NS Simulation: Scheduling Events (examples inside)

NS Simulation: Scheduling Events

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• Simulation time
  • A similation system (such as NS) must have a built-in simulation clock - it represents the "clock" in the real world.
  • You can examine (read) the value of the simulation clock by using the now command in the Simulator class
  • Example::

      set  ns  [new Simulator]
    
      $ns  now   // Returns the current simulation time   
    

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• Events
  • An event is an action made by a simulation entity
  • An event (action) in a simulation is represented a OTcl command !!!
  • Each event happens at a certain time and therefore, each event has an associated event time which is the time when the event will happen
  • In simulation, events are scheduled (to happen at some future time)
  • To schedule an event, use the at command in the Simulator class
  • Example::

      set  ns  [new Simulator]
    
      $ns  at  1.0  {puts "Point 1: Now = [$ns now]"}  
      $ns  at  8.0  {puts "Point 2: Now = [$ns now]"}
      $ns  at  4.0  {puts "Point 3: Now = [$ns now]"}
    
      $ns run  // Run simulation !
    

  • You will see the following output:

      Point 1: Now = 1
      Point 3: Now = 4      
      Point 2: Now = 8
    
    

    Notice that Point 3 happens before Point 2

  • This is what is happening:

      set  ns  [new Simulator]
    
      $ns  at  1.0  {puts "Point 1: ..."}  // Schedule event at time 1.0   
      $ns  at  8.0  {puts "Point 2: ..."}  // Schedule event at time 8.0   
      $ns  at  4.0  {puts "Point 3: ..."}  // Schedule event at time 4.0   
    
      $ns run  // Run simulation !
    

    So when the simulation is run, the events are "fired" in chronological order

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• Running the NS simulation
  • The run command in the Simulation class is used to run the network simuation.
  • Example:

        set  ns  [new Simulator]
    
        ... (set up simulation network)
    
        $ns run  // run simulation       
    

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• Stopping an NS simulation
  • There is no "stop" command available.
  • To stop a running simulation, you must schedule a termination command before you start running the simulation.
  • The termination command is usually a Tcl procedure (to wrap up things)
  • Example:

        set  ns  [new Simulator]
    
        #Define the 'finish' procedure
        proc finish {} {
           exit 0
        }
    
    
        ... (set up simulation network)
    
        
        #Set simulation end time
        $ns at 100.0 "finish"
    
        $ns run           // run simulation       
    

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• A simple simulation....
  • To give you a taste of simulation, here is a NS script that simulates 2 person "talking" to each other:

      proc person1 {x} {
        global ns
    
        puts "Person 1:"
        puts " Hey, $x, time is [$ns now], it's your turn to say something"    
    
        $ns  at  [expr [$ns now] + 0.4]  "$x person1"
      }
    
      proc person2 {x} {
        global ns
    
        puts "Person 2:"
        puts " Hey, $x, time is [$ns now], it's your turn to say something"
    
        $ns  at  [expr [$ns now] + 0.6]  "$x person2"
      }
    
      set  ns  [new Simulator]
    
      $ns  at  0  "person1 person2"
    
      $ns  at  4.5  "exit 0"
    
      $ns run
    
    

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  • Example Program: (Demo above code)                                                
    • Prog file: click here
  • Output when you run this simulation:

    Person 1:
     Hey, person2, time is 0, it's your turn to say something
    Person 2:
     Hey, person1, time is 0.4, it's your turn to say something  
    Person 1:
     Hey, person2, time is 1, it's your turn to say something
    Person 2:
     Hey, person1, time is 1.4, it's your turn to say something
    Person 1:
     Hey, person2, time is 2, it's your turn to say something
    Person 2:
     Hey, person1, time is 2.4, it's your turn to say something
    Person 1:
     Hey, person2, time is 3, it's your turn to say something
    Person 2:
     Hey, person1, time is 3.4, it's your turn to say something
    Person 1:
     Hey, person2, time is 4, it's your turn to say something
    Person 2:
     Hey, person1, time is 4.4, it's your turn to say something     
    

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• Example NS Simulation Script
  • We will construct an NS simulation script that simulate the following network:

    

  • The following figure is a break down of the NS components that make up the above network:

    

  • Here is the NS (OTcl) Script that creates the above simulation:

      #Make a NS simulator   
      set ns [new Simulator]	
    
      # Define a 'finish' procedure
      proc finish {} {
         exit 0
      }
    
      # Create the nodes:
      set n0 [$ns node]
      set n1 [$ns node]
      set n2 [$ns node]
      set n3 [$ns node]
      set n4 [$ns node]
      set n5 [$ns node]
    
      # Create the links:
      $ns duplex-link $n0 $n2   2Mb  10ms DropTail
      $ns duplex-link $n1 $n2   2Mb  10ms DropTail
      $ns duplex-link $n2 $n3 0.3Mb 200ms DropTail
      $ns duplex-link $n3 $n4 0.5Mb  40ms DropTail
      $ns duplex-link $n3 $n5 0.5Mb  30ms DropTail
    
      # Add a TCP sending module to node n0
      set tcp1 [new Agent/TCP/Reno]
      $ns attach-agent $n0 $tcp1
    
      # Add a TCP receiving module to node n4
      set sink1 [new Agent/TCPSink]
      $ns attach-agent $n4 $sink1
    
      # Direct traffic from "tcp1" to "sink1"
      $ns connect $tcp1 $sink1
    
      # Setup a FTP traffic generator on "tcp1"
      set ftp1 [new Application/FTP]
      $ftp1 attach-agent $tcp1
      $ftp1 set type_ FTP               (no necessary)
    
      # Schedule start/stop times
      $ns at 0.1   "$ftp1 start"
      $ns at 100.0 "$ftp1 stop"
    
      # Set simulation end time
      $ns at 125.0 "finish"		    (Will invoke "exit 0")   
    
      # Run simulation !!!!
      $ns run
    
    

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Problems with the above simulation
  • It simulates alright...
  • But without producing any data that we can examine !!!
  • What we still need to learn is how to add commands in the simulation program to output state variables that we are interested in !!!

 http://www.mathcs.emory.edu/~cheung/Courses/558-old/Syllabus/90-NS/2-NS-Prog/events.html

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