Subject: [manet] Non-zero NAV frame transmission in 802.11

From: "Ehsan Hamadani" <[email-masked]>
Date: Wed, 29 Mar 2006 16:06:07 +0100

Dear all.
 
As part of my simulation results with 802.11 in ad hoc networks, I have
come across the following scenario which looks odd regarding my
understanding of 802.11 DCF channel access algorithm.
 
Consider the following scenario where node A has a packet larger than
RTS_Threshold for B and node D has a packet smaller than RTS_Threshold
for C. 
 
A-->B-->C<--D 
 
In this case, node A first starts sending a RTS packet towards B, and
consequently receives a CTS back from node B. Once node A initiates its
data transmission, node D also starts its own data transmission towards
C (without any prior channel reservation). Despite the fact that node C
has been made silent for the duration of A-->B data transmission, node C
will issue an link layer ACK after it successfully receive the packet
from node D !! In other words, node C sends a link layer ACK regardless
of the its non-zero NAV value. 
 
Is this an expected 802.11 behavior in the case of link layer
acknowledgment? If so, what is the idea behind it considering the
collision it causes to A-->B transmission?
 
Thanks
 
Ehsan
 
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