Understanding Distributed Radio Resource Provisioning

To prevent interference between APs, the FortiOS WiFi Controller features Distributed Automatic Radio Resource Provisioning (DARRP). Through DARRP, each FortiAP unit autonomously and periodically determines the channel that is best suited for wireless communications. FortiAP units select their channel so that they do not interfere with each other in large-scale deployments where multiple access points have overlapping radio ranges. Channel selection is optimized by monitoring neighboring AP channels and by performing periodic background scans to collect signal strength.

DARRP has two phases:

• Channel Planing Phase
  • Sub-phase 1: Find channels to be excluded from consideration.
  • Sub-phase 2: If all channels are excluded during sub-phase 1, select a channel to use based on an assigned channel score.
• Channel Quality Monitoring Phase: The AP monitors the channel quality using monitor-period to check for TX and RX retries and errors. If the threshold is crossed, the AP changes channels as needed.

Channel Planning

Sub-phase 1

The AP first identifies channels with AP scanning and spectral scanning. The AP then excludes channels that exceed the following configured threshold values:

• threshold-ap 250
• threshold-noise-floor "-85"
• threshold-channel-load 60
• threshold-spectral-rssi "-65"

DARRP will also exclude the DFS channel and weather channel if they are disabled.

After excluding channels, channels are selected based on the following criteria:

• If there is only one channel left, that channel is picked.
• If there are multiple channels left, a random channel is picked.
• It there are no channels left, the AP Controller proceeds to sub-phase 2.

Sub-phase 2

If all channels are excluded after sub-phase 1, the AP Controller calculates a channel score and selects the channel with the lowest score. The channel score is based on a combination of the following factors:

channel_score = weight-managed-ap * rssi_score_managed_ap +
                weight-rogue-ap * rssi_score_rogue_ap +
                weight-noise-floor * noise_floor +
                weight-channel-load * channel load +
                weight-spectral-rssi * spectral RSSI +

The channel with the lowest score is then selected. If no channel is available, the AP disables the radio.

Channel Quality Monitoring

Once a channel is picked, the AP performance on that channel is periodically monitored by the AP and switched if required. If a channel switch occurs, the AP reports the new channel to the controller.

Channel quality is calculated with the following:

If (current tx-retries > threshold-tx-retries) or (current rx-errors > threshold-rx-errors), then the AP will select a new channel to use. This is similar to how channels are selected in Channel Planning sub-phase 2.

The current tx-retries and current rx-errors is averaged over the configured time under monitor-period.