Installation & Configuration of NS2 in Linux environment and building basic LAN topology in NS2.


Aim: Installation & Configuration of NS2 in Linux environment and building basic LAN topology in NS2. 
 
Apparatus (Software): NS2 simulator.

Procedure: Following should be studied to understand this practical.

NS2 (Network Simulator version 2)

1. What is Network Simulator?

Setting up a network to do some real experiments is the best way for studying about communication in internet. However, setting a network is not easy and costly. For this reason, a virtual network provided by network simulator is used for experiment in only one computer. Specially, NS2 which is free and easy to use is the popular all over the world.

For example, to observe the communication between A,B,C,D in a network as shown in Fig. 1(a), we can set up a network topology as shown in Fig.1(b) for simulator to do experiment.
(a) The reality network
(b) The network for simulation 
 

 
2. NS2 Manual

NS2 use Tcl language for creating simulation scenario file (for example, sample.tcl). Network topology, transmission time, using protocol etc... are defined in scenario file. If we execute this scenario file, the simulation result will be output to out.tr and out.nam file.

out.tr all the information about communication is written in this file. We can find out the way a packet was forwarded. This file is called as trace file.
out.nam contains the data for animation of the experiment result. This file can be execute by Nam, an animation software. The state of forwarding packet in Nam is shown in Fig.2.

If simulation use TCP, we can also observe the state of TCP congestion control by a trace file.
Figure 2: Animation soft Nam

out.tcp Record the change of TCP parameters by time. Using Gnuplot software to plot a graph, it is easy to observe the appearance of congestion control. This file is called as TCP trace file.

3. The following is steps to creating a scenario file.

Step 0: Declare Simulator and setting output file
Step 1: Setting Node and Link
Step 2: Setting Agent
Step 3: Setting Application
Step 4: Setting Simulation time and schedules
Step 5: Declare finish.


3.1 Step 0: Declare Simulator and setting output file

Declare simulator and setting output file is written as below.
--------------------------------------------------
set ns [new Simulator]
set file [open out.tr w]
$ns trace-all $file
set namfile [open out.nam w] 
$ns namtrace-all $namfile
set tcpfile [open out.tcp w]
Agent/TCP set trace_all_oneline_ true
--------------------------------------------------

3.2 Step1: Setting Node and Link

Setting a node is shown as below.
-------------------------------------------------
set n0 [$ns node]
--------------------------------------------------
then the node which have name as n0 is ready to use. (in Tcl file it will refer to $n0). Node is numbered from 0. Link is declared as below.


--------------------------------------------------
$ns duplex-link $n0 $n2 3Mb 5ms DropTail
$ns duplex-link-op $n0 $n2 orient right-down
--------------------------------------------------
In line 1, the link between two nodes n0 and n2 has bandwidth 3Mbps and delay 5ms. A biconnection link between n0 and n2 is declared. DropTail is a waiting queue type, if the queue overflow then the new entered packet will be dropped, similar to buffer of general realistic network. Line 2 setting position of node and link for Nam. It does not affect to the result of simulation. The length of queue is set as below
--------------------------------------------------
$ns queue-limit $n2 $n3 20
$ns duplex-link-op $n2 $n3 queuePos 0.5
--------------------------------------------------
In line 1, the length of Queue on the link from n2 to n3 is 20[packets]. In line 2, the position of Queue is set for executing simulation result on Nam, 0.5 is the angle between link and queue, it equal to (0.5π).

3.3 Step2: Setting Agent

UDP Agent

Using UDP for simulation, the sender set the Agent as UDP Agent while the receiver set to Null Agent. Null Agents do nothing except receiving the packet. Setting of Agent as flowing.
--------------------------------------------------
set udp [new Agent/UDP]
$ns attach-agent $n0 $udp
set null [new Agent/Null]
$ns attach-agent $n3 $null
$ns connect $udp $null
$udp set fid_ 0
$ns color 0 blue
--------------------------------------------------
In first 4 lines, udp, null Agent is set for n0, n3. Line 5 declared the transmission between udp and
null. Line 6 set the number for data flow of udp. This number will be recorded to all packet which are sent from udp. By using this number, we can easily observer the flow that packet is belong to by looking up trace file. Similarly, in line 7, we mark the color to discrete packet for showing result on Nam.

TCP Agent

Using TCP for simulation, the sender set the Agent as TCP Agent while the receiver set to TCPSink
Agent. When receiving a packet, TCPSink Agent will reply an acknowledgment packet (ACK). Setting Agent for TCP is similar to UDP.
--------------------------------------------------
set tcp [new Agent/TCP]
$ns attach-agent $n1 $tcp
set sink [new Agent/TCPSink]
$ns attach-agent $n3 $sink
$ns connect $tcp $sink
$tcp set fid_ 1
$ns color 1 red
--------------------------------------------------

Confirm that fid and color is differ from udp.

Trace file of TCP is written as below.
--------------------------------------------------
$tcp attach-trace $tcpfile
$tcp trace cwnd_
--------------------------------------------------

3.4 Step3: Setting Application

In general, UDP Agent use CBR Application while TCP Agent use FTP Application.
--------------------------------------------------
set cbr [new Application/Traffic/CBR]
$cbr attach-agent $udp
set ftp [new Application/FTP]
$ftp attach-agent $tcp
--------------------------------------------------

3.5 Step4: Setting time schedule for simulation

Time schedule of a simulation is setting as below:
--------------------------------------------------
$ns at 1.0 "$cbr start"
$ns at 1.5 "$ftp start"
$ns at 3.0 "$ftp stop"
$ns at 3.5 "$cbr stop"
$ns at 4.0 "finish"
proc finish {} { 
global ns file namfile tcpfile
$ns flush-trace
close $file
close $namfile
close $tcpfile
exit 0
}
--------------------------------------------------
cbr transmit data from 1.0[sec] to 3.5[sec] and ftp transmit data from 1.5[sec] to 3.0[sec]. The finish
function is use for output data file which always at the end of simulation.

3.6 Step5: Declare finish

After finish setting, declare finish is written at the end of file.
--------------------------------------------------
$ns run
--------------------------------------------------




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