802.11 Protocol Architecture
Duration Values
For example, a station transmitting a data frame would include a
duration value that includes the time needed for one SIFS and to
receive the acknowledgement frame from the receiving station.
Stations receive and monitor the duration field of all frames sent within
their own BSS and by nearby stations, which might not be part of their
BSS, on the same or overlapping channel. Stations place the duration
field contents in their network allocation vector (NAV). The NAV
operates like a timer, starting with a value equal to the duration field
value of the last frame transmission sensed on the medium and
counting down to zero. Once the NAV reaches zero, the station
proceeds with the physical medium access process of sensing the
wireless medium. Examples of the duration values carried by frames
are shown in figures 2.11 and 2.12 below.
FIGURE 2.11
Duration values, no fragmentation in use
FIGURE 2.12
Duration values, fragmentation in use
