WebCamp Ljubljana

27.04.2014 20:18

Yesterday I attended WebCamp Ljubljana held at the Faculty of Computer and Information Science. It was expertly organized by Jure and his team of volunteers as the conclusion to the OCWC global conference on open education.

Kiberpipa table at WebCamp Ljubljana 2014

Image by jskrablin

I managed to attend two talks, but otherwise I was mostly hanging around the Kiberpipa table where I was invited to show off some of my hardware projects. VESNA spectrum sensor, an interactive animated pony on an OLED display and a few other tidbits attracted a surprising amount of attention at the otherwise web-development focused event.

I recorded a trace of the 2.4 GHz band during the event to compare it with my earlier measurements at home. As you can see on the visualization below, it looks a bit more lively than my previous recordings that were made in a quiet residential area.

At the conclusion of the conference I made a hurriedly prepared lightning talk about these measurements and how they can help you choose the best channel for a Wi-Fi access point. I proposed to setup a crowd-funding campaign for a cheap spectrum sensor for that sole purpose if there is enough interest. From the feedback I got though I think I failed to present the topic in an understandable way for this audience.

Incidentally, the wireless network at the event was not particularly reliable. Although as I understand, the physical layer was not a problem in this case.

2.4 GHz use at WebCamp Ljubljana 2014

(Click to watch 2.4 GHz use at WebCamp Ljubljana 2014 video)

In general, the event went by in a very positive atmosphere. I think 20 minutes per talk gave it just the right pace for a gathering like this. I enjoyed the presentations about IPv6 and Apache Solr. I talked with a lot of nice people and I was surprised by the gender balance among the speakers and audience. Unfortunately I was too tired from getting up early on a Saturday and the hectic week before to check out the social event in Kiberpipa that followed in the evening.

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Wi-Fi monitoring

15.04.2014 17:25

Ever since we setup the wireless testbed in Logatec I wanted to use VESNA spectrum sensors to monitor Wi-Fi and other unlicensed radio usage in the 2.4 GHz band.

Unfortunately, it turned out that the low-power mesh network is way too slow to do any kind of real-time transfers. It was designed for reading battery-powered temperature sensors every once in a while, not streaming radio spectrum data. The 2.4 GHz CC2500-based radio generates around 2 kB per second. That maybe doesn't sound like much these days, but in ideal circumstances a raw data stream from one radio alone more than saturates the mesh, much less 50 of them.

There is a wired Ethernet interface in the works that should take care of the limited bandwidth problem. Meanwhile, motivated by rants about unusable Wi-Fi in some parts of Ljubljana, I improvised and made a pair of stand-alone devices that simply record spectrum data to a SD card.

Stand-alone VESNA RF spectrum sensors.

Here are two particularly colorful visualizations of measurements I took from a balcony in a residential district. They show changes in a two-dimensional histogram of RF power samples over time. This display is similar to the persistence mode on expensive spectrum analyzers (it only becomes feasible on this hardware though when you have relatively long-term measurements).

Each individual sample recorded the total received power, averaged over 1 ms, in a 400 kHz wide channel. Samples were taken at 255 different central frequencies, continuously covering the band between 2.4 GHz and 2.5 GHz. Color on the picture shows how often a sample with that frequency (X axis) and power (Y axis) was encountered.

Following is a recording of a little more than one day, compressed down to 40 seconds:

A day in Wi-Fi

(Click to watch A day in Wi-Fi video)

You can see here several IEEE 802.11 networks. The nice thing about this kind of monitoring is that you can detect access points as well as devices that connect to them. Devices using 802.11b standard with direct sequence spread spectrum modulation have a slightly rounded footprint. Newer and faster 802.11g/n standards with OFDM leave a more sharply square shape.

Some narrow-band transmissions correspond to wireless keyboards and mice. Others I haven't identified yet. Regularly spaced bumps in the noise floor are due to internal interference from the receiver.

Similarly here is the complete week. Each video frame comprises of 5070 scans of the spectrum.

A week in Wi-Fi

(Click to watch A week in Wi-Fi video)

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Thoughts on Bullet Journal

12.04.2014 20:04

Somewhere around November last year Jure introduced me to Bullet journal. Despite its chilling name, it's simply a set of recommendations on how to organize day-to-day notes and to-do lists in a plain old notebook. I gave it a try and experimented a bit with the way I put things down on paper. Here are some thoughts on the topic after a few months.


I've always kept a notebook with me. Before I started using the Bullet journal, I usually went through one 80 page notebook per year. Now I use up around 20 pages per month.

I don't think I write that much more than before though. I now leave much more space around notes for later additions and I typically start a new topic on a new page. This means that notes are less crowded and more readable after a while, but also sometimes a page will remain three-quarters empty if I don't return to some particular thought.

Previously I did a lot of random notes on various scraps of paper and printouts which invariably accumulated on my desk until they got lost or thrown away. I now tend to do all notes directly into the notebook. Sometimes with pencil if I predict a lot of corrections, but usually with ink. Annotated printouts get taped between the pages of the notebook so they don't get lost.

Every once in a while I tend to do a brain-dump page with lots of assorted tasks that sometimes pile up. But normally I keep notes and tasks organized under a common heading that spans one, two or three pages.

Monthly index page from my Bullet journal.

The biggest improvement that came from Bullet journal are the indexes. I number the pages and I write out an index page once per month. I group individual topics in the index by projects. I still keep dates in the margin of pages. I don't keep per-project indexes, but I plan to make a yearly index of projects. Page numbers don't restart when starting a new physical notebook.

I think indexes are really what made the notebook read-write and not only write-only. It used to be that in my old notebooks I only kept going back to a few pages with the most important notes or recipes that I had to keep looking up. Now I find myself daily browsing back to read a thought I have written down a month ago.


Using square boxes for marking to-do items is also a very good idea. I used to mark to-do items with arrows, but boxes are much more visually distinct and allow for quickly scanning the page for un-checked items. I don't use all of the marks described in the Bullet journal tutorial: I either check the box to mark it done, cross over the line to mark it not relevant or draw an arrow over the box to mark that the item has been moved to another page. This last one is fairly rare, since it's easy to look up past unfinished tasks.

Contrary to the Bullet journal proper, most of my notes are not organized by bullets. They tend to be a mix of sketches, bits of text, diagrams, and calculations. Often my notes start in the paper notebook and continue into a IPython Notebook or some other digital file.

For now I usually write down which digital file is connected to the paper notes and vice versa, add a comment in the file pointing to a paper page number. I try to keep duplication to a minimum, but between the notebook, digital files and project documentation which has to be shared or filed separately, there is necessarily some overlap.

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