NS simulation: scheduling events (examples inside)

NS simulation: scheduling events

 

• Simulation time

   

  • A similation system (such as NS) must have a built-inSimulation Clock-It represents the "Clock" in the real world.

     

  • You canExamine(Read) the value of the Simulation Clock by usingNowCommand inSimulatorClass

     

  • Example::

     

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

     

   

   

• Events

   

  • AnEventIs an action made by a simulation entity

     

  • An event (action) in a simulation is representedOTCL command!!!

     

  • Each event happens at a certainTimeAnd therefore, each event has an associatedEvent TimeWhich is the time when the event will happen

     

  • In simulation,EventsAreScheduled(To happen at some ure time)

     

  • To schedule an event, useAtCommand inSimulatorClass

     

  • 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 thatPoint 3Happens beforePoint 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"InChronological order

   

   

• Running the NS simulation

   

  • TheRunCommand inSimulationClass is used to run the network simuation.

     

  • Example:

     

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

     

   

   

• Stopping an NS simulation

   

  • There is no"Stop"Command available.

     

  • To stop a running simulation, you mustScheduleATerminationCommandBeforeYou start running the simulation.

     

  • TheTerminationCommand is usuallyTCL 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

     

   

• 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

     

     

  • Example program:(Demo abve code)

     

    • Prog file: click here

     

  • Output when you run this simulation:

     

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

     

   

   

• 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

     

     

  • Example program:(Demo abve code)

     

    • Prog file: click here

   

   

   

• Problems with the above Simulation

   

  • It simulates alright...

     

  • ButWithout producing any dataThat we can examine !!!

     

  • What we still need to learn is howAdd commandsIn the simulation programOutput State VariablesThat we are interested in !!!

     

Http://www.mathcs.emory.edu /~ Cheung/courses/558-old/syllabus/90-ns/2-ns-prog/events.html

NS simulation: scheduling events (examples inside)