2. Network Protocol Simulation: A look at Discrete Event Simulation Grant D. Lanterman 5/21/2004

3. Overview  Network Protocol Example  Simulation  Overview  Why?  Discrete Event Simulation  Why?  Example  Software

4. Example: TCP/IP  Driving force of the internet  Layered protocol

5. TCP/IP Layers  Physical layer  Network access layer  Internet layer  Host-to-host, or transport layer  Application layer

6. Network Protocols  Difficult to simulate activity with small test networks  Dynamic

7. The Answer!  Simulation  Imitating real-world situations  Two types  Analytical modeling  Computer simulation

8. Analytical Modeling  Use of mathematical equations to represent a network  May lead to oversimplification  Can’t effectively model the dynamic nature of a network  Less Practical

9. Computer Simulation  Preferred simulation type for networks  Can adapt to dynamic nature of networks  Use of a computer to simulate what will happen in real-world events  Discrete Event Simulation usually used  Example:  What would be the effect of adding an extra cashier to a bank

10. Simulation Terminology  Simulation Model  Attributes  Entities  Activities  Delays  States  Events

11. Simulation Model  Set of assumptions about the system being simulated  Includes assumptions based on:  Algorithms  Mathematical equations  Conditions imposed by the system  Simplified  Replace real parts of the system by concepts

12. Entities  Objects that will participate in the simulation  Part of the model  Examples from a Bank Simulation:  Customers  Employees  Managers

13. Attributes  Properties of entities within the system  Examples from Bank Simulation:  Gender of a customer  Name of an employee

14. Activities and Delays  Activity is a duration of known time  A delay is a length of unknown time  Can change state of system  Activity Example:  A customer checking their account balance  Delay Example:  Time it takes for n customers to enter the bank

15. States  The current configuration of the system  Sate variables represent the current system  Example:  The length of the line of customers in the bank

16. Events  Instantaneous in time  Often change the state of the system  Example:  A customer enters the bank

17. Simulation Process

18. Discrete Event Simulation (DES)  Asks the questions:  What events are possible and do they change the state?  What activities and what are their duration?  What events begin/end each activity?  How are delays defined?  How do we start the simulation?

19. DES  Models a system as it evolves  Represents changes as separate discrete events  Simulation executive  Next event  Time slicing

20. DES Approaches  Event  Commonly used  Describes an instantaneous change  Activity  Examines as a duration of time  Easy to understand but not efficient  Process  Groups activities to describe the life cycle of an entity  Most efficient and effective but difficult to impliment

21. Event Types  Primary  Scheduled at a specific time  New cashier comes to work  Conditional  Triggered by a condition  Customer moving from the line to a cashier

22. Event List  Often called the FEL or future event list  Main data structure of a DES  Ordered in increasing time of event notice  Contains only primary events  Contains all information needed to execute the events in it  Efficiency of simulator depends on efficiency of FEL

23. DES Algorithm  Remove first event(l, E) from FEL  Advance simulation to time t  Update sate variables according to E  Insert new events into FEL according to E  Compute statistics

24. Bank Queuing Example  Owner asks: What would the effects of adding another cashier to the bank have?  Customer wait time  Bank congestion

25. Bank (Cont.)  Can use DES to simulate the randomness of customers arriving  Calculate wait times with n cashiers  Gather statistics on multiple runs

26. Network Simulators  Many software packages available  Example:  OPNET  Assist in building simulation model  Run simulation and collect results

27. Questions?  Comments?  Hearing None…