Basics of NS2 and Otcl/tcl script

Simulation is the process of learning by doing. Whenever there is something new in the world, we try to analyse it first by examining it and in the process get to learn a lot of things. This entire course is called as Simulation.

Correlating to this process, in order to understand all the complexities one need to model the entire role-play in form of computer simulation, the need is to build artificial objects and assign them roles dynamically.

Computer simulation is the designing of a theoretical physical system on a digital computer with emphasis on model designing, execution and analysis. After creation of the mathematical model the most important step is to create a computer program for updating the state and event variables through time (by time slicing or event scheduling). If this simulation is carried out successively in parallel computers, it is called Parallel or Distributed simulation.

Network simulation (NS) is one of the types of simulation, which is used to simulate the networks such as in MANETs, VANETs etc. It provides simulation for routing and multicast protocols for both wired and wireless networks. NS is licensed for use under version 2 of the GNU (General Public License) and is popularly known as NS2. It is an object-oriented, discrete event-driven simulator written in C++ and Otcl/tcl.
NS-2 can be used to implement network protocols such as TCP and UPD, traffic source behavior such as FTP, Telnet, Web, CBR and VBR, router queue management mechanism such as Drop Tail, RED and CBQ, routing algorithms and many more. In ns2, C++ is used for detailed protocol implementation and Otcl is used for the setup. The compiled C++ objects are made available to the Otcl interpreter and in this way, the ready-made C++ objects can be controlled from the OTcl level.
 
Install NS-2 using this command :

sudo apt-get install ns2

Nam is also needed to install. Nam (Network Animator) is an animation tool to graphically represent the network and packet traces. Use this command :

sudo apt-get install nam

Some basic Otcl script syntax :

• Basic Command :

  set a 8 set b [expr $a/8]

  Explanation : In the first line, the variable a is assigned the value 8. In the second line, the result of the command [expr $a/8], which equals 1, is then used as an argument to another command, which in turn assigns a value to the variable b. The “$” sign is used to obtain a value contained in a variable and square brackets are an indication of a command substitution.

 

• Define new procedures with proc command

  proc factorial fact {        if {$fact <= 1} {            return 1     } expr $fact * [factorial [expr $fact-1]]    }

 

• To open a file for reading :

  set testfile [open hello.dat r]

  Similarly, put command is used to write data into the file

  set testfile [open hello.dat w]    puts $testfile “hello1”

 

• To call subprocesses within another process, exec is used, which creates a subprocess and waits for it to complete.

  exec rm $testfile

   

• To be able to run a simulation scenario, a network topology must first be created. In ns2, the topology consists of a collection of nodes and links.

  set ns [new Simulator]

 

• The simulator object has member functions which enables to create the nodes and define the links between them. The class simulator contains all the basic functions. Since ns was defined to handle the Simulator object, the command $ns is used for using the functions belonging to the simulator class.

  In the network topology nodes can be added in the following manner :

  set n0 [$ns node]    set n1 [$ns node]

 

• Traffic agents (TCP, UDP etc.) and traffic sources (FTP, CBR etc.) must be set up if the node is not a router. It enables to create CBR traffic source using UDP as transport protocol or an FTP traffic source using TCP as a transport protocol.

  CBR traffic source using UDP :

  set udp0 [new Agent/UDP]    $ns attach-agent $n0 $udp0 set cbr0 [new Application/Traffic/CBR] $cbr0 attach-agent $udp0 $cbr0 set packet_size_ 512

   
  FTP traffic source using TCP :

  set tcp0 [new Agent/TCP] $ns attach-agent $n0 $tcp0 set ftp0 [new Application/FTP] $ftp0 attach-agent $tcp0 $tcp0 set packet_size_ 512

Below is the implementation of creating links between the source and destination using both ftp and tcp :

Output :