LINUX
Linux refers to the family of Unix-like computer operating systems using the
Linux kernel. Linux can be installed on a wide variety of computer hardware, ranging from mobile phones, tablet computers and video game consoles, to mainframes and supercomputers. Desktop use of Linux has increased in recent years, partly owing to the popular Ubuntu, Fedora, and openSUSE distributions and the emergence of netbooks and smart-phones running an embedded Linux.
The development of Linux is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed, both commercially and non-commercially, by anyone under licenses such as the GNU General Public License. Typically Linux is packaged in a format known as a Linux distribution for desktop and server use. Linux distributions include the Linux kernel and all of the supporting software required to run a complete system, such as utilities and libraries, the X Window System, the GNOME and KDE desktop environments, and the Apache HTTP Server. Commonly used applications with desktop Linux systems include the Mozilla Firefox web-browser, the
OpenOffice.org office application suite and the GIMP image editor.
The name "Linux" comes from the Linux kernel, originally written in 1991 by
Linus Torvalds. The main supporting user space system tools and libraries from the
GNU Project (announced in 1983 by Richard Stallman) are the basis for the Free
Software Foundation's preferred name GNU/Linux.
NETWORK SIMULATOR NS-2
The network simulator covers a very large number of application, protocols, network types, network elements, and traffic models. We call these as “simulation objects.” Our goals are to learn how to use ns, and to become familiar and to understand the operations of some of the simulated objects using ns. We will not provide only some basis and description of ns, but a description of the simulated

objects will be provided as well. Finally, we will focus on the analysis of the behavior of the simulated objects using ns.
This course is intended to help students who want to learn through simple examples how to analyze some simulated object using ns. In that purpose, we will provide a number of tcl scripts that can be used so as to start programming immediately. For students who are interested to learn from examples, we should mention that a very large number of examples are already available in the software packages of the simulator (it typically appears in the directory ns-2/tcl/ex). Other tutorials containing many examples are available electronically: Marc Greis
( http://www.isi.edu/nsnam/ns/tutorial/index.html
), and the tutorial by Jae Chung and
Mark Claypool ( http://nile.wpi.edu/NS/ ). For a much deeper study of the ns simulator, one should refer to the ns manual which is maintained up-to-date at http://www.isi.edu/nsnam/ns/ .
Background on The Network Simulator ns-2
During the last decade there were a lot of studies and works produced pertaining to the application, protocols, network types, network elements, and traffic models. Most of these studies and works have been based on the use of simulators. Network simulator ns-2 has been the most used network simulator for these studies. Most mobile ad-hoc research groups employ the ns-2 to implement and validate various algorithms, since it provides a kind of integration environment. The virtual environment provided by simulation makes it a highly beneficial approach for an assortment of favorite characteristics, such as network modeling depending on specified criteria and analyzing its performance under different scenarios.
Network simulator ns-2 is an open-source event-driven simulator designed specifically for research in computer communication networks. Since its inception in
1989, NS2 has continuously gained tremendous interest from industry, academia, and government. The network simulator ns-2 is developed by the VINT project research group at the University of California of Berkley. The simulator has been extended by the Monarch Research group at the Carnegie Mellon University to support node mobility including a realistic physical layer, medium access control, and ad-hoc routing

protocols. Having been under constant investigation and enhancement for years, NS2 now contains modules for numerous network components such as routing, transport layer protocol, application, etc.
Network simulator ns-2 can be defined as a simulator that is written in two languages, C++ and OTcl (object-oriented tool command language), with the concept of object oriented. The topology of simulation is written with Tcl, and it has been linked with the modules of the simulator that are written in C++ through the use of
OTcl linkages (the universal functioning could be represented in the figure below). The process of error debugging became more complicated with ns-2, and it is considered to be a drawback with the use of this simulator, since there is a combination of two languages with this simulation.
The overall functioning of ns-2
The attractive issue with simulation that makes this environment convenient rather than the real-world environment is the ability of the simulation to be carefully controlled, while with the real-world we might encounter numerous difficulties that restrict efficient protocol implementation and testing.

Tcl: Tool Command Language is used for millions of people in the real world. It is a language with very simple syntaxes and it allows a very easy integration with other languages. Tcl was created by Jhon Ousterhout. The characteristics of this language are:
 It allows a fast development.
 It provides a graphic interface.
 It is compatible with many platforms.
 It is flexible for integration.
 It is free and easy to use. otcl: OTcl usually refers to an object oriented extension of Tcl created by David
Wetherall. It is used in network simulator (ns-2) and usually run under Unix environment. sudo: Super-User DO: it is a program for some Unix and Unix-like computer operating systems that allows users to run programs with the security privileges of another user
(normally the superuser, or root). apt: The Advanced Packaging Tool, or APT, is a free user interface that works with core libraries to handle the installation and removal of software on the Debian
GNU/Linux distribution and its variants. APT simplifies the process of managing software on Unix-like computer systems by automating the retrieval, configuration and installation of software packages, either from binary files or by compiling source code.
Tclcl : (Tcl with classes) is a Tcl/C++ interface used by Mash, vic, vat, rtp_play, ns, and nam. It provides a layer of C++ glue over OTcl.