A lightweight access point, in normal operational conditions, periodically goes off channel and scans another channel. This
is in order to perform RRM operations such as the following:
Transmitting and receiving NDP packets with other APs.
Detecting rogue APs and clients.
Measuring noise and interference.
During the off channel period, which normally is about 70 milliseconds, the AP is unable to transmit or receive data on its
serving channel. Therefore, there is a slight impact on its performance and some client transmissions might be dropped.
While the AP is sending and receiving important data, it is possible to configure off-channel scanning deferral so that the
AP does not go off channel and its normal operation is not impacted. You can configure off-channel scanning deferral on a
per-WLAN basis, per WMM UP class basis, with a specified time threshold in milliseconds. If the AP sends or receives, on a
particular WLAN, a data frame marked with the given UP class within the specified threshold, the AP defers its next RRM off-channel
scan. For example, by default, off-channel scanning deferral is enabled for UP classes 4, 5, and 6, with a time threshold
of 100 millseconds. Therefore, when RRM is about to perform an off-channel scan, a data frame marked with UP 4, 5, or 6 is
received within the last 100 milliseconds, RRM defers going off-channel. If a voice call, which is sending and receiving audio
samples, marked as UP 6, every 20 milliseconds is active, then the AP radio does not go off channel.
Off-channel scanning deferral does come with a tradeoff. Off-channel scanning can impact throughput by 2 percent or more,
depending on the configuration, traffic patterns, and so on. Throughput can be slightly improved if you enable off-channel
scanning deferral for all traffic classes and increase the time threshold. However, by not going off-channel, RRM can fail
to identify AP neighbors and rogues, resulting in negative impact to security, DCA, TPC, and 802.11k messages.
We recommend that you do not change the default off-channel scanning deferral settings.