# Create a new simulator instance
set ns [new Simulator]
 
# Open NAM trace file for network animation
set nf [open lab1.nam w]
$ns namtrace-all $nf
 
# Open trace file for analysis
set tf [open lab1.tr w]
$ns trace-all $tf
 
# Define finish procedure
proc finish {} {
    global ns nf tf
    $ns flush-trace
    close $nf
    close $tf
    exec nam lab1.nam &
    exit 0
}
 
# Create 4 nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
 
# Create duplex links between nodes
$ns duplex-link $n0 $n2 200Mb 10ms DropTail
$ns duplex-link $n1 $n2 100Mb 5ms DropTail
$ns duplex-link $n2 $n3 1Mb 1000ms DropTail
 
# Set queue limits
$ns queue-limit $n0 $n2 10
$ns queue-limit $n1 $n2 10
 
# Create UDP agents
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0
 
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1
 
set udp2 [new Agent/UDP]
$ns attach-agent $n2 $udp2
 
# Create CBR traffic generators
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0
 
set cbr1 [new Application/Traffic/CBR]
$cbr1 attach-agent $udp1
 
set cbr2 [new Application/Traffic/CBR]
$cbr2 attach-agent $udp2
 
# Create null agent (sink)
set null0 [new Agent/Null]
$ns attach-agent $n3 $null0
 
# Connect UDP agents to null agent
$ns connect $udp0 $null0
$ns connect $udp1 $null0
 
# Schedule events
$ns at 0.1 "$cbr0 start"
$ns at 0.2 "$cbr1 start"
$ns at 1.0 "finish"
 
# Start simulation
$ns run

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