LinuxQuestions.org - tcl code for ns2 simulation

hiiii..i am trying to write a code in ns2 where 2 nodes act as access point and a station and exchange of frames take place between them one after the other.Frames like probe req response authentication and association.But for the code i have written the exchange is takin place simultaneously.Below is my code.Can u plz help me out such that frames are transferred one after the other.
Thanks in advance.

#Creating a new Simulator object
set ns [new Simulator]

#Opening file to be used as nam trace file
set nf [open shot.nam w]
$ns namtrace-all $nf

#Finish procedure
proc finish {} {
global ns nf
$ns flush-trace
close $nf
exec nam shot.nam &
exit 0
}

#Defining two nodes
set n0 [$ns node]
set n1 [$ns node]

#Connecting two nodes
$ns duplex-link $n0 $n1 1Mb 10ms DropTail

#CreateUDP agent and connect to node0
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0

#Create CBR traffic source and attach to UDP agent
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 1
$cbr0 attach-agent $udp0

set null0 [new Agent/Null]
$ns attach-agent $n1 $null0

$ns connect $udp0 $null0

#CreateUDP agent and connect to node1
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1

#Create CBR traffic source and attach to UDP agent
set cbr1 [new Application/Traffic/CBR]
$cbr1 set packetSize_ 500
$cbr1 set interval_ 1
$cbr1 attach-agent $udp1

set null1 [new Agent/Null]
$ns attach-agent $n0 $null1

$ns connect $udp1 $null1

for {set i 0} {$i < 10} {incr i} {

set q [expr $i%2]
if {$q==0} {
set a i
set b [expr $i+1]
$ns at a "$cbr0 start"
$ns at b "$cbr0 stop"
}
if {$q!=0} {
set a i
set b [expr $i+1]
$ns at a "$cbr1 start"
$ns at b "$cbr1 stop"
}
}

#Stop after 10 seconds
$ns at 10.0 "finish"

#Running simulation
$ns run