- The simulation system contains
a scheduling facilty
that schedules the runnable
processes
according to their
ready time
- We are not going to learn
how to build such a
scheduler,
but how to use one
- The Simulator class in
NS
contains the code of
the simulation system
In order to use the scheduling functionality of the simulation system, we must create a Simulator object:
new Simulator
Usually: set ns [ new Simulator ]
- A similation system
(such as NS) always have a
simulation clock
- The simulation clock represents the "time" that has elapsed in the simulation
- Simulation clock in NS:
- The method now in a Simulator object returns the simulation clock value
- You can examine (read)
the value of the
simulation clock
by invoking
the
now
method in the
Simulator
class
- Example::
set ns [new Simulator] set curr_time [$ns now] // Returns the current simulation time
- An event
is an action made by a
simulation entity
- Events in NS:
- An event (action) in a NS simulation is a OTcl command !!!
- Each event
happens
at a certain (simulation)
time
- Therefore,
each event has an
associated
event time which
is the time
when the
event will happen
I.e., the time when the OTcl command is run !!!
- Simulation system:
- In a simulation system,
the events
are
ordered by their event time
- The event et
with the
smallest event time t
is run first
- When event et is run, the simuation clock (now) is set to t (i.e., the simulation time advances)
- In a simulation system,
the events
are
ordered by their event time
- To schedule an event in
NS, use the
at
command in the
Simulator object
- Syntax:
set ns [new Simulator] $ns at Time ActionNotes:
- Time is a
floating point number
representing the event time
- Action is a OTcl command representing the event
- Time is a
floating point number
representing the event time
- 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
Note:
- Notice that Point 3 happens before Point 2
- Explanation:
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 !When the simulation is run, the scheduled events are "fired" (run) in chronological order (given by their event time !!!
- 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
- There is
no "stop" command
available in NS
- Stopping a simulation in NS:
- To stop a running simulation,
you must
schedule
a
exit command
- Note that you must
schedule the exit command
before
you
start running the simulation.
(Once you start the simulation, it's too late to schedule the exit command)
- To stop a running simulation,
you must
schedule
a
exit command
- Usually, you want to
gather performance data
at the end of the simulation
- The exit command
is usually part of a
Tcl procedure
used to wrap up things
- Example:
set ns [new Simulator] # ----------------------------------------- # The 'finish' procedure wraps things up # ----------------------------------------- proc finish {} { ... ... (print performance information gathered) ... exit 0 } ... (set up simulation network) # ------------------------------------------- # Schedule the wrapup action at time 100 # ------------------------------------------- $ns at 100.0 "finish" $ns run // run simulation
- Example event scheduling:
NS script that simulates 2 entities passing control 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