A channel access timer at a communication device is set to a duration for which a shared communication medium is expected to be busy in a basic service set (BSS) heard by the communication device. The channel access timer is counted down. Before the channel access timer reaches zero, respective indications of respective contention free period durations in multiple basic service sets (BSSs) heard by the communication device are recorded in a BSS record maintained by the communication device. Recording the respective indications includes recording, for each particular BSS of the multiple BSSs, (i) an identifier of the particular BSS and (ii) an offset value corresponding to an offset of an end of a contention free period in the particular BSS from an end of a contention free period in the BSS based on which the channel access timer is currently set at the communication device.