network simulation motivation: r learn fundamentals of evaluating network performance via simulation...
TRANSCRIPT
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Network Simulation
Motivation: learn fundamentals
of evaluating network performance via simulation
Overview: fundamentals of
discrete event simulation
analyzing simulation outputs
ns-2 simulation
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The evaluation spectrum
numericalmodels
simulation
emulation
prototype
operational system
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What is simulation?
system under study(has deterministic rules governing its behavior)
exogenous inputsto system
(the environment)
system boundary
observer
“real” life
computer programsimulates deterministic rules governing behavior
psuedo random inputsto system
(models environment)
program boundary
observer
“simulated” life
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Why Simulation?
goal: study system performance, operation real-system not available, is complex/costly or
dangerous (eg: space simulations, flight simulations)
quickly evaluate design alternatives (eg: different system configurations)
evaluate complex functions for which closed form formulas or numerical techniques not available
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Simulation: advantages/drawbacks
advantages:
drawbacks/dangers:
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Programming a simulationWhat ‘s in a simulation program? simulated time: internal (to simulation program)
variable that keeps track of simulated time system “state”: variables maintained by
simulation program define system “state” e.g., may track number (possibly order) of packets in
queue, current value of retransmission timer events: points in time when system changes state
each event has associated event time• e.g., arrival of packet to queue, departure from
queue• precisely at these points in time that simulation must
take action (change state and may cause new future events)
model for time between events (probabilistic) caused by external environment
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Discrete Event Simulation
simulation program maintains and updates list of future events: event list
simulator structure: initialize event list
get next (nearest future)event from event list
time = event time
process event(change state values, add/delete
future events from event list
update statistics
done?n
Need: well defined set of
events for each event:
simulated system action, updating of event list
y
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Simulation: example
packets arrive (avg. interrarrival time: 1/ ) to router (avg. execution time 1/) with two outgoing links
arriving packet joins link i with probability i
state of system: size of each queue system events:
job arrivals service time completions
define performance measure to be gathered
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Simulation: example
Simulator actions on arrival event choose a link
if link idle {place pkt in service, determine service time (random number drawn from service time distribution) add future event onto event list for pkt transfer completion, set number of pkts in queue to 1}
if buffer full {increment # dropped packets, ignore arrival}
else increment number in queue where queued
create event for next arrival (generate interarrival time) stick event on event list
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Simulation: example
Simulator actions on departure event remove event, update simulation time,
update performance statistics decrement counter of number of pkts in
queue If (number of jobs in queue > 0) put next pkt
into service – schedule completion event (generate service time for put)
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Simulation: example
Simulator initialization event list: state variables:
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Gathering Performance Statistics
avg delay at queue i: record Dij : delay of customer j at queue i. Let Ni be # customers passing through queue i
iii TN Average queue length at i:
iNtotal simulated time
throughput at queue i, i =
i
N
jij
i N
D
T
i
1
Little’s Law
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Analyzing Output Results
Each time we run a simulation, (using different random number streams), we will get different output results!
distribution of random numbersto be used during simulation(interarrival, service times)
random number sequence 1 simulation output results 1input output
random number sequence 2 simulation output results 2input output
random number sequence M simulation output results Minput output
… … … … … …
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Analyzing Output Results
W2,n: delay of nth departing customer from queue 2
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Analyzing Output Results
each run shows variation in customer delay
one run different from next
statistical characterization of delay must be made expected delay of n-
th customer behavior as n
approaches infinity average of n
customers
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Transient Behavior
simulation outputs that depend on initial condition (i.e., output value changes when initial conditions change) are called transient characteristics “early” part of simulation later part of simulation less dependent on initial
conditions
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Effect of initial conditions histogram of delay of 20th
customer, given initially empty (1000 runs)
histogram of delay of 20th customer, given non-empty conditions
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Steady state behavior output results may converge to limiting “steady state”
value if simulation run “long enough”
discard statistics gathered during transient phase, e.g., ignore first n0 measurements of delay at queue 2
avg delayof packets
[n, n+10]
0
0
nN
D
Ti
N
njij
i
i
pick n0 so statistic is “approximately the same” for different random number streams and remains same as n increases
avg of 5 simulations
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Confidence Intervals
run simulation: get estimate X1 as estimate of performance metrics of interest
repeat simulation M times (each with new set of random numbers), get X2, … XM – all different!
which of X1, … XM is “right”?
intuitively, average of M samples should be “better” than choosing any one of M samples
M
X
X
M
jj
1
How “confident”are we in X?
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Confidence Intervals Can not get perfect estimate of true mean, , with
finite # samples Look for bounds: find c1 and c2 such that:
Probability(c1 < < c2) = 1 – c1,c2confidence interval100(1-): confidence level
One approach for finding c1, c2 (suppose =.1) take k samples (e.g., k independent simulation
runs) sort find largest value is smallest 5% -> c1
find smallest value in largest 5% -> c2
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Confidence Intervals: Central Limit Thm
Central Limit Theorem: If samples X1, … XM independent and from same population with population mean and standard deviation then
M
X
X
M
jj
1
is approximately normally distributed with mean u and standard deviation
sample mean:
M
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Confidence Intervals .. more
Still don’t know population standard deviation. So we estimate it using sample (observed) standard deviation:
M
mmX XX
M 1
22 )(1
1
Given we can now find upper and lower tails of normal distributions containing 100% of the mass
2, XX
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Interpretation of Confidence Interval
If we calculate confidence intervals as in recipe, 95% of the confidence intervals thus computed will contain the true (unknown) population mean.
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Confidence Intervals .. the recipe
M
mmX XX
M 1
22 )(1
1
Given samples X1, … XM, (e.g., having repeated simulation M times), compute
M
X
X
M
jj
1
95% confidence interval:M
X X96.1
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Generating confidence intervals forsteady state measures independent replications with deletions method of batch means autoregressive method spectrum analysis regenerative method
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Independent replications
1. generate n independent replications with m samples, remove first l0 samples from each to obtain
2. calculate sample mean and variance from 3. use t-distribution to compute confidence
intervals4. can combine with sequential stopping rule to
obtain confidence interval of specified width.
),(,),,( 001 lmXlmX n ),( 0lmX i
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0ther methods
batch means: take single run, delete first l0 observations, divide remainder into n groups and obtain Xi for i-th, i = 1,…,n follow procedure for independent replications complication due to nonindependence of Xis
potential efficiency due to deletion of only l0 observation
autoregressive method: spectrum analysis: based on study of correlation of observations
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regenerative method: applicable to systems with regeneration points regeneration point future independent of
past can construct observations for intervals
between regeneration points that will be iid use of CLT provides confidence intervals
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Comparing two different systems
Example: want to compare mean response times of two queues where arrival process remains unchanged but speed of servers are different.
run each system n times to get {X1,j} and {X2,j} and take Zj = (X1,j ,X2,j) as the observations to determine confidence intervals for
note that no assumption is made regarding whether
{X1,j} and {X2,j} are independent of each other using the same streams to generate rvs for j-th runs
of both systems usually results in smaller sample variance of {Zj}
referred to as method of common random number
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ns-2, the network simulator
discrete event simulator modeling network
protocols wired, wireless, satellite TCP, UDP, multicast,
unicast web, telnet, ftp ad hoc, sensor nets infrastructure: stats,
tracing, error models, etc.
prepackaged protocols and modules, or create your own
Our goal: flavor of ns: simple
example, modification, execution and trace analysis
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“ns” components
ns, the simulator itself (this is all we’ll have time for)
nam, the Network AniMator visualize ns (or other) output GUI input simple ns scenarios
pre-processing: traffic and topology generators
post-processing: simple trace analysis, often in Awk, Perl, or Tcl
tutorial: http://www.isi.edu/nsnam/ns/tutorial/index.html ns by example: http://nile.wpi.edu/NS/