Wireless Measurement Probe

Images for Wireless Probe based on 4G SD card

It is recommended to clone to a 8G SD cards in order to avoid issues with new cards being a few sectors smaller. Image files are encypted using GPG, and the key will be provided by email/sms to those requesting it. The images below are for the two most recent Raspberry PI models. Note that version 2 has a different processor than version 1 so the images can not be mixed. The kernvel version used is 3.18.9 based on Raspbian release from Feb 2015.

Image for Raspberry PI I (15.04.2015)

Image for Raspberry PI 2 (15.04.2015)

The GPG encryption is supported on most OS versions (https://www.gnupg.org/). On Linux the decryption of the image files are achieved by issuing the simple command “gpg <imagefilename>.gpg” which will prompt you for the password.

Ordering additional probes

The WiFi device currently supported is the D-Link DWA-171, HW Ver. A1. Note that there are different HW version of this device so make sure to order HW Ver. A1 (Realtek chipset). The order version have a different WiFi chipset, and thereby also a different driver. Uninett has ordered these units from Imento, but I am sure there are other sources for this as well.

There are many alternative casing solutions for the Raspberry PI. The one I have tried, which is really good can be ordered from https://www.modmypi.com/ using partnumber: RB-BLK. The optional SD card cover is worth the money as it makes it impossible to even touch the SD card.

Registering new probes

After assembly of the Raspberry PI B+ Starter Kit, SD card cloning completed and inserted and D-Link WiFi device inserted – the unit is almost ready for use. The final step is just to register it on the following webpage http://ytelse.labs.uninett.no/probe_eng.html

Verifying that a new probe is working

Or course, the ultimate verification is to see that the device is reporting data to Zabbix (cf. section on Measurement Results below). However, the immediate after-boot verification of that it is working would be to check the files on the probe which stores measurement results. This is best done if you connect a keyboard and monitor directly to the Raspberry Pi and executes the following command: cat /home/pi/probefiles/zabbix_report The output of this will have one line for each metric collected, all lines starting with the MAC address of the D-Link, the next column contains the metric ID as defined in Zabbix and the last column is the metric value. If things are working you should expect to see some values here.

Measurement Results

Log into https://zabbix.uninett.no with the provided read-only username and password. The one given in the presentation below will be changed and information has been sent out to those of you which are registered as pilot customers. Additional pilot customers are of course welcome, just drop me an email (bv@uninett.no).

Local logging

If you are using the local logging facility, i.e. you have inserted a USB flash disk (VFAT formatted) all measurements collected by the device is also stored here. The disk is mounted as /mnt/usbdisk and there you will find two files. The first is named index.log and it just contains the metric names and order. This is a rather static file, and will only change if changes are made in the scripts so that new metrics are collected. The second file contains one line for each measurement cycle, and each line starts with the date and time – followed by the metric values. This file can be inspected in addition to the data reported to Zabbix, in cases where the reporting to Zabbix is not complete.

Notes on file system

Although not 100% complete yet, I have made attempts to minimize the amount of write operations to the SD card for the purpose of extending its lifetime. Thus, all bash scripts used in the operation of the probe is copied over from the SD card automatically after boot over to a defined ramdisk directory ( /home/pi/probefiles) and this is where they are executed from as well. Further on, the same directory is used for temporary storing of measurement results awaiting the actual zabbix report at the end of each measurement cycle.

Scheduled reload

Just because there may be some hick-ups in the probe operation, there is a cronjob defined which reloads the probe each midnight.


Presentation of “Trådløs Probe held at Trådløssamling 28-28 Okt. 2014.
Presentation of “WiFi Probe held at Sunet Dagarna 20. April 2015.


WPA_supplicant info:

SD card cloning with Win32DiskImager

Setup of RAMDISK

Dynamic Frequency Selection – DFS (reasons why 5Ghz may disappear in short intervals)







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