for {set i 1} {$i < 21} {incr i} {

    set udpAgent($i) [new Agent/UDP]
    $ns attach-agent $W1 $udpAgent($i)
    set udpSink($i) [new Agent/Null]
    $ns attach-agent $node($i) $udpSink($i)
    $ns connect $udpAgent($i) $udpSink($i)
    set cbr($i) [new Application/Traffic/CBR]
    $cbr($i) set packetSize_ 500
    $cbr($i) set interval_ 0.005
    $cbr($i) attach-agent $udpAgent($i)
}
#code for the end
for {set i 1} {$i < 21} {incr i} {
    $ns at 5.0 "$cbr($i)
}