How to write Publisher Subscriber in ROS Noetic | Step by Step instructions

ROS1 Noetic -Publisher and Subscriber Example

This post tells about the implementation of a publisher and a subscriber in ROS Noetic.  

In this example, we have considered the following:

Example 1: 

One publisher (pub.py) and one Subscriber (Sub.py)

Topic: first_lecture

Message: String Message 

Example 2:

Two publishers (pub.py and pub1.py) and

Two subscribers (sub.py and sub1.py)

Topic: first_lecture 

See the following video for step by step instructions for running this example:

The following picture shows the nodes (The picture is taken by running the rqt_graph command)

Publisher Subscriber in ROS Noetic

How to run in ROS Noetic

Prerequsities 

1. Ubuntu Basic commands

2. PATH Setting in Ubuntu

3. Basic knowledge of Linux 

4. Programming fundamentals

To Install ROS1 Noetic in Ubuntu 20.04, you can refer the following video in my channel.

ROS is enabled with a catkin make system which perform the easier build operation and cross compilation enablement.

Create a workspace to store all our files.

/home/pradeepkumar/pradeep_ws 

To create a workspace:

Open a terminal 

$ mkdir -p pradeep_ws/src

$ cd pradeep_ws/

$ catkin_make 

Then set the path 

Copy the following line in the /home/pradeepkumar/.bashrc file 

source /home/pradeepkumar/pradeep_ws/devel/setup.bash

]alternative way to set the environment, but for every terminal to be opened, we need to do the following step.

$ source $HOME/pradeep_ws/devel/setup.bash

To create a Catkin package, here is the command

$ cd pradeep_ws/src

$ catkin_create_pkg ros_tutorial1 rospy std_msgs

$ cd pradeep_ws/src/ros_tutorial1

$ mkdir script

create the pub.py, pub1.py, sub.py and sub1.py (You can download the source code from https://github.com/tspradeepkumar/ROSNoteic 

$ chmod 777 *.py

ROS Noetic runs always with a master that means it should run roscore to run all the ros applications.

You need to open 5 Terminals and give the commands in each of the terminals

• $ roscore
• $ rosrun ros_tutorial1 pub.py
• $ rosrun ros_tutorial1 pub1.py
• $ rosrun ros_tutorial1 sub.py
• $ rosrun ros_tutorial1 sub1.py

Open a terminal and run the command roscore

$ roscore

Open another terminal for publisher  and run the following command 

$ rosrun ros_tutorial1 pub.py

You will get the output as ROS Noetic World

If ros_tutorial1 is not found, then set the following command and try again 

$ source $HOME/pradeep_ws/devel/setup.bash

Open one more ternminal for the subscriber and run the following command 

$ rosrun ros_tutorial1 sub.py

You will get the output as I learn about ROS Noetic World. 

Run the following command to know about the graph 

$ rqt_graph 

You can see the following graph (You need to untick the Debug checkbox, to see the complete nodes running in ROS Noetic) 

ROS Noetic Pub Sub

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Telnet and FTP in Wired networks using NS2 (Network Simulator 2)

Telnet and FTP for a Wired Networks

This post tells about the use of Telnet and FTP in Network Simulator 2 for a wired network. This can be extended to a wireless network as well.

The following is the source code for implementing Telnet and FTP application in a Wired network scenario. 

The network looks like this:

Telnet in NS2

Node 0 to Node 2 is enabled with Telnet Application and Node 1 to Node 3 is enabled with FTP Application.  Save the following file as wired.tcl

#===================================
#     Simulation parameters setup
#===================================
set val(stop)   10.0  ;# time of simulation end

#===================================
#        Initialization        
#===================================
#Create a ns simulator
set ns [new Simulator]

#Open the NS trace file
set tracefile [open out.tr w]
$ns trace-all $tracefile

#Open the NAM trace file
set namfile [open out.nam w]
$ns namtrace-all $namfile

#===================================
#        Nodes Definition        
#===================================
#Create 5 nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]

#===================================
#        Links Definition        
#===================================
#Createlinks between nodes
$ns duplex-link $n0 $n4 100.0Mb 10ms DropTail
$ns queue-limit $n0 $n4 50
$ns duplex-link $n1 $n4 100.0Mb 10ms DropTail
$ns queue-limit $n1 $n4 50
$ns duplex-link $n2 $n4 100.0Mb 10ms DropTail
$ns queue-limit $n2 $n4 50
$ns duplex-link $n3 $n4 100.0Mb 10ms DropTail
$ns queue-limit $n3 $n4 50
$ns duplex-link $n3 $n0 100.0Mb 10ms DropTail
$ns queue-limit $n3 $n0 50

#        Agents Definition        
#===================================
#Setup a UDP connection
set tcp0 [new Agent/TCP]
$ns attach-agent $n0 $tcp0
set sink1 [new Agent/TCPSink]
$ns attach-agent $n2 $sink1
$ns connect $tcp0 $sink1

#Setup a UDP connection
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1
set null3 [new Agent/Null]
$ns attach-agent $n3 $null3
$ns connect $udp1 $null3
$udp1 set packetSize_ 1500


#===================================
#        Applications Definition        
#===================================
#Setup a FTP Application over TCP connection
set ftp1 [new Application/FTP]
$ftp1 attach-agent $tcp0
$ns at 1.0 "$ftp1 start"
$ns at 10.0 "$ftp1 stop"

#Setup a Telnet Application over UDP connection
set telnet0 [new Application/Telnet]
$telnet0 set interval_ 0.001
$telnet0 attach-agent $udp1
$ns at 1.0 "$telnet0 start"
$ns at 10.0 "$telnet0 stop"
#$ns at 10.0 "$cbr1 stop"
$telnet0 set type_ Telnet

#===================================
#        Termination        
#===================================
#Define a 'finish' procedure
proc finish {} {
    global ns tracefile namfile
    $ns flush-trace
    close $tracefile
    close $namfile
    exec nam out.nam &
    exit 0
}
$ns at $val(stop) "$ns nam-end-wireless $val(stop)"
$ns at $val(stop) "finish"
$ns at $val(stop) "puts \"done\" ; $ns halt"
$ns run

When you run the above file using the command 

$ ns wired.tcl 

and the output generated is out.nam and out.tr 

The packet transmission is shown like this in the animation window:

Telnet in ns2

Node 0 to Node 2 is enabled with Telnet Application and Node 1 to Node 3 is enabled with FTP Application, which is shown in the above picture.

To get the throughput of the above file in bits per second, here is the awk script: 

Save the following in a file called telnet.awk  and store in the same place where the wired.tcl is also stored. 

BEGIN {
numTCP1=0;
tcpSize1=0;
numTCP2=0;
tcpSize2=0;
totaltcp1=0;
totaltcp2=0;
}
{
event=$1;
pkttype= $5;
fromnode=$9;
tonode=$10;
pktsize=$6;
if(event == "r" &&pkttype == "udp" &&fromnode == "1.0" &&tonode == "3.0")
{
numTCP1++;
tcpSize1 = pktsize;
}
if(event == "r" &&pkttype == "tcp" &&fromnode == "0.0" &&tonode == "2.0")
{
numTCP2++;
tcpSize2 = pktsize;

}

}

END {
totaltcp1=numTCP1*tcpSize1*8;
totaltcp2=numTCP2*tcpSize2*8;
throughputtcp1= totaltcp1/24; # because simulation time is 24.5   0.5 = 24
throughputtcp2= totaltcp2/24; # because simulation time is 24.5   0.5 = 24
printf("The Throughput of FTP application is %d \n", throughputtcp1);
printf("The Throughput of TELNET application is %d \n", throughputtcp2);
}

The above file can be run using the command:

$ gawk -f telnet.awk out.tr

The above command will print the following two lines which informs the throughput of using Telnet and FTP

The Throughput of FTP application is 8962000 
The Throughput of TELNET application is 3058293 

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