Experiment 3: Simulating the effect of Queuing disciplines on network performance

Experiment-3 :

Simulating the effect of Queuing disciplines on network performance


#  Write a simple tcl script that creates the simple network configuration and runs the simulation scenario shown below:

Figure 3.1: A Network Topology and Simulation Scenario


At the end of this experiment students will be able to:

  1. Understand the algorithm
  2. Configure the network and associated traffic
  3. develop the code using NS2
  4. Analyse the data collected using AWK scrip


  1. This network consists of 4 nodes (n0, n1, n2, n3) as shown in above figure 3.1.
  2. The duplex links between n0 and n2, and n1 and n2 have 2 Mbps of bandwidth and 10 ms of delay.
  3. The duplex link between n2 and n3 has 1.7 Mbps of bandwidth and 20 ms of delay.
  4. Each node uses a Drop-Tail queue, of which the maximum size is 10. A “tcp” agent is attached to n0, and a connection is established to a tcp “sink” agent attached to n3.
  5. As default, the maximum size of a packet that a “tcp” agent can generate is 1KByte.
  6. A tcp “sink” agent generates and sends ACK packets to the sender (tcp agent) and frees the received packets.
  7. A “udp” agent that is attached to n1 is connected to a “null” agent attached to n3.
  8. A “null” agent just frees the packets received.
  9. A “ftp” and a “cbr” traffic generator are attached to “tcp” and “udp” agents respectively, and the “cbr” is configured to generate 1 KByte packets at the rate of 1 Mbps.
  10. The “cbr” is set to start at 0.1 sec and stop at 4.5 sec, and “ftp” is set to start at 1.0 sec and stop at 4.0 sec.

TcL Script


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

Figure 3.2: Output of Network Simulator


AWK Script to analyse the performance


pktDrop = 0;


pktRcd = 0;

ByteRcd =0;



if ($1==”d” && $5==”tcp” || $5==”cbr”)


pktDrop += 1;

ByteDrop += $6;


if ($1==”r” && $5==”TCP” || $5==”cbr”)


                pktRcd += 1;

ByteRcd += $6;




printf(“\nNo of Packet Drop = %d \nNo of Bytes Drop = %d \nNo of Packet Received = %d \nNo of Bytes Received = %d \n”, pktDrop, ByteDrop, pktRcd, ByteRcd);


Figure 3.3: Analysis using AWK





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