Wi-Fi is half duplex, only one device can communicate on the physical medium at any given time. Ie if a client hears another client it will wait until it is clear to send its own packets on the air. This is true for lower rates with newer features in 802.11ac/ax aiming to reduce this overhead but may still use management and control frames in a way that is half duplex.
High utilization can be caused by a few things:
Supporting low basic rate speeds.
Multiple APs on the same channel interfere with each other.
Multiple SSIDs on the same channel.
AP transmit power is too high.
Too many rogue APs on the network using up the channel time.
High client traffic.
Non-802.11 sources such as microwave ovens, or 5Ghz wireless camera systems.
Measuring channel utilization
Pre-6Ghz Sensors
Older sensors report channel utilization seen during clear channel assessment (CCA) which is constantly run before the sensor TX's any frame. The total time the medium is busy is divided by the total time we used the medium. Where busy is defined by the energy detection level of the specific Wi-Fi driver in use, along with signal detect energy level (For preamble packets).
More information can be found in the IEEE802.11-2016 amendment which defines the physical carrier sense mechanism.
Note: Different UXI devices have different receive sensitivities and different vendor Wi-Fi silicon chipsets. As a result, the channel utilization measurement across UX-F, UX-5x, and UX-6x may differ slightly.
6Ghz Sensors
UX-6Ex Sensors that support Wi-Fi 6E have further information available on channel utilization. As above our baseline measure is a percentage of time the medium is busy with energy and in which we would not be able to transmit. However, there is further information on the type of energy driving the channel utilization.
In-BSSID
Measures only the channel usage caused by frames between the BSSID we are currently connected to and testing and other clients. This usage metric shows you direct BSSID usage and contention.
Other-BSSID
Measures channel usage caused by frames between other BSSIDs to which we are not currently connected to, which are contending for airtime with the sensor. This can be seen as a measure of co-channel interference.
Non-Wi-Fi(NoPKT)
Measures energy that has triggered the carrier sense and caused the device to not be able to transmit. This is typically Non 802.11 energy. It could be caused by severely malformed packet collisions*
Note: These measures may not sum up to the top-level value as we do not break out the measurement for frames that have only partially been decoded but are still recognized as 802.11 frames. This could be from frame collisions, a bad Cyclic Redundancy Check (CRC), or a bad Frame Check Sequence (FCS).% Usage by Malformed 802.11 packets = Channel utilization - (InBSSID + OtherBSSID + NonWi-Fi(NoPKT)). The extra channel utilization is coming from "uncategorized traffic," which is utilization contributed by the sensor itself.
There is the sensor's contribution (which is small) and uncategorized traffic. The two radios are not limited to listening / monitoring and operation. Both are used in operation, and the measured channel utilization is determined by the radio that is transmitting (TX) and receiving (RX) on the BSSID being tested by the sensor. The two radios cannot operate on the same band (let alone the same channel,) so if one radio is testing a BSSID, the second radio cannot monitor that. The monitored stats come from our proprietary monitor mode implementation, and the uncategorized traffic is not from the sensor itself. It's just Wi-Fi frames that the radio itself cannot categorize.