MSM7xx Controllers Configuration Guide v6.4.0

Mitigation of poor RF performance
RRM provides several features that help to mitigate wireless performance issues.
AP/radio down detection and mitigation
Each AP in the network maintains a list of neighboring APs, with information gathered from the
beacons it receives. These beacons may be received on the current operating channel and also
by scanning non-operating channels in both frequency bands (2.4 GHz or 5 GHz).
Each AP monitors the state of its neighbors to detect radio-down transitions. An AP only monitors
nearby neighbors, those whose beacons are received reliably and with a high RSSI (received
signal strength indicator).
When an AP stops receiving beacons from a nearby neighbor for a period of time, it informs the
controller. Subsequently, if the AP starts receiving beacons from that neighbor, it will inform the
controller that the neighbor radio is back.
The controller maintains a list of all neighbors for RRM purposes. Based on the radio-down
indications it receives, the controller analyzes the situation to determine if the radio-down (or
AP-down) condition is valid. Basically, the controller waits for indications from several neighboring
APs before deciding that an AP radio has failed. It then sends messages to neighbors of the failed
AP to mitigate the problem. Actions that might be taken include:
• Increase the power of neighbor APs (if any are operating at less than maximum power) to
cover the area that was serviced by the failed radio.
• Accept new client stations with below-normal RSSI so clients that were serviced by the failed
radio can reconnect.
The controller reports the radio-down condition as an alarm. Additional diagnostic information is
also logged.
This feature also detects channel changes by neighboring radios, and generates an event for each
occurrence. In these cases, no mitigation is needed because the radio is still operating.
Severe interference detection and mitigation
For RRM purposes, each AP radio maintains channel-quality information for all potential operating
channels. The information for the current operating channel is derived using performance statistics
(packet retry rates, error rates, per-client data-rates), beacons received from in-channel neighbor
APs, spectrum analysis samples, etc.
When an AP detects a severe degradation in the channel quality of the current operating channel
on a radio (that persists for tens of seconds), the AP informs the controller that it is experiencing
severe channel interference and wants to initiate a channel change. Before doing this, the AP does
an intensive spectrum analysis scan to identify the type of interference. This information is included
in the report to the controller.
The controller responds to the AP and provides it with a prioritized list of alternative channels that
are optimal from a system-wide perspective. The AP does a quick check of each channel in priority
order to verify that interference is not present. Assuming a new channel is usable, the AP initiates
a channel switch to the alternate channel. The AP then informs the controller that it has switched
channels.
After switching to an alternative channel, the AP continues to monitor the channel quality of the
non-operating channels. Eventually, it is expected that the interference will go away. (Most
interference sources are temporary.) At this point the AP informs the controller that the original
channel is clear. The controller then decides whether the AP should switch back to the original
channel or continue operating on the alternate channel. To configure this option, see “Severe
interference detection/mitigation” (page 97).
Mitigation of poor RF performance 183