The Network Allocation Vector (NAV) is a timer in WLANs according to 802.11. It is generated by a WLAN station when it detects by means of carrier sensing that the medium is occupied. The NAV vector is decremented independently of the state of the transmission medium and reduces possible collisions.
The NAV timer operating on the Medium Access Control( MAC) indicates the time the WLAN is expected to be busy. The NAV vector is supported by the stations operating with Prioritized Channel Access( PCA). During the NAV time, no transmission attempts are made by the connected stations, regardless of whether the radio medium is free or busy. This significantly reduces collisions in the CSMA/ CA access procedure.