A method of determining whether a wireless communication medium is clear is carried out on a client device, and includes: associating with a first virtual access point (AP) of a plurality of virtual APs implemented by a physical AP; receiving, from a physical AP, a message identifying the basic service set (BSS) color of each of the plurality of virtual APs implemented by on the physical AP; detecting a packet data unit; measuring the energy of the packet data unit; decoding a BSS color from the packet data unit; if the decoded BSS color is the same as a BSS color in the message from the physical AP, then setting an energy threshold to a first level; if the decoded BSS color is not the same as any BSS color in the message from the physical AP, then setting an energy threshold to a second level, wherein the second level is higher than the first level; and transmitting or refraining from transmitting a packet data unit based on a comparison of the measured energy and the energy threshold.