1. Network Simulation with Opnet

2. Simulation of Networks
Computer experiment, alternative to building a network
Input network and traffic details
Specify performance stats to be collected
Run the simulation
Examine the results – use graphs, spreadsheet
Evaluate averages, min/max, deviation

3. Input details
Components – PCs, workstations, LANs, links, link speeds, protocols
User profiles – applications, start/stop time, destination servers
Application details – transaction interarrival time, transaction size
►Complex behaviour of networks
Difficult to predict (even simple sub-networks and single links can have complex behaviour due to the large number of interacting dynamic aspects).

4. C/S Database Application – link load example
Ratio of query to update 50-50
Mean (average) transaction interarrival time = Exponential (2 sec)
(Constant, Exponential are statistical distributions – discussed later)
Try calculating the load in bits/sec each way.
Query (C to S): 128 bytes,
Response (S to C) 2400 bytes
Client (C)
Server (S)
Client to server: one query (128) and one update (2400) every 4 seconds.
Total traffic = 8 * ( ) bits in 4 seconds = 5056 bits per sec.
Update request (C to S) 2400 bytes,
Confirmation (S to C) 128 bytes

5. Video conferencing bandwidth requirements example
30 frames per second
Frame size: 800 x 640 pixels
Pixel resolution 12 bits
Load = [30 x 800 x 640 x 12] bits/sec
= 184,320,000 bits / second (approx 200Mb/s)

6. Performance statistics - examples
Output from the simulation experiment:
Application response time, delay
Voice/video packet end-to-end delay
Network access delay
No. of collisions
Link/server utilisation, throughput
Packet loss, retransmission

7. OPNET – Simulation Package
Facilitates simulation of communications networks
Enables determination of performance figures
Is a complex and powerful package with many features:
Get started by doing some of the tutorials.
Then modify the library models, and compare
different scenarios.
Save your own version of the library model under a
different name (your-initials_LANs).
Save to your O: drive.
Then develop your own models, code etc.

8. OPNET Start up screen Project may have multiple (related) scenarios.
A scenario is a specific configuration of a model.
You can open projects, nodes etc. from the file menu.
Documentation and tutorials from the ‘help’ menu.

9. See ‘help’ menu for tutorials

11. Small Internetworks Introduction
This tutorial shows how you can do organizational scaling to solve a typical "what if" problem.
You will learn how to use OPNET Modeler features to build and analyze network models. The tutorial focuses on the use of the Project Editor, and how it will be used with the Node and Process editors in later lessons.
Tutorial activities include:
Build a network quickly
Collect statistics about network performance
Analyze these statistics

12. Tutorials for this course
Start with:
Small Internetworks: Create, simulate & analyse the (star network, client server) model within the project editor. Compare two scenarios.
M/M/1 Queue: Build a simple queuing system and simulate it.

13. Workspace & documentation
Opnet Workspace (various GUI components, inc.)
Menu bar
Action buttons
Message display
Tool Tips
Documentation
On line documentation (see ‘help’ menu)
Comments within model components
From the Opnet web site
OPNET Website:

14. Work Sequence when using OPNET
Create the network model – nodes, links, processes – always save to your drive (O:/)
Specify traffic types (e.g. database, web browsing) and volumes (how many packets, what size packets)
Specify stats to collect – e.g. delays, buffer use, queue size, load, utilisation for servers, links.
Run the simulation – how long? Seed?
Estimate performance parameters (utilisation, delays, throughput, buffer use, loss/error rates).
Average, min/max, standard deviation
Export to Excel spreadsheet for better analysis and graphs

15. Main Components of OPNET
Project Editor: build a network model using various model libraries; specify statistics (results) to collect, run the simulation and examine results
Node Editor: Nodes have modules (e.g. MAC, IP, TCP, ARP) & processes (module behaviour, described by State Transition Diagrams STD & Finite State Machines FSM).
Process Model Editor: Process functionality (C++).
Link Model Editor: Create data links, specify speed, protocol etc.
Packet Format Editor: Create different types of packets & frames
Probe Editor: To specify simulation statistics
Simulation Tool: To run multiple simulations, with different run parameters, and other sophisticated features
Analysis Tool: More detailed analysis, comparisons etc.

16. Scenarios The project describes the model
Scenarios are slight variations of a basic model.
E.g. Change in link speed, traffic parameters etc.

17. Object Attributes right click and edit attributes
Attributes of node-30 (Central Switch)
3Com model, type etc. included in the name
Explanation (‘details’) of attributes can be obtained

18. User Profiles Each profile represents a typical user
– a mix of applications
Start/stop times are defined for profiles

19. Applications Standard applications are defined
Here is a list of what is available
Actual parameters can be modified

20. Application Parameters

21. Example traffic specification
Telnet (remote login) example shown on previous slide.
Enter command (interarrival) time (secs)
Command size (bytes) for terminal traffic
Command size (bytes) for host traffic
Distribution needed too – constant, exponential, uniform etc.
Note / recall:
Load (bits/sec) = commands/sec x command size (bytes) x 8
Commands/sec (rate) = 1/interarrival_time
Always remember:
If the interarrival time = t secs, Arrival rate = 1/t commands/packets/messages per second.

22. Running the simulation
Run length – enough to generate enough packets.
Values per statistic – I suggest 40-50
Seed – determines the sequence of random numbers used to generate packet arrival events etc.
A given value of the seed will gave the same results every time.
A different seed will give slightly, but not drastically different results
Calculate the average over 3-4 runs with different seeds to get a more accurate estimate.
Use optimised mode for quick execution.

23. Guidelines
Duration or run length? Enough for 20k-30k messages of each type.
Values per statistic: if duration = 3600 seconds, values/statistic = 60, each value will be averaged over 3600/60 = 60 seconds.
Independent runs? Change the seed each time.
Results from different runs will be close, but not identical.
Enter parameters in the right units (e.g. seconds, bytes)
Report results, using appropriate units (e.g. 5 ms rather than sec).  

24. Results
Graphs of delays, utilisation etc. can be obtained (Opnet graphs are poor quality for coursework reports)
Graph properties can be changed if you wish
You can also export to Excel spreadsheet to analyse, such as calculate averages, and also generate much better quality graphs

25. More Results Multiple graphs for comparison
Separate (stacked) or overlaid (same frame)
Various types of plots – as is, average, time average, histograms

26. Checking Results
Estimate or calculate a sample of results to confirm your model, its parameters etc. are correct.
Example: Message inter-arrival time = 10 seconds, message size = 1000 bytes.
Selected output statistics:
Messages/sec sent and/or received
Bits/second sent and/or received
Results should be:
Messages/sec sent or received = 0.1
Bits/sec sent or received = 800

27. Example calculation of utilisation ( (Rho) )
Messages
Pkt inter-arrival time 0.2s
Pkt arrival rate = 1/0.2 = 5 packets per sec
Pkt size = 1600 bytes
Link/channel bitrate = 128 Kbits/s  
Number of bytes per sec = 5 x 1600
Number of bits per second = 8 x 5 x 1600
Link (channel) utilisation = (8 x 5 x 1600)/128000
= 0.5 or 50%