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,
That's exactly it Ken, and 5 for you too...
Just a last complement of information to make it perfect, machines count in slots, a slot can be 20 microseconds or 9 microseconds depending on if you use 802.11g/802.11a or 802.11b.
The rest of the process is exactly how you describe it. You pick up a number, say 16 (slots, so for example 320 us). countdown from that, checking the air at each slot.
Someone sends for 300 us, which is 15 slots, you add them and restart as you describe.
Thanks for getting to help people, the more we are, the clearer it all becomes!