What is NS2? NS2 is an event driven network simulator developed at UC Berkeley that simulates variety of IP networks. It implements network protocols such as TCP and UDP, traffic source behavior such as FTP, Telnet, Web, CBR and VBR, router queue management mechanism such as Drop Tail, RED and CBQ, routing algorithms such as Dijkstra, and more. NS also implements multicasting and some of the MAC layer protocols for LAN simulations. You can find additional information about Ns2 in the NS by example Site

Now we will see the installation procedure of Ns2. Download the Network Simulator Ns2 to your local drive and copy it to the location: /class/cs164/cs164_09win/your_user_name/. Untar it and then go inside the ns-allinone-2.31 folder and type ./install, in order to begin installation of ns2.

After the installation of Ns2 has finished you will see in your terminal that you have to make some corrections, in order the simulator to work. The message that you will see will be like the following:

Please put: /class/cs164/cs164_09win/jning/ns-allinone-2.31/bin:/class/cs164/cs164_09win/jning/ns-allinone-2.31/tcl8.4.14/unix:/class/cs164/ cs164_09win/jning/ns-allinone-2.31/tk8.4.14/unix into your PATH environment; so that you'll be able to run itm/tclsh/wish/xgraph. IMPORTANT NOTICES: (1) You MUST put /class/cs164/cs164_09win/jning/ns-allinone-2.31/otcl- 1.13, /class/cs164/cs164_09win/jning/ns-allinone-2.31/lib, into your LD_LIBRARY_PATH environment variable. If it complains about X libraries, add path to your X libraries into LD_LIBRARY_PATH. If you are using csh, you can set it like: setenv LD_LIBRARY_PATH <paths> If you are using sh, you can set it like: export LD_LIBRARY_PATH=<paths> (2) You MUST put /class/cs164/cs164_09win/jning/ns-allinone-2.31/tcl8. 4.14/library into your TCL_LIBRARY environmental variable. Otherwise ns/nam will complain during startup. (3) [OPTIONAL] To save disk space, you can now delete directories tcl8.4.14 and tk8.4.14. They are now installed under /class/cs164/cs164_09win/j ning/ns-allinone-2.31/{bin,include,lib} After these steps, you can now run the ns validation suite with cd ns-2.31; ./validate For trouble shooting, please first read ns problems page http://www.isi.edu/nsnam/ns/ns-problems.html. Also search the ns mailing list ar chive for related posts.

In order to do these modifications you will have to add some lines of code into the .bashrc file, which is located in your home directory. WARNING:Before you make any modifications to the .bashrc file have a backup, in case something goes wrong

In detail you will have to add the following lines:

• Inside the file you will find a line that says export PATH. Before this line add the following line of code:
  PATH=$PATH:/class/cs164/cs164_09win/jning/ns-allinone-2.31/bin:/class/cs164/cs164_09win/jning/ns-allinone-2.31/tcl8.4.14/unix:/class/cs164/cs164_09win/jning/ns-allinone-2.31/tk8.4.14/unix
• After the export PATH add the following line of code:
  LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/class/cs164/cs164_09win/jning/ns-allinone-2.31/otcl-1.13:/class/cs164/cs164_09win/jning/ns-allinone-2.31/lib
  export LD_LIBRARY_PATH
• After the export LD_LIBRARY_PATH add the following line of code:
  TCL_LIBRARY=$TCL_LIBRARY:/class/cs164/cs164_09win/jning/ns-allinone-2.31/tcl8.4.14/library
  export TCL_LIBRARY

In the above lines, replace jning with your username. That's all that you have to change in the .bashrc file.

Note:In the terminal do NOT run the command ./validate because it takes a long time to run .... Additional resources and information you can find in the Official Site of Ns2.
That's about it :-). Now we can experiment with a simple simulation in ns2.

Figure 1. A Simple Network Topology and Simulation Scenario

This network consists of 4 nodes (n0, n1, n2, n3) as shown in above figure. The duplex links between n0 and n2, and n1 and n2 have 2 Mbps of bandwidth and 10 ms of delay. The duplex link between n2 and n3 has 1.7 Mbps of bandwidth and 20 ms of delay. Each node uses a DropTail queue, of which the maximum size is 10. A "tcp" agent is attached to n0, and a connection is established to a tcp "sink" agent attached to n3. As default, the maximum size of a packet that a "tcp" agent can generate is 1KByte. A tcp "sink" agent generates and sends ACK packets to the sender (tcp agent) and frees the received packets. A "udp" agent that is attached to n1 is connected to a "null" agent attached to n3. A "null" agent just frees the packets received. A "ftp" and a "cbr" traffic generator are attached to "tcp" and "udp" agents respectively, and the "cbr" is configured to generate 1 KByte packets at the rate of 1 Mbps. The "cbr" is set to start at 0.1 sec and stop at 4.5 sec, and "ftp" is set to start at 1.0 sec and stop at 4.0 sec.

#Create a simulator object set ns [new Simulator] #Define different colors for data flows (for NAM) $ns color 1 Blue $ns color 2 Red #Open the NAM trace file set nf [open out.nam w] $ns namtrace-all $nf #Define a 'finish' procedure proc finish {} { global ns nf $ns flush-trace #Close the NAM trace file close $nf #Execute NAM on the trace file exec nam out.nam & exit 0 } #Create four nodes set n0 [$ns node] set n1 [$ns node] set n2 [$ns node] set n3 [$ns node] #Create links between the nodes $ns duplex-link $n0 $n2 2Mb 10ms DropTail $ns duplex-link $n1 $n2 2Mb 10ms DropTail $ns duplex-link $n2 $n3 1.7Mb 20ms DropTail #Set Queue Size of link (n2-n3) to 10 $ns queue-limit $n2 $n3 10 #Give node position (for NAM) $ns duplex-link-op $n0 $n2 orient right-down $ns duplex-link-op $n1 $n2 orient right-up $ns duplex-link-op $n2 $n3 orient right #Monitor the queue for link (n2-n3). (for NAM) $ns duplex-link-op $n2 $n3 queuePos 0.5 #Setup a TCP connection set tcp [new Agent/TCP] $tcp set class_ 2 $ns attach-agent $n0 $tcp set sink [new Agent/TCPSink] $ns attach-agent $n3 $sink $ns connect $tcp $sink $tcp set fid_ 1 #Setup a FTP over TCP connection set ftp [new Application/FTP] $ftp attach-agent $tcp $ftp set type_ FTP #Setup a UDP connection set udp [new Agent/UDP] $ns attach-agent $n1 $udp set null [new Agent/Null] $ns attach-agent $n3 $null $ns connect $udp $null $udp set fid_ 2 #Setup a CBR over UDP connection set cbr [new Application/Traffic/CBR] $cbr attach-agent $udp $cbr set type_ CBR $cbr set packet_size_ 1000 $cbr set rate_ 1mb $cbr set random_ false #Schedule events for the CBR and FTP agents $ns at 0.1 "$cbr start" $ns at 1.0 "$ftp start" $ns at 4.0 "$ftp stop" $ns at 4.5 "$cbr stop" #Detach tcp and sink agents (not really necessary) $ns at 4.5 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink" #Call the finish procedure after 5 seconds of simulation time $ns at 5.0 "finish" #Print CBR packet size and interval puts "CBR packet size = [$cbr set packet_size_]" puts "CBR interval = [$cbr set interval_]" #Run the simulation $ns run

 

Example 1. A Simple NS Simulation Script

The following is the explanation of the script above. In general, an NS script starts with making a Simulator object instance.

• set ns [new Simulator]: generates an NS simulator object instance, and assigns it to variable ns (italics is used for variables and values in this section). What this line does is the following:
   
  • Initialize the packet format (ignore this for now)
  • Create a scheduler (default is calendar scheduler)
  • Select the default address format (ignore this for now)
     

  The "Simulator" object has member functions that do the following:
   

  • Create compound objects such as nodes and links (described later)
  • Connect network component objects created (ex. attach-agent)
  • Set network component parameters (mostly for compound objects)
  • Create connections between agents (ex. make connection between a "tcp" and "sink")
  • Specify NAM display options
  • Etc.
     

  Most of member functions are for simulation setup (referred to as plumbing functions) and scheduling, however some of them are for the NAM display. The "Simulator" object member function implementations are located in the "ns-2/tcl/lib/ns-lib.tcl" file.
   

• $ns color fid color: is to set color of the packets for a flow specified by the flow id (fid). This member function of "Simulator" object is for the NAM display, and has no effect on the actual simulation.
   
• $ns namtrace-all file-descriptor: This member function tells the simulator to record simulation traces in NAM input format. It also gives the file name that the trace will be written to later by the command $ns flush-trace. Similarly, the member function trace-all is for recording the simulation trace in a general format.
   
• proc finish {}: is called after this simulation is over by the command $ns at 5.0 "finish". In this function, post-simulation processes are specified.
   
• set n0 [$ns node]: The member function node creates a node. A node in NS is compound object made of address and port classifiers (described in a later section). Users can create a node by separately creating an address and a port classifier objects and connecting them together. However, this member function of Simulator object makes the job easier. To see how a node is created, look at the files: "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns-node.tcl".
   
• $ns duplex-link node1 node2 bandwidth delay queue-type: creates two simplex links of specified bandwidth and delay, and connects the two specified nodes. In NS, the output queue of a node is implemented as a part of a link, therefore users should specify the queue-type when creating links. In the above simulation script, DropTail queue is used. If the reader wants to use a RED queue, simply replace the word DropTail with RED. The NS implementation of a link is shown in a later section. Like a node, a link is a compound object, and users can create its sub-objects and connect them and the nodes. Link source codes can be found in "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns-link.tcl" files. One thing to note is that you can insert error modules in a link component to simulate a lossy link (actually users can make and insert any network objects). Refer to the NS documentation to find out how to do this.
   
• $ns queue-limit node1 node2 number: This line sets the queue limit of the two simplex links that connect node1 and node2 to the number specified. At this point, the authors do not know how many of these kinds of member functions of Simulator objects are available and what they are. Please take a look at "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns-link.tcl", or NS documentation for more information.
   
• $ns duplex-link-op node1 node2 ...: The next couple of lines are used for the NAM display. To see the effects of these lines, users can comment these lines out and try the simulation.

Now that the basic network setup is done, the next thing to do is to setup traffic agents such as TCP and UDP, traffic sources such as FTP and CBR, and attach them to nodes and agents respectively.

• set tcp [new Agent/TCP]: This line shows how to create a TCP agent. But in general, users can create any agent or traffic sources in this way. Agents and traffic sources are in fact basic objects (not compound objects), mostly implemented in C++ and linked to OTcl. Therefore, there are no specific Simulator object member functions that create these object instances. To create agents or traffic sources, a user should know the class names these objects (Agent/TCP, Agnet/TCPSink, Application/FTP and so on). This information can be found in the NS documentation or partly in this documentation. But one shortcut is to look at the "ns-2/tcl/libs/ns-default.tcl" file. This file contains the default configurable parameter value settings for available network objects. Therefore, it works as a good indicator of what kind of network objects are available in NS and what are the configurable parameters.
   
• $ns attach-agent node agent: The attach-agent member function attaches an agent object created to a node object. Actually, what this function does is call the attach member function of specified node, which attaches the given agent to itself. Therefore, a user can do the same thing by, for example, $n0 attach $tcp. Similarly, each agent object has a member function attach-agent that attaches a traffic source object to itself.
   
• $ns connect agent1 agent2: After two agents that will communicate with each other are created, the next thing is to establish a logical network connection between them. This line establishes a network connection by setting the destination address to each others' network and port address pair.

Assuming that all the network configuration is done, the next thing to do is write a simulation scenario (i.e. simulation scheduling). The Simulator object has many scheduling member functions. However, the one that is mostly used is the following:

• $ns at time "string": This member function of a Simulator object makes the scheduler (scheduler_ is the variable that points the scheduler object created by [new Scheduler] command at the beginning of the script) to schedule the execution of the specified string at given simulation time. For example, $ns at 0.1 "$cbr start" will make the scheduler call a start member function of the CBR traffic source object, which starts the CBR to transmit data. In NS, usually a traffic source does not transmit actual data, but it notifies the underlying agent that it has some amount of data to transmit, and the agent, just knowing how much of the data to transfer, creates packets and sends them.

After all network configuration, scheduling and post-simulation procedure specifications are done, the only thing left is to run the simulation. This is done by $ns run.

 

The above example and information come from the Worcester Polytechnic Institute and from the site Ns by Example. There you can find many examples for Ns Simulator and additional resources.
 

Now that we saw a simple example of how Ns work, try to change the bandwidth of the links between the nodes and watch the drops of the packets in comparison with the bandwidth of the links.