NS-2 The first part: pre-definition

1.建立一個模擬器物件 (Must)
   set ns [new Simulator]

2.開啟一個nam trace 檔案 (Option)
   set nf [open out.nam w]
   ns namtrace-all $nf

3.宣告一個finish程序(Must)
   proc finish{}
   {
        global ns nf    #設nf為全域變數
        $ns flush-trace
        #close the trace file
        close $nf
        #execute nam on the trace file
        exec nam out.nam &
        exit 0
   }

data source: http://www.scribd.com/doc/76224756/Network-Simulator-NS-2