Configure Dynamic Channel Assignment

In this post we will learn about DCA and it’s a really cool feature of RRM.

DCA is managed by RF Group Leader (How to define RF leader, we saw in one of my last post)

DCA used to determine the optimal AP channel based on these parameters.

Load: Percentage of time spent transmitting 802.11 frames

Noise: Measurement of non-802.11 signals on every serviced channel

Interference: Percentage of radio time used by neighbor 802.11 transmissions

Signal strength: Received signal strength indication (RSSI) measurement of the received neighbor messages

These values are then used by the Group Leader to determine if another channel schema will result in at least a bettering of the worst performing AP by 5dB (SNR) or in other words: Based on these metrics, if the worst performing AP will benefit by at least 5 dB or more, a channel change will take place. The decision to change the channel of an AP is also weighted to prevent a mass change within the RF group. We would not want to have a single AP change channel and have that change result in 20 other APs having to change their channel. The controller also takes into account how heavily an AP is used. A less utilized AP is more likely to have a channel change instead of a heavily used neighbor (isn’t it an interesting feature?). This helps mitigate client disassociations during a DCA event because a radio channel change disconnects all associated clients.

***Note: When an AP first boots up out the box, it transmits on channel 1 on the 802.11b/g radio and channel 36 for the 802.11a radio. The channels change according to any DCA adjustments if necessary. If a reboot occurs, the APs remain on the same channel they were using before the reboot until a DCA event occurs. If an AP is on channel 152 and reboots, it will continue to use channel 152 when it comes back up.

***Note: Radios using 40-MHz channels in the 2.4-GHz band or or 80MHz channels are not supported by DCA.

The RRM startup mode is invoked in the following conditions:

  • In a single-controller environment, the RRM startup mode is invoked after the controller is rebooted.
  • In a multiple-controller environment, the RRM startup mode is invoked after an RF Group leader is elected.

Configure DCA:

***We must disable 802.11a and b radio before changing the config. for DCA and then enable it again. Simplest way to enable/disable the radio is via CLI:

(WLAN1) >config 802.11a disable network
(WLAN1) >config 802.11a enable network

Go to Wireless > 802.11a/n or 802.11b/g/n > RRM > DCA

DCA1

DCA2

There is three type of Channel Assignment Method.

Channel Assignment Mode:

  • Automatic: This mode will cause the controller to periodically evaluate and, if necessary, update the channel assignment for all joined access points.
  • Freeze: It will Causes the controller to evaluate and update the channel assignment for all joined access points, but only when we click Invoke Channel Update Once.
  • OFF: Turns off DCA and sets all access point radios to the first channel of the band.

Avoid Foreign AP Interference:  It detect foreign AP and take into consideration while changing the channel.

Avoid Cisco AP Load: When its enabled then the AP load is taken into account before result in which AP will change the channel (least loaded AP will change the channel first.

Avoid Non-802.11a (802.11b) Noise: It cause the controller’s RRM algorithms to consider noise (non-802.11 traffic) in the channel when assigning channels to lightweight access points.

Avoid Persistent Non-Wi-Fi Interference:  Its enable the controller to ignore or avoid from persistent non-Wi-Fi interference.

Channel Assignment Leader: The IP address of the RF group leader, which is fully responsible for channel assignment.

Last Auto Channel Assignment: The last time RRM evaluated the current channel assignments.

DCA Channel Sensitivity: We have 3 levels (Low, Medium and High)

Channel Width:  depends on the 802.11a or b radios:  5GHz select 40MHz. In 2.4 GHz it will be 20MHz.

Avoid check for non-DFS channel: Enabled then the controller avoid checks for non-DFS channels. (Apply only for outdoor APs)

DCA Channel List: This option shows the selected channel on this radio.

 

Dont forget to enable both radios after changing the parameter in this section by using these commands 🙂

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1 Comment

  1. “Avoid check for non DFS channel”: Cisco did not recommend us to use this feature for our indoor application but nobody could tell me the exact meaning so far.
    My Suggestion is it will exclude non-DFS channels 36 – 48 and 149 – 165 from the DCA channel list but so reducing the no. of available channels i.e. System capacity/thruput. One need to know that DFS channels 52-64 and 100 – 140 may be exposed to Weather radar signals forcing the WLC to swap this channel immediately resulting in a Service loss or degradation.

    Using Fixed channels (Freeze) should be done only if you elaborated yourself a reasonable channel plan for your facility. I recommend to use only non-DFS channels for this because in “Freeze” mode all AP´s encountering a DFS Trigger will always be re-set to channel 36 even if adjacent neighbor AP´s already utilize this channel. This may result in a severe Service Degradation.

    We decided to use a fixed 40MHz BW in 5 GHZ band because there a still new low-cost clients devices in the market not supporting 80MHz ac Standard. Ist a good compromise between optimal throughput, connectivity and stability. Using “Fixed 80MHz BW” will leave only 4 non overlapping a-band channels which is a Problem in case of DFS Triggers (no alternative channels possible if 3 or more APs Close together are used) or strong Rogue AP´s.
    “Best BW”: the DCA will select the BW according to the capabilities of associated clients. If you have e.g. 95% ac clients it will go to 80MHz BW. If you have mixed clients (n+ac) it will choose 40MHz. If there are legacy clients supporting only 20MHz DCA will choose 20MHz which will decrease your throughput seriously. From our test experience we would recommend this only for very limited use scenarious.
    FCC and ETSI will soon implement further WiFi channels in 5GHz band. Eg. channels 68 – 94, 144, 169 – 173 and even above this. So in future the usage of 80MHz BW may be better justifiable.

    rgds. RadioX

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