NS-2 is a Free Open Source Network Simulator.

Definition:
A simulation imitates the behaviour of a system.
Simulate when actual system is too:
Expensive, dangerous or time consuming to learn
Complicated to mould analytically

Many areas use simulation:
Aerospace engineering
Electronic circuit design
Structural design
Computer networks!

NS - The Network Simulator
Started as variant of REAL simulator (1989)
Main participants:
VINT project at LBNL
Xerox PARC
UCB
USC/ISI

Development funded by DARPA, NSF

Significant contributions from user community
UCB Deadelus project
CMU Monarch project
Sun Microsystems

Severals contributions from all around the world that are not part of the distribution NS - Basics
NS is a discrete-event based simulator.
A network is composed from:
Nodes
Links
Agents
Demo: building a simple network with NAM editor
Examples of events:
An agent sends a packet
A node moves to another place
A link fails
Installing and running NS
NS: Internal Organization
NS uses a split programming approach
C++ when speed is required
Processing packets Implementing algorithms
OTcl for combining components and configuration
Specifying a simulation

NS: Overview of a Simulation
1 Specify network configuration in an OTcl script
2 Specify traffic
3 Any other events at network components
4 Run NS
5 NS writes a log of events to a trace file
6 Analyze trace file for events of our interest.
7 Optionally, use a network visualizer like NAM
NS is used in . . .
Research
Education
Network emulation
Introducing simulator elements into a real network
Network protocol implementation
AODV-UU - simulator code formed basis for actual
Implementation