Carrier-Sensing Functions and the Network Allocation Vector

用于CSMACA的机制

 

Two types of carrier-sensing functions in 802.11 manage this process:

 the physical carrier-sensing and virtual carrier-sensing functions.

 

 

Virtual carrier-sensing is provided by the Network Allocation Vector (NAV). Most 802.11 frames carry a duration field, which can be used to reserve the medium for a fixed time period. The NAV is a timer that indicates the amount of time the medium will be reserved, in microseconds. Stations set the NAV to the time for which they expect to use the medium, including any frames necessary to complete the current operation. Other stations count down from the NAV to 0. When the NAV is nonzero, the virtual carrier-sensing function indicates that the medium is busy; when the NAV reaches 0, the virtual carrier-sensing function indicates that the medium is idle.

 

在RTS/CTS frame里面就有NAV。