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Intro to NS2: Scheduling Events

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• The NS Simulator
  • NS2:

    NS2 = network simulator 2 NS2 is the version 2 of a popular publicly available network simulator

    The NS2 system is:

    Written as C++ objects The C++ objects are then incorporated as: Extension to the Tcl programming language

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  • Therefore:

    The NS2 simulation program must be written as Tcl program (script)

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• Creating a Simulator object
  • Syntax to create a simulator object:

    set Variable [ new Simulator ]; Example: set ns [ new Simulator ]

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  • The simulator object contains:

    A virtual system (simulation) time variable called now A scheduler that execute "event" actions in the order of the virtual time A number of arrays of network objects that the scheduler will run based of their virtual time schedule

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• The simulation time
  • Simulation time:

    Simulation time = a variable in the Simulator object

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  • Syntax to read the simulation time:

    set ns [new Simulator] ; # Create a simulator object set curr_time [$ns now] ; # Returns the current simulation time

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• How is the simulation time updated ?
  • Fact:

    The scheduler has a list of objects that will be active at a given (simulation) time: The object (entity)) that is running (active) is always the object (entity) where: actionTime == simulation time When the current active object has performed its actions (e.g.: service a client): The object with the next smallest actionTime is run The simulation time is set to the action time of that object Example:

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• Events
  • Event:

    Event = an action performed by a simulation entity         Events are represented by (program) statements or functions = collection of statements)

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  • Events in NS:

    An event (action) in a NS simulation is a Tcl statement (command) or function call !!!

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• Event time
  • Event Time:

    Event time = the simulation time when the action will take place Each event (statement) has an event time

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  • Simulation and event time:

    The events (statements) are always executed in the order of their event time The scheduler in the simulation system is responsible for the ordered execution of events !!!

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• Schduling an event in NS
  • Syntax to schedule an event in NS2:

    set ns [new Simulator] $ns at Time Action

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  • Example::

    set ns [new Simulator]; # Create a simulator object puts "Line 1"; # Schedule 3 "events" $ns at 1.0 {puts "Event 1: Now = [$ns now]"}; # puts = put string $ns at 8.0 {puts "Event 2: Now = [$ns now]"}; $ns at 4.0 {puts "Event 3: Now = [$ns now]"}; puts "Line 2"; puts "Notice line 1 & 2 preceeds all events !!!" $ns run ; # Run simulation puts "Notice that event 3 preceeds event 2 !!!"

  • You will see the following output:

    Line 1 Line 2 Notice line 1 & 2 preceeds all events !!! Event 1: Now = 1 Event 3: Now = 4 Event 2: Now = 8 Notice that event 3 preceeds event 2 !!!

    Notice that:

    Event 3 occurs before the event 2 !!! Because the event time of event 3 is smaller !!!

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  • Example Program: (Demo above code)                                                

    cd /home/cs455000/demo/NS2/intro ns intro1.ns

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• Function as events
  • Example:

    set ns [new Simulator]; # Create a netw simulator object # ----------------------------------------- # Define 2 functions # ----------------------------------------- proc event {} { global ns; puts "[$ns now]: exec: event()"; } proc finish {} { global ns; puts "[$ns now]: exec: finish()"; } # ------------------------------------------- # Schedule function execution as events # ------------------------------------------- $ns at 1.0 "event" ; $ns at 5.0 "event" ; $ns at 20.0 "event" ; $ns at 10.0 "finish" ; $ns run ;

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  • Example Program: (Demo above code)                                                

    cd /home/cs455000/demo/NS2/intro ns intro2.ns

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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
  • Fact:

    There is no "stop" command available in NS

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  • How to stop a simulation program:

    Schedule a exit command

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  • Note:

    You must schedule the exit command before you run the simulation !!!!

    (Once you run the simulation, it's too late to schedule the exit command)

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  • Example:

    set ns [new Simulator] # ----------------------------------------- # The 'finish' procedure wraps things up # ----------------------------------------- proc finish {} { ... ... (print performance information gathered) ... exit 0 } ... # ------------------------------------------- # Schedule the wrapup action at time 100 # ------------------------------------------- $ns at 100.0 "finish" $ns run // run simulation

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• A "self-scheduling" example
  • Example:

    set ns [new Simulator]; # Create a netw simulator object # --------------------------------------------- # This function schedule itself 3.0 sec later # --------------------------------------------- proc event {} { global ns; puts "[$ns now]: exec: event()"; $ns at [expr [$ns now] + 3.0] "event" ; # Self-scheduling !!! } # ------------------------------------------- # Schedule function execution as events # ------------------------------------------- $ns at 1.0 "event" ; $ns at 30.0 "exit 0" ; $ns run ;

    Result:

    1: exec: event() 4: exec: event() 7: exec: event() 10: exec: event() 13: exec: event() 16: exec: event() 19: exec: event() 22: exec: event() 25: exec: event() 28: exec: event()

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  • Example Program: (Demo above code)                                                

    cd /home/cs455000/demo/NS2/intro ns intro4.ns