The Duration / ID field is 16 bits in length . The Duration ID field can be used for three difference reasons.
- Virtual Carrier Sense: The main purpose of this field is to rest the NAV timer of other stations.
- Legacy power management: PS-Poll frames use the field as an association identifier (AID)
- Contention-free period: Point coordination function (PCF) process has begun
Above the picture from CWAP
Virtual Carrier Sense uses a timer mechanism known as the Network Allocation Vector (NAV).
As you con see the following picture, when the receiver hears a frame transmission from the sender, it looks at the header of the frame and determines whether the Duration/ID field contains a Duration value or an ID value. If the field contains a Duration value, the listening station will set its NAV timer to this value. The receiver will then use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0.
Above the picture from CWAP
The second way for Duration/ID field is used for power management process. The access point uses this AID to keep track of the stations that are associated and the members of the BSS. If
the access point is buffering data for a station in Power Save mode, when the access point
transmits its next beacon, the AID of the station will be seen in a field of the beacon frame
known as the traffic indication map (TIM). The TIM field is a list of all stations that have
undelivered unicast data buffered on the access point waiting to be delivered. The following picture shows that PS-Poll frame the Duration/ID field will now be used as an AID value.
Above the picture from CWAP
Above the picture from CWAP
As you can see that 2^15- 1 == 32767, 2^14 -1 == 16383 and from 2008 - 16383 are reserved
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