Random backoff timing.

Unanswered Question
Nov 9th, 2008

Hi All,

Was going thro this post

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&topicID=.ee6e8c2&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cc12631

And iam confused. Have some queries.

1. When does the random backoff timer start?

a) whenever the station wants to transmit after the completion of its NAV

b) Starts when Station wants to transmit irrespective of the NAV, later NAV gets added to backoff timer some other station is transmitting before completion of its backoff timer to 0

2. Who sets this Random number for the backoff time?

3. Can two devices on the same WLAN set the same random backoff time?

4. In the scenario that two stations finish their NAVs and start their ramdom backoff time. Is it possible that they both set the same time? if nope... how is this acheived? if yes then how r colisions avoided in such a scenario?

5. the above post spoke of intervals like 20ns etc

Can someone tel me the intervals for 802.11 a, b , b and n explicitely?

6. When calculating NAV, does it consider a whole frame or if there is frangments to that frame. does it consider only 1 fragment.

meaning... will the other stations have to wait for the whole frame to be transmitted or if there is fragmentation of that frame/data, after the 1st fragment, he other clients will get a chance to transmit data?

Pl help. any inputs r welcome.

I have this problem too.
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tstanik Fri, 11/14/2008 - 14:18

When Both stations should have the same NAV values, and both will physically sense when the medium is idle. There is a high probability that both stations will attempt to transmit when the medium becomes idle, causing a collision. To avoid this situation, DCF uses a random backoff timer.

The random backoff algorithm randomly selects a value from 0 to the contention window (CW) value. The default CW values vary by vendor and are value-stored in the station NIC. The range of values for random backoff start at 0 slot times and increment up to the maximum value, which is a moving ceiling starting at CWmin and stopping at a maximum value known as CWmax. For the sake of this example, assume that the CWmin value begins at 7 and CWmax value is 255

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