Ph.D. (Dr. rer. nat.) Thesis

Content & User Behavior
in Anonymous Hyperlocal Online Platforms*

Brandenburg University of Technology, RNKS, 2022, * submitted

Examiner(s): Oliver Hohlfeld, Markus Strohmaier, Andriy Panchenko
Advisor: Oliver Hohlfeld


Nowadays day-to-day digital communication and social life has only fortified with the ongoing pandemic. People enjoy communication and information across various (direct) messaging platforms and accepted its ever-increasing impact on public discourse, and thus society. While most traditional platforms implement user profiles enabling social credit, the new landscape now also includes anonymity. Yet, a new type of applications combining anonymity with a strong spatial focus, hyperlocality, emerged over recent years. To this point, platform implications of both uniquely combined design properties largely remain unknown.

In this thesis (with minor exceptions) we provide a first data-driven holistic view on Jodel that combines both properties. We leverage unbiased and complete ground truth information to dissect a plethora of communities across two different countries: Germany & the Kingdom of Saudi Arabia. This work follows a platform perspective identifying four major essentially important areas revolving around the individual.

That is, we begin with a broad analysis of three User Adoption processes along three different applications. After discussing our measurements of the start and user base adoption of the German COVID-19 digital contact tracing application, we provide evidence of well-established platforms being re-purposed as a side channel to evade censorship in the ongoing Russo-Ukrainian hybrid war. We further showcase that even the very same platform ingredients may yield vastly different outcomes on the messaging app Jodel.

While any online platform builds upon user User Interactions, we structurally characterize Jodel behavior across both countries. We discuss structural disparities and detail platform implications - solely induced by local user behavior. An in-depth look into the Saudi community landscape closes a research gap to platform usage in non-Western societies, identifying fundamental differences.

Further, we discuss User Content analyzing information diffusion. Taking content to the next level, we developed a multidimensional classification scheme for intents (why) and topics (what) of social media messages and provide details of a crowdsourced campaign for Saudi Arabian contents. With neural word embeddings as a tool for making text tangible and the prevalence of emoji in social media communication, we discuss quantitative & qualitative insights to word-emoji embeddings reflecting semantics. Additionally, we make such embeddings interpretable and provide evidence that our method is well in line with human judgement.

In terms of User Management, we detail insights to distributed moderation processes and model the threat of abusive content. In the long term, platforms need to establish a sustainable, preferably growing, environment. That is, we next discuss user lifetime and possibly early churn factors, while modeling user lifetime from metadata. We finish with a blueprint of data-driven long term quality of experience analyzes in a controlled massively multiplayer online game (MMOG) environment.

* subject to change

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Master Thesis

Attitude Independent Movement Determination using MEMS for Screen-less Pedestrian Navigation

RWTH Aachen University, COMSYS, 2013

Examiner(s): Klaus Wehrle, Bernhard Rumpe
Advisor: Jó Ágila Bitsch Link

PDF Slides

While indoor navigation systems attract more and more attention, smartphone-based dead reckoning systems heavily depend on the user holding the device in aspecific way. As soon as a user puts the device in her trouser pocket or jacket, they fail. In this thesis, we enable smartphones to accurately determine steps and the current bearing of a user even in these conditions.

We build a general model of the walking motion with specific instances for different device locations (trousers pocket, jacket pocket). Slicing the measured data within a step to only extract the segment which provides the most information on the user’s walking direction, our approach has a median absolute error of only 12° (mean: 22°, q75: 25°) on a total of 15 participants completing a total of 49 test runs. Therefore, together with proposed feedback generation methods, this thesis lies the foundation of true hands-free indoor navigation without the need for any infrastructure.

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  • FootPath. Bitsch et al. International Conference on Indoor Positioning and Indoor Navigation IPIN'2011 PDF
  • Student Worker

Bachelor Thesis

Implementation of a OneAPI-REST interface for integrating web services in an IMS-based telecommunication network

RWTH Aachen University, COMSYS, 2011

Examiner(s): Klaus Wehrle, Bernhard Rumpe
Advisor: Dirk Thissen

PDF Slides

Latest development shows that telecommunication providers merge their networksinto all-IP-based networks. For this pupose, IMS (IP-Multimedia Subsystem) has been introduced. The architecture allows to deploy application servers providing several telecommunication services ontop of the IMS.

Motivated by the rapidly growing popularity of Web 2.0 applications and the emerging market with value added services on mobile devices, there have been introduced standards to combine the telecommunication and internet domain. The most recent standard is OneAPI which defines some key services in a RESTful design. RESTful Web Services are kept small by directly using the HTTP’s protocol mechanisms which makes them very efficient.

This thesis shows a prototype implementation of a generic REST interface, and a gateway module translating between specific OneAPI and REST messages on a JAIN SLEE Application Server being an event driven platform. To perfect an example of the complete vertical communication through this AS, we also present an SMS service implementation.

A brief overview of a performance evaluation is given and the results are discussed.

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Seminars · Laboratories


  1. Computer Graphics
    Physically-Based Real-Time Lens Flare Raytracing
    RWTH Aachen University
    Computer Graphics Group, Prof. Dr. Leif Kobbelt
    PDF slides
    Lens flares are an effect in imaging caused by light traversing a camera lens system. Due to unwantedreflections, ghosting artifacts appear on the resulting image. Besides such ghosting artifacts, glare isanother observable phenomenon due to scattered light and diffraction. Both effects are normally undesiredin photography and cinematography, but have become very important for realistic imagery. Moreover they can dramatically increase the perceived dynamic range of a range-limited medium. This seminar work introduces both effects and the relevant physical background. In order to simulate lens flares and glare, 2D as well as physically-based approaches are discussed. Current physical approaches use ray-tracing techniques on a commonly used camera model. Most of these ray tracing techniques arevery costly to compute. Hullin et al. have presented a new method which combines many approaches and approximations into a sparse ray tracing algorithm for very efficient simulation. Furthermore their approach provides sophisticated artistic control and a user defined trade-off between computational costs and quality.
  2. 2011

  3. Automata Theory
    Equivalence of Unambiguous Nondeterministic Finite Automata (DE)
    RWTH Aachen University
    Logic and Theory of Discrete Systems, Prof. Dr. Wolfgang Thomas
    PDF slides

    Das Inklusionsproblem sowie das Äquivalanzproblem von regulären Sprachen, regulären Grammatiken und endlichen Automaten sind fundamenta Fragen in der Informatik. Diese Probleme haben sich als PSCPCE vollständig erwiesen. Das heißt, dass sie nach heitigen Kenntnissen algorithmisch nicht praktisch entscheidbar sind. Es gibt viele Ansätze, die durch verschiedene Voraussetzungen an der Struktur der Sprache, Grammatik oder Automaten versuchen diese Probleme effizient zu lösem, doch umfassen diese immer Familien von echten regulären Teilmengen. DIe Hauptquelle dieser Seminararbeit [Stearns et al., 1985] betrachtet die strukturelle Voraussetzung der Eindeutigkeit (die Eindeutigkeit beschreibt die Anzahl verschiedener Läufe von einem Automaten auf akzeptierten Wörtern bzw. Anzahl der Ableitungen einer Grammatik zu einem Wort). Diese Restriktion erscheint sehr natürlich, da durch die Semantik hinter Sprachen und Grammatiken diese häufig eindeutig sind. So sind bspw. alle LR(0) Grammatiken, wie sie im Compilerbau vorkommen, immer eindeutig.

    Es wird gezeigt, dass das Inklusions- und Äquivalenzproblem eindeutiger Automaten effizient entscheidbar ist. Diese Ergebnisse werden dann auf Uneindeutigkeit eines fest beschränkten Grades erweitert. Die Resultate beziehen sich sowohl auf reguläre Sprachen und Grammatiken wie auch Automaten. Da der Umfang dieser Arbeit beschränkt ist, werden hier die Beweisführungen zu Automaten im Detail aufgegriffen. Diese lassen sich entsprechend auf reguläre Sprachen und Grammatiken übertragen.

Jens Helge Reelfs Photo

Jens Helge Reelfs
Brandenburg University of Technology
Chair of Computer Networks
Konrad-Wachsmann-Allee 5
03046 Cottbus, Germany
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