Basics of NS2 and Otcltcl script

NS2 (Network Simulator version 2) is a discrete event network simulator widely used to simulate and analyze computer network behavior. It is open-source software written in C++ and Otcl (Object-oriented Tool Command Language). Otcl is an extension of Tcl (Tool Command Language) used to create and control network entities and configure network scenarios in NS2.

NS2 has a dual-language architecture: the C++ part provides the underlying simulation engine for scheduling events, maintaining network state, and handling low-level packet processing, while the Otcl part provides the user interface for creating and configuring network entities like nodes and links, and specifying network scenarios.

Basic Otcl Script Example

Here's a basic example of an Otcl script that creates a simple network scenario with two nodes and a duplex link:

# Create a Simulator object
set ns [new Simulator]

# Create two nodes
set n0 [$ns node]
set n1 [$ns node]

# Create a duplex link between the nodes
$ns duplex-link $n0 $n1 1Mb 10ms DropTail

# Start the simulation
$ns run

This script creates a Simulator object to control the simulation, two nodes (n0 and n1), and a duplex link between them with 1 Mbps bandwidth, 10 ms delay, and DropTail queue management. The $ns run command starts the simulation.

Essential NS2 Commands

• set ns [new Simulator] Creates a new Simulator object to control the simulation

• set n[i] [$ns node] Creates a new node with unique identifier i

• $ns duplex-link $n0 $n1 bw delay queue-type Creates bidirectional link between nodes

• $ns simplex-link $n0 $n1 bw delay queue-type Creates unidirectional link between nodes

• $ns at time "command" Schedules command execution at specified simulation time

• $ns run Starts and executes the simulation

Command Examples

Creating Network Topology

set ns [new Simulator]

# Create nodes
set n0 [$ns node]
set n1 [$ns node] 
set n2 [$ns node]

# Create links
$ns duplex-link $n0 $n1 1Mb 10ms DropTail
$ns simplex-link $n0 $n2 2Mb 15ms DropTail

Event Scheduling and Simulation Control

# Schedule events
$ns at 1.0 "puts "Simulation started""
$ns at 10.0 "$ns stop"

# Run simulation
$ns run

Key Features

• Protocol Support Implements TCP, UDP, routing protocols like DSR, AODV, DSDV

• Queue Management Supports DropTail, RED, FQ queuing algorithms

• Mobility Models Built-in support for wireless and mobile network simulation

• Trace Analysis Generates detailed trace files for performance analysis

Conclusion

NS2 combines C++ simulation engine with Otcl scripting interface to provide powerful network simulation capabilities. Understanding basic Otcl commands and NS2 architecture enables effective simulation of various network scenarios and protocol implementations.