Simulation is the process of learning by doing. Whenever there is something new in the world, we try to analyse it first by examining it and in the process get to learn a lot of things. This entire course is called as Simulation.
Correlating to this process, in order to understand all the complexities one need to model the entire role-play in form of computer simulation, the need is to build artificial objects and assign them roles dynamically.
Computer simulation is the designing of a theoretical physical system on a digital computer with emphasis on model designing, execution and analysis. After creation of the mathematical model the most important step is to create a computer program for updating the state and event variables through time (by time slicing or event scheduling). If this simulation is carried out successively in parallel computers, it is called Parallel or Distributed simulation.
Network simulation (NS) is one of the types of simulation, which is used to simulate the networks such as in MANETs, VANETs etc. It provides simulation for routing and multicast protocols for both wired and wireless networks. NS is licensed for use under version 2 of the GNU (General Public License) and is popularly known as NS2. It is an object-oriented, discrete event-driven simulator written in C++ and Otcl/tcl.
NS-2 can be used to implement network protocols such as TCP and UPD, traffic source behavior such as FTP, Telnet, Web, CBR and VBR, router queue management mechanism such as Drop Tail, RED and CBQ, routing algorithms and many more. In ns2, C++ is used for detailed protocol implementation and Otcl is used for the setup. The compiled C++ objects are made available to the Otcl interpreter and in this way, the ready-made C++ objects can be controlled from the OTcl level.
Install NS-2 using this command :
sudo apt-get install ns2
Nam is also needed to install. Nam (Network Animator) is an animation tool to graphically represent the network and packet traces. Use this command :
sudo apt-get install nam
Some basic Otcl script syntax :
- Basic Command :
set
a
8
set
b [expr $a
/
8
]
Explanation : In the first line, the variable a is assigned the value 8. In the second line, the result of the command [expr $a/8], which equals 1, is then used as an argument to another command, which in turn assigns a value to the variable b. The “$” sign is used to obtain a value contained in a variable and square brackets are an indication of a command substitution.
- Define new procedures with proc command
proc factorial fact {
if
{$fact <
=
1
} {
return
1
}
expr $fact
*
[factorial [expr $fact
-
1
]]
}
- To open a file for reading :
set
testfile [
open
hello.dat r]
Similarly, put command is used to write data into the file
set
testfile [
open
hello.dat w]
puts $testfile “hello1”
- To call subprocesses within another process, exec is used, which creates a subprocess and waits for it to complete.
exec
rm $testfile
- To be able to run a simulation scenario, a network topology must first be created. In ns2, the topology consists of a collection of nodes and links.
set
ns [new Simulator]
- The simulator object has member functions which enables to create the nodes and define the links between them. The class simulator contains all the basic functions. Since ns was defined to handle the Simulator object, the command $ns is used for using the functions belonging to the simulator class.
In the network topology nodes can be added in the following manner :
set
n0 [$ns node]
set
n1 [$ns node]
- Traffic agents (TCP, UDP etc.) and traffic sources (FTP, CBR etc.) must be set up if the node is not a router. It enables to create CBR traffic source using UDP as transport protocol or an FTP traffic source using TCP as a transport protocol.
CBR traffic source using UDP :
set
udp0 [new Agent
/
UDP]
$ns attach
-
agent $n0 $udp0
set
cbr0 [new Application
/
Traffic
/
CBR]
$cbr0 attach
-
agent $udp0
$cbr0
set
packet_size_
512
FTP traffic source using TCP :set
tcp0 [new Agent
/
TCP]
$ns attach
-
agent $n0 $tcp0
set
ftp0 [new Application
/
FTP]
$ftp0 attach
-
agent $tcp0
$tcp0
set
packet_size_
512
Below is the implementation of creating links between the source and destination using both ftp and tcp :
# Create a simulator object
set
ns [new Simulator]
# Define different colors
# for data flows (for NAM)
$ns color
1
Blue
$ns color
2
Red
# Open the NAM trace file
set
nf [
open
out.nam w]
$ns namtrace
-
all
$nf
# Define a 'finish' procedure
proc finish {}
{
global
ns nf
$ns flush
-
trace
# Close the NAM trace file
close $nf
# Execute NAM on the trace file
exec
nam out.nam &
exit
0
}
# Create four nodes
set
n0 [$ns node]
set
n1 [$ns node]
set
n2 [$ns node]
set
n3 [$ns node]
# Create links between the nodes
$ns duplex
-
link $n0 $n2
2Mb
10ms
DropTail
$ns duplex
-
link $n1 $n2
2Mb
10ms
DropTail
$ns duplex
-
link $n2 $n3
1.7Mb
20ms
DropTail
# Set Queue Size of link (n2-n3) to 10
$ns queue
-
limit $n2 $n3
10
# Give node position (for NAM)
$ns duplex
-
link
-
op $n0 $n2 orient right
-
down
$ns duplex
-
link
-
op $n1 $n2 orient right
-
up
$ns duplex
-
link
-
op $n2 $n3 orient right
# Monitor the queue for link (n2-n3). (for NAM)
$ns duplex
-
link
-
op $n2 $n3 queuePos
0.5
# Setup a TCP connection
set
tcp [new Agent
/
TCP]
$tcp
set
class_
2
$ns attach
-
agent $n0 $tcp
set
sink [new Agent
/
TCPSink]
$ns attach
-
agent $n3 $sink
$ns connect $tcp $sink
$tcp
set
fid_
1
# Setup a FTP over TCP connection
set
ftp [new Application
/
FTP]
$ftp attach
-
agent $tcp
$ftp
set
type_ FTP
# Setup a UDP connection
set
udp [new Agent
/
UDP]
$ns attach
-
agent $n1 $udp
set
null [new Agent
/
Null]
$ns attach
-
agent $n3 $null
$ns connect $udp $null
$udp
set
fid_
2
# Setup a CBR over UDP connection
set
cbr [new Application
/
Traffic
/
CBR]
$cbr attach
-
agent $udp
$cbr
set
type_ CBR
$cbr
set
packet_size_
1000
$cbr
set
rate_
1mb
$cbr
set
random_ false
# Schedule events for the CBR and FTP agents
$ns at
0.1
"$cbr start"
$ns at
1.0
"$ftp start"
$ns at
4.0
"$ftp stop"
$ns at
4.5
"$cbr stop"
# Detach tcp and sink agents
# (not really necessary)
$ns at
4.5
"$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink"
# Call the finish procedure after
# 5 seconds of simulation time
$ns at
5.0
"finish"
# Print CBR packet size and interval
puts
"CBR packet size = [$cbr set packet_size_]"
puts
"CBR interval = [$cbr set interval_]"
# Run the simulation
$ns run
Output :
This article is attributed to GeeksforGeeks.org
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