1 measuring and explaining differences in wireless simulation models dheeraj reddy, george f. riley,...
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Measuring and Explaining Differences in Wireless Simulation Models
Dheeraj Reddy, George F. Riley,Yang Chen, Bryan Larish
Georgia Institute of Technology
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Outline
Introduction Simulation Experiments
Parameters Topology Description
Results and Discussion Conclusions
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Introduction
Discrete-Event Simulations Why wireless simulations ? GTNetS, ns-2 and GloMoSim IEEE 802.11 (MAC and PHY) Motivated by Cavin et. al (POMC 2002) Same specification but different results
Why is this important ?
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Introduction
Quantifiable differences Goals
Not to point correct/incorrectness Why wireless simulations will often
provide differing results Precautions when drawing conclusions
from simulations
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Simulation Experiments
Two experiments Concentration at MAC layer only Ideal Behavior Verification under
no contention Behavior during contention
resolution
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Simulation Experiments
Parameter Value
Basic Rate 2Mbps
Data Rate 11Mbps
Preamble Rate 1 Mbps
RTS Size 20 bytes
CTS Size 14 bytes
ACK Size 14 bytes
DIFS 50 us
SIFS 10 us
Slot time 20 us
UDP Header 8 bytes
IP Header 20 bytes
Parameter Value
LLC/SNAP Header 8 bytes
Preamble 24 bytes
Data Header 34 bytes
Payload 512 bytes
Forwarding delay 500 us
Initial CW 31 Slot times
Node Spacing 100 meters
Speed of Light 300 meters/us
Radio Range 250 meters
Hops per round 100
Number of Rounds 10
IEEE 802.11 parameters used in simulations
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Simulation Experiments
Experimental Methodology Simple forwarding protocol
Node n forwards packet to Node (n+1)%100
A round is finished when node 0 receives its packet from node 99
Experiment finishes at the end of 100 rounds
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Simulation Experiments
Simulation Network Topology
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Simulation Experiments
First Experiment Node 0 originates a packet Experiment finishes at the end of 100
rounds Deterministic results ?
There is contention even when a single packet is making rounds
500 us forwarding delay
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Simulation Experiments
802.11 M A C
IP /F o rw ard ing Layer
802.11 M A C
G lu e La y e r
R T S
C T S
D A T A
A C K
R T S
C T S
D A T A
A C K
Implementation artifact resulting in contention (all three simulators)
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Results and Discussions Pristine Simulators GloMoSim sends
control frames as well as data frames at the same rate
ns-2 uses ARP ns-2 and GloMoSim
ignore LLC/SNAP layer
Additional random delays in ns-2
Experiment 1 using default Simulation Parameters
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Results and Discussions
Theoretical Analysis DR=BR=11Mbps Deterministic results
Duration (usec)
Cumulative Time (usec)
RTS Rx Time 257 257
CTS Rx Time 213 470
Data Rx Time
620 1090
Forward Delay
500 1590
Per Round (100 hops)
0.1590 seconds
100 Rounds 15.90 seconds
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Results and Discussions
Set control frame rate as well as data frame rate to 11 Mbps
Ignore 1st round in ns-2 Adjust payload size in
ns-2 and GloMoSim to account for missing headers
Extra random delays removed in ns-2
Closely matches with theoretical analysis
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Simulation Experiments
Second Experiment Exercise the contention resolution
mechanisms All nodes create and send a packet to
their neighbor at time picked from [0,10ms)
100 packets contending for medium Significant packet loss Widely varying results
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Results and Discussions
Code Inspection and Testing Causes for differing behavior
Sample the contention window before/after incrementing when initiating a backoff
Contention window increment while sending back-to-back packets
Detecting a busy medium (VCS vs. PCS) Interface between MAC and higher layers
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Results and Discussions
100 simulation runs GTNetS and GloMoSim
have some overlap ns-2 takes longest to
finish its rounds Later rounds finish
significantly faster Data ignored when
packet originated by node 0 is dropped
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Results and Discussions
Backoff behavior (Contention Window Sampling)
Average number of rounds completed per run/100
Bin GTNetSGloMoSim
0-31 10287 21016
31-63 2719 3328
63-127 1840 1023
127-255 1325 510
255-511 867 274
511-1023 746 253
Simulator GTNetSGloMoSim
NS-2
Avg.Completions
78.00 62.18 56.97
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Results and Discussions
Complicated Scenarios still have considerable variations Analyses/Corrections needed to verify
if identified variations help bridge the differences
Undetermined variations ? Identify sources of differences and
effects of them in a big picture
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Conclusions
Simple Experiments eliminating effects of mobility, path-loss and modulation choices.
Simple scenarios have been fixed to provide identical results.
Complicated scenarios with channel contention still have considerable variation
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Questions ?