NS by example

NS by Example

Jae Chung

and

difficult for a first time user, because there are few user-friendly manuals.

Even though there is a lot of documentation written by the developers

which has in depth explanation of the simulator, it is written with the

depth of a skilled NS user. The purpose of this project is to give a new

user some basic idea of how the simultor works, how to setup simulation

networks, where to look for further information about network

components in simulator codes, how to create new network components,

etc., mainly by giving simple examples and brief explanations based on

our experiences. Although all the usage of the simulator or possible

network simulation setups may not be covered in this project, the project

and CBQ, routing algorithms such as Dijkstra, and more. NS also

implements multicasting and some of the MAC layer protocols for LAN

simulations. The NS project is now a part of the VINT project that

develops tools for simulation results display, analysis and converters that

convert network topologies generated by well-known generators to NS

formats. Currently, NS (version 2) written in C++ and OTcl (Tcl script

language with Object-oriented extensions developed at MIT) is available.

This document talks briefly about the basic structure of NS, and explains

in detail how to use NS mostly by giving examples. Most of the figures

that are used in describing the NS basic structure and network

Documentation"), modified little bit as needed. For more information

about NS and the related tools, visit the VINT project home page.

As shown in Figure 1, in a simplified user's view, NS is Object-oriented Tcl

(OTcl) script interpreter that has a simulation event scheduler and

network component object libraries, and network setup (plumbing)

module libraries (actually, plumbing modules are implemented as

member functions of the base simulator object). In other words, to use

NS, you program in OTcl script language. To setup and run a simulation

network, a user should write an OTcl script that initiates an event

scheduler, sets up the network topology using the network objects and

the plumbing functions in the library, and tells traffic sources when to

start and stop transmitting packets through the event scheduler. The

appropriate object. When a user wants to make a new network object, he

or she can easily make an object either by writing a new object or by

plumbing OTcl modules actually make the job very easy. The power of

NS comes from this plumbing.

Another major component of NS beside network objects is the event

scheduled time and the pointer to an object that handles the event. In

NS, an event scheduler keeps track of simulation time and fires all the

events in the event queue scheduled for the current time by invoking

appropriate network components, which usually are the ones who issued

the events, and let them do the appropriate action associated with

packet pointed by the event. Network components communicate with

one another passing packets, however this does not consume actual

switched to the scheduler as an event 20 microsecond later. The

scheduler after 20 microsecond dequeues the event and fires it to the

switch component, which then passes the packet to an appropriate

out for retransmission (transmission of a packet with the same TCP

packet number but different NS packet ID). Timers use event schedulers

in a similar manner that delay does. The only difference is that timer

measures a time value associated with a packet and does an appropriate

action related to that packet after a certain time goes by, and does not

simulate a delay.

NS is written not only in OTcl but in C++ also. For efficiency reason, NS

separates the data path implementation from control path

implementations. In order to reduce packet and event processing time

(not simulation time), the event scheduler and the basic network

component objects in the data path are written and compiled using C++.

member functions and variables to a C++ linked OTcl object. The objects