Table of Contents

Sharing the testbed with other users

This tutorial is based off of the GNU Radio Benchmark example to show how to enable the sharing of the testbed with other users.

Why would you want to do this?

While CorteXlab was made to allow a single user to operate the testbed at a time, mainly to avoid harmful interference to the experimenter, a few situations might require sharing the testbed:

It is important to know that while sharing, there are no guarantees on interference in the room.

Reserving the testbed

First, we need to reserve the CorteXlab room for the shared session. If you're teaching a course or if you're hosting a challenge, in order to guarantee the testbed will be free when you need, the best thing to do is to reserve it prior to the date of usage:

you@srvairlock:~/examples$ oarsub -l nodes=BEST,walltime=4:00:00 -t container -r '2018-07-21 14:00:00'

Beware that the walltime parameter has to span the whole session.

Logging into the SSH front-end

Assuming your account has been correctly created, you can start by logging into the “srvairlock” SSH front-end:

you@yourpc:~$ ssh -X -v [-i path/to/the/key] -p 2269 username@gw.cortexlab.fr 

If this doesn't work, either you still don't have an account (you can get one by going here) or your SSH keys aren't present in your computer, in which case you need to copy them from the computer you used to generated them. For more information on the connection process, please check here.

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

Inspecting the example task

Retrieve the examples repository on GitHub :

you@srvairlock:~$ git clone https://github.com/CorteXlab/examples.git
you@srvairlock:~$ cd examples

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

you@srvairlock:~/examples$ cd my_task
you@srvairlock:~/examples/my_task$ ls
benchmark_rx.py  receive_path.py  transmit_path.py
benchmark_tx.py  scenario.yaml    uhd_interface.py

Let's go over each one of these files:

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

you@srvairlock:~/examples/my_task$ less scenario.yaml

Which will give you this:

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


# Scenario textual description
#   simple string (a one liner)
description: base scenario for CorteXlab

# 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)

nodes:

  node4:
    command: ./benchmark_rx.py --antenna="TX/RX" --rx-gain=25 -v -W 2M -f 2.49G
    passive: true

  node6:
    command: ./benchmark_tx.py --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:

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

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

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:

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

you@srvairlock:~/examples/my_task$ cd ..
you@srvairlock:~/examples$

And now, instruct Minus to create a task file:

you@srvairlock:~/examples$ minus task create my_task
you@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.

Submitting the task

Now we need to give the task to Minus, so that it can operate its magic.

First we need to reserve the CorteXlab room:

you@srvairlock:~/examples$ oarsub -l nodes=BEST,walltime=0:30:00 -I

(If you're running your reservation in a container reservation):

you@srvairlock:~/examples$ oarsub -t inner=<id of container> -l {"network_address in ('mnode4.cortexlab.fr', 'mnode6.cortexlab.fr')"}/nodes=2,walltime=0:30:00 -I

(Be sure that no one else is using the same node as you)

This will run a 30 minute job and open a subshell in which you can run minus tasks. This subshell will be killed after 30 minutes, and if you leave the shell earlier, it will terminate the corresponding oar job. More documentation on oar can be found here. You can also monitor the current jobs in the gantt web interface.

We then submit the minus task:

you@srvairlock:~/examples$ minus task submit my_task.task
Task with id 15 enqueued user <login>.

You'll see that Minus recognizes you as the submitter of the task and 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.

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

you@srvairlock:~/examples$ minus testbed status
num total tasks:   2540
num tasks waiting: 0
num tasks running: 0
tasks currently running:
  (none)

These information are returned:

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:

you@srvairlock:~/examples$ cd ../results
you@srvairlock:~/results$ ls
task_15
you@srvairlock:~/results$ cd task_15
you@srvairlock:~/results/task_15$ ls
node4.tgz  node6.tgz
you@srvairlock:~/results/task_15$

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:

you@srvairlock:~/results/task_15$ tar -zxf node4.tgz
you@srvairlock:~/results/task_15$ ls 
node4  node4.tgz  node6.tgz
you@srvairlock:~/results/task_15$ cd node4
you@srvairlock:~/results/task_15/node4$ ls
benchmark_rx.py  receive_path.pyc  stdout.txt        uhd_interface.pyc
benchmark_tx.py  scenario.yaml     transmit_path.py
receive_path.py  stderr.txt        uhd_interface.py

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:

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

you@srvairlock:~/results/task_15/node4$ less stdout.txt

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:

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:

you@srvairlock:~/results/task_15/node4$ less stderr.txt

What next?

Congratulations! You have finished your first tutorial on 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.