[ns] [NS2]What is meaning in Ns-default.tcl (tcl\lib):LL set delay_

[¤終於aware]

Hi,
     What is  meaning in Ns-default.tcl (tcl\lib):LL set delay_                 
  25us? I analyzed the trace file and found that the difference value between 
the time when a packet is receive on MAC and the time when the packet is 
forwarded on RTR.
      Looking forward to your reply,thanks
  
 yours,
 Bruce