NAV(Network Alloction Vector)

NAV is a virtual carrier sensing mechanism. What does this mean ? I assume that you know what 'Carrier Sensing' mean. Carrier Sensing is a process that a WLAN station physically measure(sense) the energy level in a medium (channel) to figure out whether there is any communication already being done by other stations (Refer to CSMA/CA). In real CSCM/CA process, the station perform physical measurement to figure out whether the channel is available or not.

However there is a case where a station can figure out the availability of channel without performing the physical measurement.

How ?

Some of WLAN MAC Frame carries(e.g, RTS, CTS, ACK) the field called 'Duration'. If a station decode these frame and extract Duration field, it can figure out when the channel will be available without performing any physical measurement. Once it decoded this 'Duration' info, the station store the information into the internal configuration field so that it does not try carrier sensing for that period.

Simply put, NAV is a period during which a station should not transmit anything.

Following is an example showing how NAV is configured.

• i) Station (Src) is sending RTS frame to another station (Dest)
• ii) Station (Dest) detects the RTS from Station Src and send CTS frame to the Station Src.
• iii) All other stations around Src and Dest can detect the RTS and CTS between Src and Dest because these frames are broadcast in a shared medium and decode the information contained in RTS / CTS / ACK frame and contsruct NAV information.