A network node may use carrier sensing to determine if a shared medium is available so as to avoid packet collision. In a wireless network, the two types of carrier sensing that are typically used are physical carrier sensing, by a physical layer sampling the radio frequency (RF) energy level in the medium, and virtual carrier sensing, by a media-access control layer that updates a network node's network allocation vector (NAV). A network node maintains its NAV to indicate the period(s) during which the medium is reserved by other network nodes; hence, it knows when NOT to transmit.
A network may have several service sets with network nodes communicatively coupled to other network nodes in the same service set by an RF channel. In crowded environments, more than one service set may be operating on the same channel in the same vicinity. If this is the case, a network node from one service set may pick up NAV information from a network node in a neighboring service set and be prevented from transmitting for a certain period. This may be the case even if communications from the two network nodes could take place simultaneously without interfering with one another.