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Event Driven Simulation in NSEvent Driven Simulation in NS22Event Driven Simulation in NSEvent Driven Simulation in NS22mmmm

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 1ed

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Event Driven Simulation in NS2R D E T

Event Driven Simulation in NS2• Recap: Discrete Event v.s. Time

Driven• NS2 Simulation Concept• Events and HandlersEvents and Handlers• The Scheduler

h l• The Simulator• SummarySummary


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Event-Driven v s Time-DrivenEvent Driven v.s. Time Driven• Q: Time Driven = ( )Move from one time slot to anotherQ me Dr ven ( )

• Q: Event Driven = ( )• Time Driven or Discrete Time Simulation

Move from one t me slot to another

Move from one event to another

Time Driven or Discrete Time Simulation• Example: Packet arrivals and departures

Arrivals DeparturesArrivals Departures


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Time-Driven SimulationTime Driven Simulation• Observe the buffer for every FIXEDObserve the buffer for every FIXED

period (e.g., 1 second)

No. of Packet in the Buffer

2

3

1Time (s)

1 2 3 4 5 6 7 8 9 10 11 12

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 4Ppa

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Time-Driven Simulation• Simulation event for every time slot (fixed

interval)• Example Psudo Codes:

For t = 1 to sim_time {

p

if (arrival)buffer = buffer + 1;buffer buffer 1;

if (departure) buff buff 1;buffer = buffer -1;

print(buffer);

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008.

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Event-Driven SimulationEvent Driven Simulation• Go from one event to anotherGo from one event to another• Same Example

No. of Packet in the Buffer

2

3

0 8 1 5 2 4 5 2 5 5 7 4 9 4 10 5 11 5

1Time (s)

0.8 1.5 2.4 5.2 5.5 7.4 9.4 10.5 11.5

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 6P

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Event-Driven Simulation• Use a Scheduler• Maintain a set of eventsMaintain a set of events• Example

CreateEvent();R ()

CreateEvent(){Pkt1.arr(0.8)

Run ();

Psudo Codes

( )Pkt2.arr(1.5)}Psudo Codes }

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 7ddCod

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Event Driven SimulationEvent-Driven SimulationEvent ID 1 2 3 4 5 6

CreateEvent();

Type Arrival Arrival Arrival Arrival Arrival ArrivalTime 0.8 1.5 5.2 7.4 9.4 11.5

C t CreateRun();

Create departure

Create departure

Create departure

Event ID 7 8 9Type Departure Departure Departure

p

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 8

Time 2.4 5.5 10.5

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E nt D i n Sim l ti n in NS2Event Driven Simulation in NS2R E D T D• Recap: Event Driven v.s. Time Driven

• NS2 Simulation Conceptm p• Events and Handlers

The Scheduler• The Scheduler• The Simulator• Summary


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NS2 Simulation Concept• Event-Driven Simulation• Recap: Simulation Main StepsRecap: Simulation Main Steps

– DesignSi l i– Simulation• Network Configuration Phase � CreateEvent()• Simulation Phase � Run()

– Result Compilationp


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Simulation• Network Configuration Phase

– Create topologyCreate topology– Schedule event (e.g., CreateEvent())Si l ti Ph• Simulation Phase– Simulator::run() (e.g., Run())g– Execute the scheduled events


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Phase

Event Driven Simulation in NS2R D E T• Recap: Discrete Event v.s. Time Driven

• NS2 Simulation Concept• Events and HandlersEvents and Handlers• The Scheduler• Summary


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Concepts of Events and Handlersp f• Event-driven simulation

P t t th i l ti ti li– Put event on the simulation timeline– Move forward in time– When finding an event, execute

associated actions• Main components

– Events � C++ class Event– Actions � C++ class Handler


Q: Give examples of events.13P

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Event and Handler• Examples of Events

– Packet Arrivals/Departures– Start/Stop Application

• C++ Classes:– Event: Define events (packet arrival)– Handler: Define actions associated with an


Handler Define actions associated with an event (tell the node to receive the packet)

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Class Event

• Main variables: next : Next event

handle(){<actions>

}

handle(){<actions>

}– next_: Next event– uid_: Unique ID

ti Ti

handler handler

– time_: Time– handler_: Handler

uid time

handler_ next_

uid time

handler_ next_


uid_ time_event

uid_ time_event

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Example Events

“$ftp start” “$cbr start”Simulation time0.05 0.1

h dl

handle(){send FTP packets}

h dl

handle(){send CBR packets}

handler

h ndl r t

handler

h ndl r t…

uid_ time_

handler_0.051next_

uid_ time_

handler_0.12next_


event event16

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Event: Recap• Simulation steps:1. Put events on the simulation timeline

• Use the pointers “next_” of class Event2 Execute event2. Execute event

• Also called “fire” or “dispatch”• Take default actionsTake default actions• function handle() of class Handler

3 Move to the next event3. Move to the next event• Use the pointer “next_” of class Event


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Handlersh E• Associated with an Event

• Define Default ActionsDef ne Default Act ons� C++ function handle(Event*)

What is this?What is the purpose?

E l Cl N Obj t• Example: Class NsObject


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Events and HandlersSimulation S mulat ontime0.05

• When hitting an event e, execute handler_->handle(e)

• Define the action associated with an

handle(){send FTP packets}

Define the action associated with an event in the handler, NOT in the event

handler}

• We need mechanism to – Store events

handler_0.051next_ – Execute event at the right time

THIS FUNCTIONITY BELONGS TOuid_ time_event

THIS FUNCTIONITY BELONGS TO THE SCHEDULER


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QuestionQuestion

• What is the main purpose of events ?• What happen if NS2 does not define• What happen if NS2 does not define

classes Event, Handler, and S h d l ?Scheduler?– Some events have not occurred; – Every event occurs at the same time.


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Two Types of Eventsyp1. At Event: (Derived from Class Event)

E OT l d– Action: Execute an OTcl command– Examples:p

C Cl A E– C++ Class AtEvent– Create by instproc “at” with syntaxy p y

$ns at <time> <Tcl command>


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Two Types of EventsTwo Types of Events

2. Packet: (Derived from Class Event)– Action: Receive a packetAction: Receive a packet

?? Type casting:Packet is a derived class

– C++ Class Packet

of class Event

C++ Class Packet

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 22dass

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Q tiQuestions

Q H d E h• Q: How do we put an AtEvent on the simulation timeline? ( )$ns at <time> <Tcl command>

• Q: Is it possible to put a Packet on the simulation timeline? Why or why the s mulat on t mel ne? Why or whynot? ( )

• How do we put events on theYes; Packet derived from class Event

• How do we put events on the simulation timeline? � Use THE SCHEDULERSCHEDULER


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• NS2 Simulation Concept• Events and HandlersEvents and Handlers• The Scheduler

h l• The Simulator • SummarySummary


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RecapRecap• Event = An indication of future eventEvent = An indication of future event• Handler defines the default action (i.e.,

handler(e))( ))• NS2 moves forwards in time and execute

events • Execute = Fire = Dispatch

• What’s more? – How to put an event on the simulation timeline?

H t t th t d t– How to execute the stored events– The scheduler


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The Scheduler• Maintain Simulation (virtual) time• Move forward in time (fn run())• Maintain the unique ID for eventsMaintain the unique ID for events• Put events on the timeline (fn schedule(.))

E t t th ti li (f di t h( ))• Execute events on the timeline (fn dispatch(.))event handler

uid time

handler_ handle(){…

}uid_ time_


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C++ Class Scheduler

Current time

Unique ID: incremented for??


incremented for every new event( uniqueness )

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Functions Scheduler::schedule( )Functions Scheduler::schedule(.)• schedule(h e delay)• schedule(h,e,delay)

– Associate Event “e” with a handler “h”Put the Event “e” on the simulation time with delay– Put the Event e” on the simulation time with delay “delay”

< Checking for Error > New unique ID

Bind “e” and “h”

U d t tiUpdate time

Put “e” on the time line


Put e on the time line

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Functions Scheduler::schedule(.)• schedule(h,e,delay): Checking for Error1. Null handler (i.e., h = 0)1. Null handler (i.e., h 0)2. uid_ of the event > 0 � Something wrong3 d l 0� G b k in tim3. delay < 0 � Go back in time4. uid_ < 0 � Use up the uid_

12

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Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 29PPreook: T. Iss

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Function Scheduler::dispatch(.)• Execute the event

event handler Why put negative?handler

d

handler_Why put negative? � Let have a look at

the UID concept


uid_ time_*e *h

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Two types of Unique ID (UID)Two types of Unique ID (UID)1 Scheduler: Global UID1. Scheduler: Global UID

2 Event: Individual UID assigned by the2. Event: Individual UID assigned by the Scheduler


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Global UIDGlobal UID• For the Scheduler• For the Scheduler• Always Positive• Incremented for every new event (fn

schedule(.))


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Individual UIDIndividual UID• Unique to each eventq

– Assigned by the scheduler within fn schedule(.)– Negative when being dispatched fn dispatch(.)Negat ve when be ng d spatched fn d spatch(.)


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Individual UIDIndividual UID• Unique to each eventq

– Positive: assigned by fn schedule(.)– Negative: dispatched fn dispatch(.)Negat ve d spatched fn d spatch(.)– Dynamics: uid_ is switching between +/- values

dispatch( )

•schedule(.)•If negative �( Event UID not valid )?

dispatch(.)� Positive uid_

�( Event UID not valid )?

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 34PPreook: T. Iss

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Function Scheduler::run()Function Scheduler::run()

• Start simulation


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The SimulatorM i i hi h h d• Maintain assets which are shared among simulation objectsOT l d C Cl Si l• OTcl and C++ Classes Simulator

• Instvar– scheduler_: The schduler– nullAgent_: The packet destruction object

N d ( d d ) d b– Node_(<nodeid>) stores node objects– link_(sid:did) stores link objects connecting

two nodestwo nodes

sid did


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The SimulatorThe Simulator• Suppose $ns is a simulator objectSuppose $ns s a s mulator object• Three main instprocs 1 Constructor: init{}1. Constructor: init{}2. run{}: Start simulation

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 38PPrepa

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SummarySummary

N l E D• NS2 Simulator is Event Driven• Event

– Unique ID + Time + Handler– Two derived classes: ( AtEvent and Packet )( )

• Handlers– ( default actions )( default actions )– ( function handle(e) )


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SummarySummary

h d l• Scheduler– schedule(.): ( Put an event in the list )– dispatch(.): ( Executes an event )– run(): ( Start simulation )

• Event UID Dynamics– Schedule()� +– Schedule() � +,

– dispatch() � -• Simulator

– Maintain all common objects


j– Start the simulation (e.g., “$ns run”)

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Related files• ~ns/common/schedule.h,cc• ~ns/common/simulator.h,ccns/common/simulator.h,cc• ~ns/tcl/lib/ns-lib.tcl


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