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FIT/CorteXlab 101

We will introduce the usage of FIT/CorteXlab through a simple OFDM transmission between two nodes in FIT/CorteXlab's experiment room, transmitting dummy packets between them.

Division of resources will be according to the table below:

Group SSH login Nodes to use Frequency TCP Port Connected Leader
1 tuto1 node 3 and node 4 2.400GHz 6663 X Djibrilla
2 cadjih node 7 and node 8 2.410GHz 6664 X Cedric
3 hezzeddine node 13 and node 14 2.420GHz 6665 X Ezzeddine
4 dadja node 17 and node 18 2.430GHz 6666 X Dadja
5 tuto4 node 23 and node 24 2.440GHz 6667 X Satya
6 ymestrah node 25 and node 27 2.450GHz 6668 X Yasser
7 tuto7 node 28 and node 32 2.460GHz 6669 X Thomas
8 amayouche node 33 and node 34 2.470GHz 6670 X Abdou
9 aparis node 37 and node 38 2.480GHz 6671 X Antoine
10 lmroueh node 16 and node 35 2.490GHz 6672 X Lina
11 gcallebaut node 6 and node 9 2.500GHz 6673 X Gilles

First Part: OFDM Transmission and Reception

This first part of the tutorial is based on the example digital OFDM benchmark transmitter and receiver code present in the vanilla GNU Radio. For more info on the OFDM benchmark, please refer to the GNU Radio documentation.

Start your virtual FIT/CorteXlab work environment

To speed up things, the tutorial will be run from inside your virtual FIT/CorteXlab work environment, the virtual machine you've installed before, in the Prerequisites section.

Assuming you have correctly followed the steps described in that section, you can now fire up VirtualBox

Log into the virtual machine with username:cxlbusr and with an empty password

Once the LXDE interface is fired up, open an LXTerminal from the Start Menu > System tools

We're now ready to start

Logging into the SSH front-end

You can start by logging into the serveur “airlock” SSH front-end with the following command:

you@your-pc:~$ ssh -X -v <your-login>@airlock 

If everything goes well you should get the FIT/CorteXlab welcome screen and an srvairlock prompt.

Cloning the example task

An example task is provided in the FIT/CorteXlab GitHub repositories. It is kept up-to-date and well prepared to run. First let us prepare to clone the example task.

your-login@srvairlock:~$ git clone
your-login@srvairlock:~$ cd examples
your-login@srvairlock:~/examples$ cd my_task

Inspecting the example task

Let's get inside and see what it's all about:

your-login@srvairlock:~/examples/my_task$ ls  scenario.yaml

Let's go over each one of these files:

  • the GNU Radio python script for the OFDM transmitter
  • the GNU Radio python script for the OFDM receiver
  •, and are helper codes for both benchmark python scripts
  • scenario.yaml: a scenario description file (in yaml format), which assigns the roles of “transmitter” and “receiver” to a set of nodes

Now, let's inspect the scenario description file, to understand what will happen during this experiment:

your-login@srvairlock:~/examples/my_task$ less scenario.yaml

Which will give you this:

# Example scenario description file
#   All lines starting with "#" and empty lines are ignored

# Scenario textual description
#   simple string (a one liner)
description: CorteXlab Tutorial 1

# Experiment maximum duration
#   Time after which the experiment is forced to stop
#   integer (seconds)
duration: 300

# Node list
#   format:
#   nodes:
#     (machine):
#       command: (entry point script relative to the task root)


    command: ./ --antenna="TX/RX" --rx-gain=25 -v -W 2M -f 2.49G
    passive: true

    command: ./ --antenna="TX/RX" --tx-amplitude=0.2 -v -W 2M -f 2.49G

The file is self-explanatory, but for now let's ignore all but the indented lines following node4: and node6:. You'll have the opportunity to understand the rest better later on.

Let's go over each said line now:

  • node4:: This opens the node4 declaration of options
  • command: ./ –antenna=“TX/RX” –rx-gain=25 -v -W 2M -f 2.49G: states the command to run on node4. Details of the parameters are:
    • –antenna=“TX/RX”: this indicates which antenna connector to use on the USRP radio platform connected to the node
    • –rx-gain=25: this sets the receive gain of the USRP radio platform
    • -v: this puts the script in verbose mode, printing debug information
    • -W 2M: this sets the bandwidth of the signal to 2 MHz
    • -f 2.49G: this sets the carrier frequency of the signal to 2.49 GHz
  • passive: true: the node 4 will be passive, meaning it will stop when all active nodes are finished

Same reasoning follows for the section on node6. So this is the scenario at hand:

  • Node4 behaves as the OFDM receiver
  • Node6 behaves as the OFDM transmitter

For more info on where these nodes are located inside the platform, please check the node position map at the home of this wiki.

Changing the scenario file

We'll need to change the scenario file in order to reflect the nodes that your group is going to use. Please refer to the table at the top for the two nodes and the frequency you're going to use. Use your preferred unix editor to edit the file:

your-login@srvairlock:~/examples/my_task$ pico scenario.yaml

Change node4 and node6 to the two nodes that have been assigned to your group. The order is not of importance. We will also change the default frequency of 2.49G to the one assigned to your group.

Be sure that the file remains the same aside from those three changes. Any syntax error on the files will produce an error when processed by FIT/CorteXlab.

For the task to take a while longer, we will also need to change the number of packets to transmit. For the transmitter node's parameters (line command: ./ add at the end of the line the option -M 10.

Save the file ctrl+X and then Y for the pico editor.

Creating the task file

Before submitting the task to the nodes, we need first to put the task into a format that can be readily understood by Minus. Minus is the experiment controller code, responsible for doing the dirty stuff for you:

  • Firing up the right nodes to be used
  • Copying the code onto the nodes
  • Starting everything synchronously
  • Waiting until everything finishes, and stop stubborn code from running forever
  • Copying results, error and output messages to srvairlock, so that you can access it
  • Turning off everything and cleaning up

Let's prepare the task, but first we need to go back to the folder containing the task:

your-login@srvairlock:~/examples/my_task$ cd ..

And now, instruct Minus to create a task file:

your-login@srvairlock:~/examples$ minus task create my_task
your-login@srvairlock:~/examples$ ls
my_task  my_task.task

And now we have a new file called my_task.task which is ready to be submitted. Warning, do not leave a slash after the directory task name (i.e. my_task ) or the command will fail.

Reserving the platform

We are now finally ready to give the task to Minus, so that it can operate its magic. However we need to reserve our resources, so that we guarantee that no one else will use it. The resources we will need to reserve are the nodes that were assigned to your group, the same ones that you typed into the scenario file.

For the purposes of this tutorial, a shared reservation has been done in advance. This parent reservation's 'id' will be given to you by the tutorial instructors. We will need to create a child reservation inside of the parent one.

To reserve your nodes in the FIT/CorteXlab room, use this following command. Please replace mnode4 and mnode6 with the node numbers assigned to your group. Bear in mind that the preceding letter 'm' should be kept!

your-login@srvairlock:~/examples$ oarsub -t inner=<id of container> -l {"network_address in ('', '')"}/nodes=2,walltime=1:00:00 -I

This will run a 1 hour job and open a subshell in which you can run minus tasks. This subshell will be killed after 1 hour, and if you leave the shell earlier, it will terminate the corresponding oar job. More documentation on oar can be found here.

Among the reservation messages OAR outputs, the system will give you a reservation 'id'. Be sure to write your's down. It can be used for stopping the job if necessary.

You can check if the job was properly created as well as monitor the current jobs in the gantt web interface.

Submitting the task

We are finally ready to submit the minus task:

your-login@srvairlock:~/examples$ minus task submit my_task.task

You'll see that Minus gives you a task number (15 in this example). You'll want to write down the number of the task as it will be important for checking its status or to abort it, if necessary.

Bear in mind that your task has been put on a queue and will await running tasks and other scheduled tasks to start, so it may take a while before it runs.

Using Minus to check the task's status

Minus can also help you check the status of the testbed:

your-login@srvairlock:~/examples$ minus testbed status
num total tasks:   2540
num tasks waiting: 0
num tasks running: 0
tasks currently running:

These information are returned:

  • num total tasks: This is the number of the last created task
  • num tasks waiting: This is the number of tasks currently awaiting in the queue
  • num tasks running: This is the number of tasks currently executing (most of the time, there can only be one task running at the same time. Only in special situations, such as demos, tutorials, can several users run tasks concurrently).
  • tasks currently running: This is a detailed list of tasks currently running.

If you wish the see all tasks in the waiting queue, use the option -w with the same command:

your-login@srvairlock:~/examples$ minus testbed status -w

Debugging your task

If your task didn't run, or finished too fast, then there might have been a problem with its execution. There are several ways to debug the Minus' problems. Lets concentrate on the easiest one: minus logs.

To view all minus logs execute this command:

your-login@srvairlock:~/examples$ minus log

All Minus' log messages can be seen, including the server logs as well as the node logs. Any non-gnuradio related problems with your task can be seen in these logs.

Collecting and analyzing the output

Generally the OFDM example task will take a few minutes to run. Once it is finished, Minus will take care of copying the results and output messages back to your home folder in srvairlock, so that you can analyze it.

All results are stored by task number in the results folder, inside your home folder.

Let's go and have a look at them. In the lines below, replace <your task number> by the task 'id' provided by minus:

your-login@srvairlock:~/examples$ cd ../results
your-login@srvairlock:~/results$ cd task_<your task number>
your-login@srvairlock:~/results/task_#$ ls
node4.tgz  node6.tgz

So we see that we have a folder for each task and inside each folder one compressed file per participant node. Let's extract one of those files and see what's inside:

your-login@srvairlock:~/results/task_#$ tar -zxf node4.tgz
your-login@srvairlock:~/results/task_#$ ls 
node4  node4.tgz  node6.tgz
your-login@srvairlock:~/results/task_#$ cd node4
your-login@srvairlock:~/results/task_#/node4$ ls  receive_path.pyc  stdout.txt        uhd_interface.pyc  scenario.yaml  stderr.txt

We see that all of the files we used to create the task are inside, but we have some new files. The *.pyc files are the python compiled files and we can safely ignore them. The other two are:

  • stdout.txt: all output messages from your GNU Radio python script are written here. These include GNU Radio messages as well as all “print”s you include in your code. Seeing the contents of this file is useful to assert its correct operation.
  • stderr.txt: all error messages are printed here. If you see strange things on the stdout.txt or nothing at all, it might be interesting to take a look at the stderr.txt to debug your code.

Let's take a look inside both, starting with the stdout.txt:

your-login@srvairlock:~/results/task_15/node4$ less stdout.txt

For the node which was configured as a receiver (refer to the scenario.yaml), you should get a long file that looks more or less like this:

linux; GNU C++ version 4.7.2; Boost_104900; UHD_003.007.001-84-gd99ce4ef

-- Opening a USRP2/N-Series device...
-- Current recv frame size: 1472 bytes
-- Current send frame size: 1472 bytes
-- Detecting internal GPSDO.... Found an internal GPSDO
-- found
-- Setting references to the internal GPSDO
-- Initializing time to the internal GPSDO

UHD Receiver:
UHD Args:     
Freq:         2.49GHz
LO Offset:    0Hz
Gain:         25.000000 dB
Sample Rate:  2Msps
Antenna:      TX/RX
Subdev Sec:   None
Clock Source: None
Using Volk machine: avx_64_mmx_orc

OFDM Demodulator:
Modulation Type: bpsk
FFT length:      512
Occupied Tones:  200
CP length:       128
Warning: failed to enable realtime scheduling
ok: False 	 pktno: 169 	 n_rcvd: 1 	 n_right: 0
ok: False 	 pktno: 170 	 n_rcvd: 2 	 n_right: 0
ok: False 	 pktno: 171 	 n_rcvd: 3 	 n_right: 0
ok: False 	 pktno: 173 	 n_rcvd: 4 	 n_right: 0
ok: False 	 pktno: 174 	 n_rcvd: 5 	 n_right: 0
ok: False 	 pktno: 175 	 n_rcvd: 6 	 n_right: 0
ok: False 	 pktno: 176 	 n_rcvd: 7 	 n_right: 0
ok: False 	 pktno: 177 	 n_rcvd: 8 	 n_right: 0
ok: False 	 pktno: 178 	 n_rcvd: 9 	 n_right: 0
ok: False 	 pktno: 179 	 n_rcvd: 10 	 n_right: 0
ok: False 	 pktno: 180 	 n_rcvd: 11 	 n_right: 0
ok: False 	 pktno: 181 	 n_rcvd: 12 	 n_right: 0
ok: False 	 pktno: 182 	 n_rcvd: 13 	 n_right: 0
ok: False 	 pktno: 185 	 n_rcvd: 14 	 n_right: 0
ok: False 	 pktno: 186 	 n_rcvd: 15 	 n_right: 0
ok: False 	 pktno: 191 	 n_rcvd: 16 	 n_right: 0
ok: False 	 pktno: 192 	 n_rcvd: 17 	 n_right: 0
ok: False 	 pktno: 197 	 n_rcvd: 18 	 n_right: 0
ok: False 	 pktno: 203 	 n_rcvd: 19 	 n_right: 0

Let's try to understand it:

  • The first 9 lines correspond to the UHD messages
  • The next 16 lines correspond to the GNU Radio messages and radio configuration parameters
  • There might be a warning message about realtime scheduling that can be safely ignored
  • The lines starting with ok: False pktno: 169 n_rcvd: 1 n_right: 0 are from the receiver trying to decode the OFDM packets. Let's further inspect these lines:
    • ok: False: means the packet was incorrectly decoded, i.e., it failed to pass the checksum
    • pktno: 169: is the sequence number seen at the receiver
    • n_rcvd: 1: is the number of received packets
    • n_right: 0: is the number of correctly received packets

As can be seen, there's a local oscillator (LO) mismatch between both nodes that complicates the decoding process.

You can also look into the stderr.txt:

your-login@srvairlock:~/results/task_15/node4$ less stderr.txt

What next?

Congratulations! You have finished your first tutorial on FIT/CorteXlab. Please, feel free to change the example task and try out different configurations, carrier frequencies, bands, and so on. You can always resubmit this task to test out different kinds of configurations.

One good example of what to do is to nudge the carrier frequency of approximately +/- 20 KHz and see if the decoding process works better.

To learn mode advanced concepts around creating and managing tasks on CorteXlab, please continue the tutorials.

cortexlab101.txt · Last modified: 2018/11/19 19:41 by ooubejja

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