Subject: [manet] Info about 802.11e

From: fxbaloch <[email-masked]>
Date: Mon, 20 Mar 2006 18:52:42 -0600

Hello All,

I am trying to understand the behaviour of 802.11e. I understand what is the 
purpose of the 802.11e and how the queues work in that environment. My 
questions is as follows,

1.After a packet arrives in lets say Q1 (highest) priority and there is also a 
packet in Q2 (lowest) then offcourse Q1's packet will get precedence over Q2 
and as soon node gets CTS from receiver it will send the Q1's packet. If now 
more packets come in Q1 (Q2 already has a packet waiting), does the node send 
RTS for the new packet or for Q2's packet. If Q2 is not given a chance does 
this means that lower priority packets will starve?

Thanks
Fariha Baloch

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From: Ali Hamidian <[email-masked]>
Date: Tue, 21 Mar 2006 10:31:33 +0100

Hi,

>packet in Q2 (lowest) then offcourse Q1's packet will get precedence over Q2 
(I guess you're talking about EDCA here.)

No. Sometimes it happens that Q2 sends its packet before Q1. Why? 
Because the queues wait a random number of slots (backoff time) equal to 
(assuming e.g. 802.11b PHY):
Q1: backoff time = 2 + random[0,7],  (CWmax = 15)
Q2: backoff time = 7 + random[0,31], (CWmax = 1023)

So although most of the times the random backoff time of Q1 is lower 
than the random backoff time of Q2, sometimes it can be the contrary. 
Therefore, the lower priority packets will not starve.

Kind regards
Ali