogue Access Point Detection
Lubos Zelinsky said:
I have noticed that your DCA channel sensitivity table shows values for 7.0.98 WLC software version. When looking into 22.214.171.124 (7.0MR1) I can see that values for Medium are 10dB for 2.4GHz and 15 for 5GHz. Values for Low sensitivity are 20dB.
Go with what 7.0MR1 says with respect to CCIEv2.0 exam.
anyone ever see an issue where a specific DCA channel will not disable? I am trying to turn off channel 64 using PI 2.2.0 and when i uncheck the channel and save it is automatically re-checked after save….. thanks!
Make sure to turn off 802.11a radio by going to the wireless tab. Select wireless tab on left expand bold text “802.11a/n/ac” select “Network” below that. Under General uncheck 802.11a Network Status Enabled Checkbox. Do remember this turns off all of the 5Ghz radios. Now you can change DCA channels. After change check the box back for enabled on 802.11a Network Status.
hilario martin (@hiletw) said:
Many thanks for your blog , I really enjoy it and is very helpfull for my job.
Regarding RRM and DCA channel List, is there any recomendation to use (select/desect) some channel plan like not overlapping channels?
Thanks for kind words.
Yes. In 2.4GHz – CH 1, 6 & 11 only for most part of the world (ref your local regulatory restriction, if any)
In 5GHz, UNII-1, UNII-2 and UNII-3 channels for sure. These will be supported by almost all clients.
If you need more channels (for VHD), you can select UNII-2e as well, but be aware certain end points may not support these channels.
Here is a good reference for client types and what channels they support
hilario martin (@hiletw) said:
Thanks for your response Rasika.
I was wondering if anyone knows any reference or paper where expressly indicated that DCA channel list may or may only recommends the use of these three channels (2.4) and not all available. This also applies to 5 GHz.
After multiple – though perhaps not enough – searches have not found anywhere to clearly make this recommendation.
If you choose all channels in 2.4GHz, your AP can choose any of them to operate.
As you know if you choose any other CH other than 1,6,11, that AP will interfere with two of CH1,6,11 AP. So it make your situation worse.
Hence it is only recommend to use CH1, 6 and 11 in 2.4GHz as those are non overlapping.
I have problem with DCA Channel list of WLC 2504 (software version 126.96.36.199), it cannot choose DCA channel beyond 140 (even though check box for Extended UNII-2 channels ). Could you please help me check ?
Did you try with different software code (like 8.0 ) ?
Jose Marrero said:
This blog has helped me numerous times, it’s great! Keep up the good work. I came here looking for what would be the best practices setting to configure DCA as far as running re-occurrence (time). I understand that the bigger the network the bigger the chances of seeing pinning or cascading issues, but having it configured at 12 hours on all radios doesn’t seem practical to me.
I have set it for 8 hrs in my environment
Richard M said:
Hello Nayarasi, how does one change the DCA update interval time, I have my wireless networks disabled, but when I try to chnage it from 10 minutes to 8 hrs I get an error message “DCA Channel intervalk cannot be set”
did you try it via GUI or CLI ? If only one method used, try other method & see
I have discovered that it may be a code related (188.8.131.52) bug, worked ok on 8.0.140 code, but can only set dca interval to max of 1hr on 184.108.40.206 code, only tried via guide, will try by cli tomorrow
Hi Richard, Look at this thread, as in newer codes, there is a new feature FRA, which is set to 1 hour.
You can change DCA interval only if you change it with FRA interval. See below post
Richard M said:
Guys, just found this out:
In 8.2, we have a new feature called FRA (Flexible Radio Assignment) for those newer APs 2800/3800, which is by default is disabled:
this FRA interval that you set has direct dependency on your DCA interval FRA interval must not be lesser than the DCA interval.
The default FRA interval is 1hour (even it is disabled) and this is the reason why we cannot set the DCA Interval for more than 1 hour.
in order for us to enable the DCA Assignment interval:
– We need to enable FRA and set the sensitivity and interval (Wireless>Advanced>RF Management>Flexible Radio Assignment)
– Then, go back to the DCA interval and set it lesser than what you set in FRA.
Thanks, did not see this response when I replied to earlier comment.
Informative post as always.
I am planning to enable UNII 2 extended channels and it’s recommended to have DFS in place for that. Does it mean that above channels shouldn’t be set statically on AP’s rather WLC should use them with “auto RRM” being set ?
Even if you set it to static, when radar event detected AP has to change the channel.
I would leave it to RRM to handle it rather make it static.
Can AP change it’s channel upon detecting radar if channel is set statically ?
john z said:
Question for you. I’m wondering how DCA gets information on off-channels when not using a monitor mode AP or a WSM module (for the 3600 or 3700ap).
Without either I notice the WLC only shows cleanair statistics for the serving channels. When using a monitor mode AP or the WSM module AQ statistics are shown for all channels.
Does this mean when not using a monitor mode or WSM module, DCA is essentially guessing on which channel to jump to, at which point after it jumps it will then have an AQ score for the future?
I do not know these details, probably direct it to Cisco and you will get a response.
Nebay Woldeab said:
Your blog regarding wireless technology is really amazing and thanks for that.
I have a question, I have 84 Cisco light weight access points and 5508 WLC running 220.127.116.11. I wonder can I put all my AP (84 APs) in one channel i.e. 2.4GHz to be on Chanel 1 and 5GHz on 149? what would be the consequences for this action?
If you want to make your wireless unusable, you can try that 🙂
As you know wireless operate in CSMA-CA (CA-collision avoidance) any station or AP need to sense medium before transmitting. By having different frequency in each AP, you can make this contention domain small. Once you put all your AP in same channel, it will become very large contention domain, so no body will get a free medium to tx (as you will hear large number of devices in your channel)
So it is big NO NO to put all your AP into same channel.
Hope that clear
Nebay Woldeab said:
Thanks very much Rasika, I really appreciate your response and Thanks for your time and patience.
Melvin Arnell said:
Would be able to provide the cli commands to disable and enable the radio ? Also the cli to add/remove channels from the DCA Channel List?