Subject: [manet] A question about the conditional collision probability in saturation model of IEEE 802.11 DCF

From: Hang Su <[email-masked]>
Date: Sat, 13 Dec 2003 21:38:34 +0800

hi  all,

In the paper 
[1]Giuseppe Bianchi and Ilenia Tinnirello
"Kalman Filter Estimation of the Number of Competing Terminals in an IEEE 802.11 network"
Infocom 2003

the conditional collision probabiltiy p can be obtained via the Eqution (4) 
p=(C_busy + C_coll) / B

Here the C_busy is the busy slot that is occupyed by the other nodes and the 
C_coll is the collision slot in which the channel is occupyed by more than one nodes.

To approve the saturation model, I prepare some simulations through GlomoSim.

The parameters in simulation is , 10 nodes in an area (100m x 100m). The 
transmission range of each node is 250m. That is to say each node can be reached 
through one hop. The network is in  saturation state ,in which the tx queue of every node
is assured to have at least one packet to send. DSSS PHY is adopted (Slot time is 20us).

The simulation is 50 second.

Below was the simulation result.

special node 0, total:     2500000, idle:       45883, recv:     2071694, tx:      219920, sense      162503
special node 1, total:     2500000, idle:       45530, recv:     1981407, tx:      325007, sense      148056
special node 2, total:     2500000, idle:       45515, recv:     2008268, tx:      285465, sense      160752
special node 3, total:     2500000, idle:       45609, recv:     1996199, tx:      272473, sense      185719
special node 4, total:     2500000, idle:       45627, recv:     2032096, tx:      271156, sense      151121
special node 5, total:     2500000, idle:       45633, recv:     2063502, tx:      227049, sense      163816
special node 6, total:     2500000, idle:       45723, recv:     1998867, tx:      274952, sense      180458
special node 7, total:     2500000, idle:       45701, recv:     2023372, tx:      256598, sense      174329
special node 8, total:     2500000, idle:       45557, recv:     2012076, tx:      276791, sense      165576
special node 9, total:     2500000, idle:       45647, recv:     1988906, tx:      284107, sense      181340

"total" means the total number of slot times, and so on.

According to the equation (4) in [1], p=(recv+sense)/total. For example, the p of node 0 is 
(2071694+162503)/25e5=0.8937. Obviously, the value of p obtained here is wrong.

Where did I make mistakes???

I would really appreciate any hints!

-- 

Hang Su.

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