Wireless Basics: Coverage Hole Detection and Mitigation (CHDM)

Coverage Hole Detection and Mitigation (CHDM)

Cisco APs will send an alarm to the controller if low RSSI levels are detected on clients. A client's signal coverage is continually poor in an area where there is no viable area for it to roam to, or a client will remain stuck at a weak signal AP due to lack of roaming capability.

In a WLAN with coverage holes enabled by default, pre-coverage and coverage hole alerts are still sent to the controller, however mitigation of the coverage holes is not taken.

For the Guest access WLAN, Cisco recommends disabling this feature because of the open nature of the users and their mobility. This can cause an AP radio reset due to the detection of coverage holes, so Cisco recommends disabling this feature for all WLANs that have real-time traffic (VoIP, Live video).

Fig 1.1- Coverage Hole Detection and Mitigation (CHDM)

Coverage Hole detection and Mitigation is highly tunable with the exception of the thresholds, the default settings are generally sufficient. Minimum client count and coverage exception work together and the default count of 3 clients with a coverage exception of 25% says for example that if 3 clients are below the threshold – in order to act there must be 12 clients currently associated (3=25% of 12). 

CHDM also listens for clients on every AP in order to determine if a failed client is really in a coverage whole, or if it is simply not roaming. In the event that we can hear a client from an AP better than the one it is currently associated too, this will be counted as a false positive and not count towards a coverage hole event. 

If both conditions are met, Coverage Hole Mitigation can increase the AP’s power by one power level to attempt to fix the coverage. RRM will then re-evaluate coverage requirements on the next DCA and TPC run.

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