[ns] Mac802_3 collision Simulation (fwd)

[Lakshmikantha C Pothula <chow@utdallas.edu>]

Hi,
  I work on Gigabit Ethernet simulations.So I was looking at Mac802_3
class and have a doubt on the way they simulate collisions. In the current
code, Each node waits for inter frame space after the collision occurs and
then resumes with its processing, But Packets which are involved in
Collision can come after inter frame space. Here is an example about what
I am talking. In the current implementaion, Propagation delay is same for
all nodes on lan.
Suppose There are 3 nodes(A,B,C) on a lan.
P=propagation delay
Node A places a pkt X on lan at time t and complets at t+2P
Node B places a pkt Y on lan at t+P-very small value and complets at
t+3P-very small value.
Node C places a pkt Z on lan at time t+P-very small value and complets at 
t+3P-very small value.
Node B & C recieve X from A at t+P. so Node B&C detect collision,back
off,  wait for inter frame space and then resume at t+P+inter frame
space.
Node A & B recieve the pkt z from C at t+P-very small value+P. So Node A
detects collision ,backs off and resumse processing at t+2P-very small
value+inter frame space.
Node B will accept the pkt  Z from C thought it was involved in collision
because it(Node B) resumed at t+P+inter frame space.
In the similar way Node C will accept the pkt Y from Node B at time
t+2P-very small value though it was invloved in Collision because Node C
also resumed at t+p+Inter frame space.
Node A will discard the pkt Y from Node B at time t+2p-very small value
because it is waiting for inter frame space from t+2p-very small value(
recpetion of pkt from Node c).

So Node B accepts pkt Z from Node C and Node C accepts pkt Y from Node B,
thought senders of these packets(B for Y and C for Z ) discard the packet.

Thanks
Lakshmikanth.