Wireless Basics : Dynamic Channel Assignment (DCA)

Dynamic Channel Assignment (DCA)

Dynamic Channel Assignment is responsible for monitoring the spectrum and choosing the best channel plan to place the AP’s on. 

Interference is the primary concern, the less interference there is the more bandwidth (airtime) we can use. To do this DCA monitors there are four parameters and these parameters are as below

  • Signal: Any Wi-Fi signal created by my network/RF Group
  • Noise: Any RF signal that is not identified as Wi-Fi; this includes collisions and packets to low to be demodulated as well.
  • Interference: Any Wi-Fi signal that is from Rogue devices or devices not part of my RF Group
  • Load: The relative channel utilization of AP’s in the RF Group

Suggested Parameters for DCA : 802.11a/802.11b/g 

  • Channel Assignment Method: automatic
  • Interval: 8 Hours
  • Anchor Time: 0
  • Avoid Foreign AP Interference : Enabled
  • Avoid Cisco AP Load : Disabled
  • Avoid non-802.11a/b/g Noise : Enabled
  • Avoid Persistent Non-WiFi Interference : Enabled (requires CleanAir enabled)
  • DCA Channel Sensitivity : medium
  • Channel Width: 20Mhz
  • Avoid Check for non-DFS Channel: Disabled
  • DCA Channel List: Default
Fig 1.1-Wireless: Dynamic Channel Assignment (DCA)

Note : 20Mhz channels present the best possible use of the available spectrum. When channels are bonded the management traffic which forms up to
 75% of traffic on the air from neighbor AP2 primary channel which is a secondary channel on say AP1drives utilization of the bonded pair high enough to offset any reasonable use of the bonded channels. 

While the actual user data traffic is merely 25% typically, using 20Mhz wide channels provides for higher number of channels available to design many high carriage capacity cells.

Channels bonded together to provide a single 80Mhz frequency space reduces the number of 80MHz wide pairs available in the DCA channel plan for channel reuse which can be allocated to other APs. 

For e.g. 24 non-overlapping channels in 5Ghz when bonded together to create 80Mhz wide pairs result in 6 available 80Mhz wide channel-pairs. For floor plans with greater than 6 APs this will mean high overlap with 80Mhz channels being reused. 

From a channel reuse pattern and efficient spectrum utilization perspective it is highly recommended to stick to 20 (40Mhz where necessitated by use case) MHz wide RF channels.