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Hi, I am (was?) a bit confused about the meaning of the Duration/ID field in the MAC header. I think I figured it out in the course of writing this post, but I would welcome any confirmation or clarification or comments you may have.

In the CWAP study guide, it states;

"If the field contains a Duration value, the listening station will set its NAV timer to this value. The listening station will then use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0."

and

"To summarize, the value of the Duration/ID field indicates how long the RF medium will be busy before another station can contend for the medium."

The CWNA study guide also says this;

"If the field contains a Duration value, the listening station will set its NAV timer to this value. The listening station will then use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0."

These statements (and others in such places as Matthew Gasts 802.11 book), led me to believe that the duration/NAV are used to set the total time the WLAN media will be busy (from the time at which the duration value is received).

Of course this interpretaton was inconsistant with other statements in the study guides such as;
"The Duration/ID value represents the time, in microseconds, that is required to transmit the ACK plus one SIFS interval"
Hence my confusion....

Upon further investigation, I now believe that the Duration/NAV are used to set the time the media will be busy after the current frame has completed transmission.

My assumptions based on this works are;
-- station A transmits a non-QoS data frame containing a Duration value less than 32,767
-- Non transmitting stations
  -- perform physical carrier sense to detect this frame
  -- determine its duration value
  -- set their own NAV with the duration value received
  -- continue to perform physical carrier sense to detect when this frame is finished (no carrier sensed)
  -- Then they use their NAV (obtained from the previous frame) to wait an additional period (allowing SIFS/ACK or fragmented frames to transmit)
-- Once all the above has occurred, then any station with a frame ready to send begin their random backoff timer to gain access to the media

Is my understanding correct? how would you improve this description? am I missing anything?

Am I the only one who finds it confusing that it is not explicitly mentioned that the NAV timer is used only **after** the media is free (frame has completed transmission/physical carrier sense detects no frame)?

I also note that the Phy header contains a Length field that is used to indicate "microseconds to transmit the PSDU"; does this value get used as part of this media access mechanism? Or is just physical carrier sense used?

Thanks
Bruce.

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P.S. The following sections in the 2012 802.11 standard helped me figure this out (FYI);

8.2.4.2 Duration/ID field
<<snip>>
c) In all other frames sent by non-QoS STAs and control frames sent by QoS STAs, the Duration/ID field contains a duration value as defined for each frame type in 8.3.
<<snip>>
NOTE—The reference point for the Duration/ID field is the end of the PPDU carrying the MPDU.
<<snip>>

8.3.2.1 Data frame format
<<snip>>
Within all data frames sent during the CP by non-QoS STAs, the Duration/ID field is set according to the following rules:
— If the Address 1 field contains a group address, the duration value is set to 0.
— If the More Fragments bit is 0 in the Frame Control field of a frame and the Address 1 field contains an individual address, the duration value is set to the time, in microseconds, required to transmit one ACK frame, plus one SIFS interval.
— If the More Fragments bit is 1 in the Frame Control field of a frame and the Address 1 field contains an individual address, the duration value is set to the time, in microseconds, required to transmit the next fragment of this data frame, plus two ACK frames, plus three SIFS intervals.

Have you read Marcus's white paper here ? :

http://www.cwnp.com/wp-content/uploads/pdf/802.11_arbitration.pdf

Are Qos specifics complicating your understanding?   If so, ignore QoS until after you understand the more vanilla transactions.

I would agree with Howard to fully understand DCF, then move on to QoS/EDCA. 

Also, take a look at the 802.11-2007 standard also (you can also use the 2012;  I used the 2007 standard in prepping for CWAP exam as it was the one listed in the exam objectives).  Section 9.2.5.4 discusses setting and resetting the NAV (example given for RTS/CTS) and this diagram helped me understand the process better. If you are studying for the CWAP, be sure to make the 802.11-2007 standard your friend and reference it regularly.