In an example of multi-user wireless communications, an access point may send a downlink frame to a first station. The first station may generate and transmit an uplink frame. A second station in a same or different basic service set coverage area as that of the access point performs a clear channel assessment of a medium associated with the station for transmitting a frame without interrupting a frame exchange between the access point and the first station. The second station receives the downlink frame and the uplink frame of the frame exchange. The second station determines a signal measurement of the uplink frame, and determines a status of the medium based on the signal measurement and the downlink frame. The first station may update or ignore a network allocation vector timer based on the status of the medium. Other methods, apparatus, and computer-readable media are also disclosed.