CSMA/CA NAV/Duration field and random backoff timer - Process

Question

Hi guys,
Gotta a quick one for you.
Pls see text below
As a condition to accessing the medium, the MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the amount of time that the previous frame needs to send its frame. The NAV must be zero before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame's length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. This process reserves the medium for the sending station.
An important aspect of the DCF is a random back off timer that a station uses if it detects a busy medium. If the channel is in use, the station must wait a random period of time before attempting to access the medium again. This ensures that multiple stations wanting to send data don't transmit at the same time. The random delay causes stations to wait different periods of time and avoids all of them sensing the medium at exactly the same time, finding the channel idle, transmitting, and colliding with each other. The back off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active users increases.
So, if all clients in a WLAN see a frame come from station A, and station A has set the duration field, all other stations set their NAV and wait until it expires.
THEN, i would expect all stations to start to contend for the medium at exactly the same time.
BUT you have the random backoff timer.
The question is does the NIC add a random number to the NAV field, ie, once it sees station As duration field value (lets say 300us) it adds 47us to this and the NAV is 347us? -- or do all stations decrement the NAV field to zero and then start a random backoff timer?
Many thx for the help,
Kind regards,
Ken

Answer 1

Sorry chaps, but I think you've got this wrong...
The whole point of a NAV being transmitted by the TX & RX ends is to stop hidden node issues - with both ends transmitting the NAV, you are guaranteeing that every 802.11 device within range of either end of the transaction is quiet for the duration of the NAV.
Lets say we have a TX (A) and an RX (B), and another device (C). Lets say that (A) can hear (B), and that (B) can hear (A) and (C). (A) and (C) cannot hear each other.
(A) <<>> (B) <<>> (C)
Applying that to your scenario...
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it now counts down to 310, it checks the air, and sees the same packet still being transmitted
309, 308, 307 .............
gets down to 12us, checks the ait, NOW DOES NOT see a packet being transmitted.
you count down to 11, it check the air, you donot see a packet being transmitted.
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The part where it jumps down from 312 to 11 is wrong. Just because (C) can't hear anything, doesn't mean that (B) isn't receiving something from (A).
What actually happens with NAV's is;
[Lets say (A) is transmitting to (B)]
1. (A) transmits NAV, value = "NAV-A"
2. (B) receives packet, along with everything else in range of (A), on the same channel as (A)
3. (B) responds to (A) with "NAV-B". "NAV B" = "NAV-A" [minus] "Elapsed Time"
4. (A) receives packet, along with everything in range of (B), on the same channel as (B)
At this point, every device on that channel will be silent for the duration of the NAV, regardless of whather or not they hear anything during the NAV countdown duration.
When the NAV expires, the Random Backoff Timer begins, and the lowest/ quickest client to get to zero then gets to transmit, and the whole process starts again.
Let me know what you think, but I'm pretty sure that's how it actually works.
Rgds,
Richard