CWAP – MAC Header : Duration/ID

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Duration/ID field is 2 bytes(16bits) long field in the 802.11 MAC header.

CWAP-802.11 Overview-05Here is the 16 bit of Duration/ID fieldCWAP-DurationID-01In 802.11, Duration/ID field can be used for 3 different reason
1. Virtual Carrier Sense – This is the main purpose which used to reset the NAV timer of the other stations
2. Legacy Power ManagementPS Poll frames use this field as an association identifer (AID)
3. Contention-free Period – This field is used as an indicator that a point coordination function (PCF) process has begun.

Virtual Carrier Sense
– Virtual Carrier Sense use a timer mechanism known as the NAV (Network Allocation Vector)
– When listening STA hears a frame transmission by other STA, the listening STA will set its NAV timer to the value appear in transmitted frame.
– listening STA then will use the NAV as a count down timer, knowing that RF medium will be busy until it reach 0.
– When a client transmit a unicast frame, Duration/ID field use bit 0-14 (value 0 – 32767)
Duration/ID value represent time in μS (microseconds) that is required to transmit the ACK + one SIFS interval.
– Duration values are always about frame transmission that are to follow.
– The client who transmit a frame will calculate how long it will take to receive an ACK frame & include that in the duration field.
– The ACK frame follows the transmitted frame having duration value of 0.

Below shows a frame capture of a unicast frame & ACK followed by the transmission. As you can see duration value of 44 μS (time for a ACK+SIFS) for tx frame & ACK having 0 μS value.

CWAP-DurationID-02When this type of frame is transmitting in the RF all other receiving STA set their NAV to 44 μS

CWAP-DurationID-03- NAV is not updated when the receiver address (RA) is the same as receiving station’s MAC address.
– Duration value of the Tx STA does not reset the trasmitter’s NAV timer (as it cannot hear its own tx)
– Transmitter NAV will be zero after transmitting

PS-Poll in Legacy Power Management
– When a STA associate to an AP each STA will get a unique AID (Association Identifier)
– If AP buffering  data for a station in power save mode, when AP transmit its next beacon, the AID of station will be seen in TIM (Traffic Indicator Map)
– TIM field is a list of all stations that have undelivered unicast data buffered on the AP (DTIM is used for multicast buffered data)
– Client will send a PS-Poll frame to request that AP sends the unicast buffered frame to the STA.
In PS-Poll frame Duration/ID field use as an identifier (AID) &  not being used for duration or resetting NAV timers.
– When it use in PS-Poll frame, tow Most Significant Bits (MSB) set to 1 & AID value use rest of the 14 LSB(Least Significant Bits).
But allowable value for AID is 1 – 2007.

CWAP-DurationID-05Below shows a PS-Poll frame capture that shows the AID value (3 in my case) instead of Duration value.CWAP-DurationID-06Contention-free period in PCF
When duration field used in PCF, it used a fixed value of 32768 (bit 15 is 1 & all other bits set to 0)
– In all frames transmitted in Contention-free Period (CFP) have duration value of 32768.
– In HCF (Hybrid Coordination Function) Channel Access (HCCA) that require to use CFP.
– None of the vendors not implemented HCCA

Below table summarize the  duration ID field contents & highlighted the 3 main scenarios explained above (source page 90-91 CWAP study guide)

CWAP-DurationID-04Reference
1. CWAP Official Study Guide – Chapter 3

Related Posts

1. MAC Header : Frame Control
2. MAC Header : Addresses
3. MAC Header : QoS Control
4. MAC Header : Sequence Control

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