Tutorial of OMNeT++
This document provides a quick introduction to OMNeT++. The aim is to get the interested user up and running with a simple game NIM. After finishing this, the user can start exploiting more of OMNeT++’s functionality based on the extensive user manual and the sample directories.
1.3 Where
to Get the Software?
2.2.1 Find
the folder of the Nim example.
2.2.3 Run
the program step by step
2.3.1 Where
can you find the parameters?
2.3.2 How
to change parameters?
2.3.3 Start
newly generated Nim
1 Introduction
1.1 What is OMNeT++?
OMNeT++ is an object-oriented modular discrete event simulator. The name itself stands for Objective Modular Network Testbed in C++. OMNeT++ has its distant roots in OMNeT, a simulator written in Object Pascal by dr. György Pongor. The simulator can be used for:
ü traffic modeling of telecommunication networks
ü protocol modeling
ü modeling queueing networks
ü modeling multiprocessors and other distributed hardware systems
ü validating hardware architectures
ü evaluating performance aspects of complex software systems
ü modeling any other system where the discrete event approach is suitable.
1.2 Why OMNeT++?
There are numerous network simulation tools on the market today, both commercial and non-commercial. In this section I will try to give an overview by picking some of the most important or most representative ones in both categories and comparing them to OMNeT++: PARSEC, SMURPH, NS, Ptolemy, NetSim++, C++SIM, CLASS as non-commercial, and OPNET, COMNET III as commercial tools. (The OMNeT++ Home Page contains a list of Web sites with collections of references to network simulation tools where the reader can get a more complete list.) In the commercial category, OPNET is widely held to be the state of the art in network simulation. OMNeT++ is targeted at roughly the same segment of network simulation as OPNET.
Seven issues are examined to get an overview about the network simulation tools:
- Detail Level. Does the simulation tool have the necessary power to express details in the model? In other words, can the user implement arbitrary new building blocks like in OMNeT++ or he is confined to the predefined blocks implemented by the supplier? Some tools like COMNET III are not programmable by the user to this extent therefore they cannot be compared to OMNeT++. Specialized network simulation tools like NS (for IP) and CLASS (for ATM) also rather fall into this category.
- Available Models. What protocol models are readily available for the simulation tool? On this point, non-commercial simulation tools cannot compete with some commercial ones (especially OPNET) which have a large selection of ready-made protocol models. OMNeT++ is no exception.
- Defining
Network Topology. How does the simulation tool support defining the
network topology? Is it possible to create some form of hierarchy
(nesting) or only “flat” topologies are supported? Network simulation
tools naturally share the property that a model (network) consists of
“nodes” (blocks, entities, modules, etc.) connected by “links” (channels,
connections, etc.). Many commercial simulators have graphical editors to define
the network; however, this is only a good solution if there is an
alternative form of topology description (e.g. text file) which allows one
to generate the topology by program. OPNET follows a unique way: the
network topology is stored in a proprietary binary file format which can
be generated (and read) by the graphical editor and C programs linked
against a special library. On the other hand, most non-commercial
simulation tools do not provide explicit support for topology description:
one must program a “driver entity” which will boot the model by creating
the necessary nodes and interconnecting them (PARSEC,
- Programming Model. What is the programming model supported by the simulation environment? Network simulators typically use either thread/coroutine-based programming (such as activity() in OMNeT++), or FSMs built upon a handleMessage()-like function. For example, OPNET, SMURPH and NetSim++ use FSMs (with underlying handleMessage()), PARSEC and C++SIM use threads. OMNeT++ supports both programming models; the author does not know of another simulation tool that does so.
- Debugging and Tracing Support. What debugging or tracing facilities does the simulation tool offer? Simulation programs are infamous for long debugging periods. C++-based simulation tools rarely offer much more than printf()-style debugging; often the simulation kernel is also capable of dumping selected debug information on the standard output. Animation is also often supported, either off-line (record&playback) or in some client-server architecture, where the simulation program is the “server” and it can be viewed using the “client”. Off-line animation naturally lacks interactivity and is therefore little use in debugging. The client-server solution typically has limited power because the simulation and the viewer run as independent operating system processes, and the viewer has limited access to the simulation program’s internals and/or it does not have enough control over the course of simulation execution. OPNET has a very good support for command-line debugging and provides both off-line and client-server style animation. NetSim++ and Ptolemy use the client-server method of animation. OMNeT++ goes a different way by linking the GUI library with the debugging/tracing capability into the simulation executable. This architecture enables the GUI to be very powerful: every user-created object is visible (and modifiable) in the GUI via inspector windows and the user has tight control over the execution. To the author’s best knowledge, the tracing feature OMNeT++ provides is unique among the C++-based simulation tools.
- Performance. What performance can be expected from the simulation? Simulation programs typically run for several hours. Probably the most important factor is the programming language; almost all network simulation tools are C/C++-based. Performance is a particularly interesting issue with OMNeT++ since the GUI debugging/tracing support involves some extra overhead in the simulation library. However, in a reported case, an OMNeT++ simulation was only 1.3 slower than its counterpart implemented in plain C (i.e. one containing very little administration overhead), which is a very good showing. A similar result was reported in a performance comparison with a PARSEC simulation.
- Source Availability. Is the simulation library available in source? This is a trivial question but it immediately becomes important if one wants to examine or teach the internal workings of a simulation kernel, or one runs into trouble because some function in the simulation library has a bug and/or it is not documented well enough. In general it can be said that noncommercial tools (like OMNeT++) are open-source and commercial ones are not. This is also true for OPNET: the source for simulation kernel is not available (although the ready-made protocol models come with sources).
In conclusion, it can be said that OMNeT++ has enough features to make it a good alternative to most network simulation tools, and it has a strong potential to become one of the most widely used network simulation packages in academic and research environments. The most serious shortcoming is the lack of available protocol models, but since this is mostly due to the fact that it is a relatively new simulation tool, with the help of the OMNeT++ user community the situation is likely to become much better in the future.
1.3 Where to Get the Software?
You can find OMNeT++ at http://www.omnetpp.org/. If you are too lazy to type this in, a search with www.google.com on omnet++ will give an instant hit.
The following graph is the screenshot of the homepage of OMNeT++
Since most students use windows operation system, we should download the file OMNet++ 2.3 Win32(exe) (Circled in above graph)or simple click the following link http://www.omnetpp.org/filemgmt/visit.php?lid=31 and download the file.
1.4 Install the OMNeT++
1.4.1 Prerequisites
- Windows
XP, Windows 2000, NT4.0 or other NT-derivative. OMNeT++ is not
supported by
WIN95/98/ME, because it uses the Win32 Fiber API which is only available on NT systems. Moreover, the makefiles and some batch (.cmd) files use syntax which is not supported by the old command.com. Besides the above "hard" prohibitive factors, it is anyway advisable to use a more stable operating system. - Microsoft Visual Studio 6.0, Very Important with at least Service Pack 1 installed. I recommend Service Pack 5 which can be downloaded from Microsoft's site under the name vs6sp5.exe. You should first install Visual C++ 6.0, then install OMNeT++. Otherwise, you will get frustrated by doing a lot of links with OMNeT++ and Visual C++ 6.0.
- 40MB free space on your hard disk. If your want to compile from source, you'll need about 20MB more. Generally speaking it is not a problem for most computers.
1.4.2 Begin to Install
a) Double click the downloaded file 
,
you will see the following license window
b) Choose I Agree and you will see the following window
c) Choose default configuration and go to Next step
d) You can choose current configure to Install OMNeT++
You can also choose another directory. But the directory can not have space in its name. For example please don't install under C:\Program files! (Otherwise you'll probably have problems with the makefiles.) After click Install you will find the following installation window.
e) After a while you will see the successful installation window as follow
f) Then click Yes and restart your computer and the OMNeT++ will be ready for you to use
2 Starting a simple example
In this chapter we will show you the basic concepts of OMNeT++ via the simulation model of a simple game, NIM.
2.1 Example Introduction
Nim is an ancient game with two players and a bunch of sticks (here also called tokens). The players take turns, removing 1, 2, 3 or 4 sticks from the heap of sticks at each turn. The one who takes the last stick is the loser. The game is modeled in OMNeT++ as a network with three modules: the 'game' (a manager module) and two players. The model of the Nim game is similar to 'Hello world!' by complexity, but it nicely demonstrates the modeling approach used by OMNeT++.
2.2 Start the Nim
Let us first start the Nim and have a brief review of OMNeT++.
2.2.1 Find the folder of the Nim example.
If you installed as default configuration, it should be at C:\OMNeT++\samples\nim. If you changed to other folder the position of the Nim is similar. The following figure shows the folder position in my computer.
2.2.2 Start Nim program
You can double click the EXE file nim.exe with icon
to
start the program. Then a lot of windows will appear as follow.
2.2.3 Run the program step by step
1. Since the graphics window is clear and straightforward, we first click
run
in
this window to see what happens. Then we find the TKenv
window displays some messages.
2. We continue to run the game by clicking in the graphics window. We can see the
following events happen. Player1 named “InteractivePlayer”
come out of player 1 box and go to game circle.
The Tkenv window shows the detailed event explanation. ** Event #1. T=0.0000000 ( 0.00s). Module #3 `nim.player1'
3. Continue to run the game, we
will notice a “smartplayer” comes out of player2 box
and goes to game circle. The windows is showed as
follow.
Then the Tkenv
window shows the detailed event explanation. ** Event
#2. T=0.0000000 ( 0.00s). Module #4 `nim.player2'
4. Continue to run the game by clicking , we
will see ** Event #3. T=0.0000000 ( 0.00s). Module #2 `nim.game'
appear in the Tkenv window
5. Click again
and the game will begin. The Tkenv window shows the
following message
In the graphics window we can see a very interesting animation. The ball changes its color, and the text with the ball also changes to “26 sticks left”. Then, the ball goes to player2’ box. The following graph is the final result of this step.
6. Continue to run the game by clicking ,
the smart player begins to take some sticks from the 26 sticks. By the smart
algorithm he takes 1 out of 26 sticks. If you are the smart player, how many
sticks are you going to take? The Tkenv windows
displays the message like following
The graphics window looks like following
7. Continue to run the game by clicking , it
is your turn to take sticks. The ball named 25 sticks goes to player1’s box.
The Tkenv window looks like following
8. Continue to run the game by clicking . A
new dialogue window appears and asks you how many sticks you take. You can only
input 1, 2,3,4, the new window look like the following graph
9. Input a number, for example, 3, and click Ok to continue the game. The Tkenv window will show the message
10. Now each of the two players has taken some sticks. You can easily continue the game until all the sticks are taken. Finally, you will pick up the last stick, then the game is over and the following window appears.
The Tkenv window shows the following message
The last screenshot of graphics window is shown by follow
2.2.4 Play the program again
You can continue to play the game
by clicking ,
you can also click 
in the Tkenv
window.
The following dialogue window will appear
Click Yes and you can restart the game. You will find that the number of stick may change and sometime the player who first picks up sticks may also change. If you are selected to first pick up sticks, try to think about it and be a smart player.
2.3 Change some parameters
This game is very simple and does not have many parameters to change. The number of sticks and the player who first pick up sticks are two parameters we can change. Let us find how we can change it.
2.3.1 Where can you find the parameters?
Click 
and have a look at the structure of Nim
You can expand the object tree to see every item. The two parameters we are looking for are at the following place
The variable “num_sticks” indicates the number of initial sticks and the “first_move” indicates the player who will first move the sticks
double click “num_sticks” you will see the following window
intuniform(21,31) means the number of sticks uniformly distributed between 21 and 31.
double click “num_sticks” you will see the following window
intuniform(1,2) means the number of sticks uniformly distributed between 1 and 2. ‘1’ refers to the player1 and ‘2’ refers to the player2. It means both players have equal probability to be the first player.
2.3.2 How to change parameters?
You may want to change these two parameters, but it is not straightforward to do so. You need to follow the following steps:
1. Find the nim.ned file in the directory C:\OMNeT++\samples\nim
2. Right click the file and open with notepad.
The following is the content of nim.ned. It defines all the modules and their input, output, and parameters.
//-------------------------------------------------------------
// file: nim.ned
//
(part of NIM - an OMNeT++ demo simulation)
//
// NIM is an ancient game with two
players and a bunch of sticks.
//
// The players take turns, removing
1..4 sticks from the heap
// at each turn.
//
// The player who must take the
*last* stick is the loser.
//
//-------------------------------------------------------------
// Game --
//
// Declaration of simple module
type Game
//
simple Game
parameters:
num_sticks :
numeric, // starting number of sticks
first_move :
numeric; // 1=Player1, 2=Player2
gates:
in: from_player1; // input and output gates
in: from_player2; // for connecting to Player1/Player2
out: to_player1;
out: to_player2;
endsimple
// Player --
//
// Interface declaration for
different modules implementing players
//
simple Player
gates:
in: in; // gates for connecting to Game
out: out;
endsimple
// Nim --
//
// The NIM game's network: Game and
two players interconnected
//
module Nim
parameters:
player1_type : string,
player2_type : string;
submodules:
//
create the Game module, with the initial number of
//
sticks and the starting player as random parameters
game: Game;
parameters:
num_sticks = intuniform
(21, 31),
first_move = intuniform
(1, 2);
//
for players, define module type as parameter to
//
the parent module, Nim
player1: player1_type like Player;
player2: player2_type like Player;
connections:
player1.out --> game.from_player1;
player1.in <-- game.to_player1;
player2.out --> game.from_player2;
player2.in <-- game.to_player2;
endmodule
// nim --
//
// an instance of the Nim module
//
network nim : Nim
parameters:
player1_type
= input("InteractivePlayer"),
player2_type = input("SmartPlayer");
endnetwork
3. The red text is the parameters. We change them to, for example,
num_sticks =27,
first_move = 1;
Save it and close the notepad.
open DOS command window by Start->Run->cmd
You will see the command window
Change directory to cd c:\omnet++\samples\nim
4. Input the command opp_nmakemake -f
5. Then you can build the program by typing:
nmake -f Makefile.vc
The most common problem is that nmake (which is is part of MSVC) cannot be found because it is not in the path. You can fix this by running vcvars32.bat, which can be found in the MSVC bin directory (usually C:\Program Files\Microsoft Visual Studio\VC98\Bin). After successfully execute the nmake command, you will see the following message in the command window
You will find a newly generated nim.exe in the directory.
2.3.3 Start newly generated Nim
1. Start the newly configured Nim game and see whether the parameters are changed or not. Double click game in the object tree as follow.
You will see the following window
2. Click Params to see the parameters of game
All the two parameters are listed and we notice their values have been changed. Double Click each of the parameters you can see its value as follow.
3. Run the game and it is your turn to first pick up sticks. Pick up 4, then the other player will pick 1, then, pick up 4 again, and you will win.
Now you have learned how change parameters and make file in OMNeT++
3 Understanding the code
There are various ways to understand the implementation of a new simulation. I found the following approach the most useful. First I run it, of course, to get an understanding of where the various entities reside and how they exchange jobs. Then I work through the files belonging to each entity separately. I start with the .ned file to understand what goes in and out of a module, and the parameters it needs. Then I give the header file a brief look to become familiar with the module’s specific internal variables and functions. Finally I study the C++ code belonging to the module. Once I somewhat understand what this first module does, I take the next module in the simulation chain, that is, the module that gets its messages from the one I just studied. Working this way, I gain a quicker understanding of what is going on than by first working through all .ned files, than all the C++ files, etc.
3.1 NED file
Each entity in a simulation needs to communicate via messages with itself and other entities. Messages can be used for various purposes. One is to represent sticks that are available. Another is to convey information of the entity’s state to itself in the future, or to other entities. Specifically, the Nim example contains three entities: a game module, two player modules. Clearly, all these modules need in- and output gates to receive and send messages. The .ned file specifies these gates. In our example, the game module has two input and two output gates. Furthermore these modules may need parameters such as number of sticks, first players, etc. These pa