NS Simulation: Scheduling Events

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

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

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

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

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 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

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 above code)
- Prog file: click here

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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NS Simulation: Scheduling Events (examples inside)

NS Simulation: Scheduling Events