The stations listening on the wireless medium read the Duration field and set their NAV, which is an indicator for a station on how long it must defer from accessing the medium.
The NAV may be thought of as a counter, which counts down to zero at a uniform rate.
The medium shall be determined to be busy when the station (STA) is transmitting.
A station decrements its NAV counter until it becomes zero, at which time it is awakened to sense the medium again.
The NAV virtual carrier sensing mechanism is a prominent part of the CSMA/CA MAC protocol used with IEEE 802.11 WLANs.