NS2 DIRECTORY AND LANGUAGES

NS2 DIRECTORY

Normally NS2 is installed in the directory nsallinone-2.35 and we can see that in the following figure.

Figure given below shows the directory structure under directory nsallinone-2.35.

ns2 directory structure

Here, directory nsallinone-2.35 is on the Level 1. 

On the Level 2, directory tclcl-1.18 contains classes in TclCL (e.g., Tcl, TclObject, TclClass). All NS2 simulation modules are in directory ns-2.35 on the Level 2. Here after, we will refer to directories ns-2.30 and tclcl-1.18 as  ̃ns/ and  ̃tclcl / respectively.

On Level 3, the modules in the interpreted hierarchy are under the directory tcl. Among the directories,the frequently used ones ( ns-lib.tcl,ns-node.tcl,ns-node.tcl) are stored under the directory lib on Level 4.Simulation modules in compiled hierarchy are classified in the directories on level 3.For example,directory tools contains various helper classes such as  random variable generators. Directory common contains basic modules  related to packet forwarding such as simulator,scheduler,packet and connectors.Directories queue,tcp and trace contain modules for queue, TCP [Transmission Control Protocol and tracing respectively.

Network Simulator is mainly based on two languages. They are;

    1) C++.

    2) OTCL.

NS2 ARCHITECTURE

NS2 LANGUAGES

The network simulator is a bank of of different network and protocol objects.

Each of these two languages has its own strengths and weaknesses. NS2 beautifully integrates these two languages to draw out their strengths. For most of the time, you would not need to know the integration mechanism. But you need to know the strength and weaknesses of these two languages in order to apply them properly.

OTCL [Object Oriented Tool Command Language]

OTCL is short for MIT Object TCL and it is an extension to the TCL/TK for object oriented programming.An OTcl program can run on the fly without the need for compilation. Upon execution, the interpreter translates OTcl instructions to machine code understandable to the operating system line by line.Therefore, OTcl codes run more slowly than C++ codes do. The upside of OTcl codes is that every change takes effect immediately.

OTcl helps in the following way:

1. With the help of OTcl, we can describe different network topologies.
2. It helps us to specify the protocols and their applications.
3. It allows fast development.
4. Tcl is compatible with many platforms and it is flexible for integration.
5. Tcl is very easy to use and it is available in free.

C++

Most important parts and kernal of NS2 is created by using C++. A C++ program needs to be compiled (i.e., translated) into the executable machine code. Since the executable is in the form of machine code, C++ program is very fast to run. However, the compilation process can be quite annoying. Every tiny little change like adding “int x = 0;” will take few seconds. 

C++ helps in the following way:

1. It helps to increase the efficiency of simulation.
2. Its is used to provide details of the protocols and their operation.
3. It is used to reduce packet and event processing time.

WHY TWO LANGUAGES???

The main requirement of simulator is;

• Detailed simulation of Protocol: Run-time speed.
• Varying parameters or configuration: easy to use.

C++ is fast to run but slower to code and change. Incase of OTCL, it is easy to code but runs slowly.This is the major reason that we are going behind the using of two languages for scripting and for kernal.