Information-Centric Content Retrieval for Delay-Tolerant Networks

The shift from host-centric to information-centric networking (ICN) promises seamless communication in mobile networks. However, most existing works either consider well-connected networks with high node density or introduce modifications to {ICN} message processing for delay-tolerant Networking (DTN). In this work, we present agent-based content retrieval, which provides information-centric {DTN} support as an application module without modifications to {ICN} message processing. This enables flexible interoperability in changing environments. If no content source can be found via wireless multi-hop routing, requesters may exploit the mobility of neighbor nodes (called agents) by delegating content retrieval to them. Agents that receive a delegation and move closer to content sources can retrieve data and return it back to requesters. We show that agent-based content retrieval may be even more efficient in scenarios where multi-hop communication is possible. Furthermore, we show that broadcast communication may not be necessarily the best option since dynamic unicast requests have little overhead and can better exploit short contact times between nodes (no broadcast delays required for duplicate suppression).

Dynamic Unicast: Information-Centric Multi-hop Routing for Mobile Ad-hoc Networks

Abstract Information-centric networking (ICN) offers new perspectives on mobile ad-hoc communication because routing is based on names but not on endpoint identifiers. Since every content object has a unique name and is signed, authentic content can be stored and cached by any node. If connectivity to a content source breaks, it is not necessarily required to build a new path to the same source but content can also be retrieved from a closer node that provides the same content copy. For example, in case of collisions, retransmissions do not need to be performed over the entire path but due to caching only over the link where the collision occurred. Furthermore, multiple requests can be aggregated to improve scalability of wireless multi-hop communication. In this work, we base our investigations on Content-Centric Networking (CCN), which is a popular {ICN} architecture. While related works in wireless {CCN} communication are based on broadcast communication exclusively, we show that this is not needed for efficient mobile ad-hoc communication. With Dynamic Unicast requesters can build unicast paths to content sources after they have been identified via broadcast. We have implemented Dynamic Unicast in CCNx, which provides a reference implementation of the {CCN} concepts, and performed extensive evaluations in diverse mobile scenarios using NS3-DCE, the direct code execution framework for the {NS3} network simulator. Our evaluations show that Dynamic Unicast can result in more efficient communication than broadcast communication, but still supports all {CCN} advantages such as caching, scalability and implicit content discovery.

Fine-grained indoor positioning and tracking systems

Indoor positioning has attracted considerable attention for decades due to the increasing demands for location based services. In the past years, although numerous methods have been proposed for indoor positioning, it is still challenging to find a convincing solution that combines high positioning accuracy and ease of deployment. Radio-based indoor positioning has emerged as a dominant method due to its ubiquitousness, especially for WiFi. RSSI (Received Signal Strength Indicator) has been investigated in the area of indoor positioning for decades. However, it is prone to multipath propagation and hence fingerprinting has become the most commonly used method for indoor positioning using RSSI. The drawback of fingerprinting is that it requires intensive labour efforts to calibrate the radio map prior to experiments, which makes the deployment of the positioning system very time consuming. Using time information as another way for radio-based indoor positioning is challenged by time synchronization among anchor nodes and timestamp accuracy. Besides radio-based positioning methods, intensive research has been conducted to make use of inertial sensors for indoor tracking due to the fast developments of smartphones. However, these methods are normally prone to accumulative errors and might not be available for some applications, such as passive positioning. This thesis focuses on network-based indoor positioning and tracking systems, mainly for passive positioning, which does not require the participation of targets in the positioning process. To achieve high positioning accuracy, we work on some information of radio signals from physical-layer processing, such as timestamps and channel information. The contributions in this thesis can be divided into two parts: time-based positioning and channel information based positioning. First, for time-based indoor positioning (especially for narrow-band signals), we address challenges for compensating synchronization offsets among anchor nodes, designing timestamps with high resolution, and developing accurate positioning methods. Second, we work on range-based positioning methods with channel information to passively locate and track WiFi targets. Targeting less efforts for deployment, we work on range-based methods, which require much less calibration efforts than fingerprinting. By designing some novel enhanced methods for both ranging and positioning (including trilateration for stationary targets and particle filter for mobile targets), we are able to locate WiFi targets with high accuracy solely relying on radio signals and our proposed enhanced particle filter significantly outperforms the other commonly used range-based positioning algorithms, e.g., a traditional particle filter, extended Kalman filter and trilateration algorithms. In addition to using radio signals for passive positioning, we propose a second enhanced particle filter for active positioning to fuse inertial sensor and channel information to track indoor targets, which achieves higher tracking accuracy than tracking methods solely relying on either radio signals or inertial sensors.

Wo sind die Schülerinnen? Eine Bild- und Textanalyse zu Geschlechter(un)gerechtigkeit in deutschen Schulbüchern

Kinder- und Schulbücher erreichen eine große Anzahl von Leserinnen und Lesern. Sie dienen nicht nur der Vermittlung von Wissen, sondern transportieren auch gesellschaftliche Werte, Normen und Stereotype über verschiedene soziale Gruppen (z.B. Markom & Weinhäupl, 2007). Vor diesem Hintergrund haben sich seit mehr als 40 Jahren viele Studien der Frage gewidmet, inwiefern Kinder- und Schulbücher zum gesellschaftlichen Konsens von Geschlechter(un)gerechtigkeit beitragen (z.B. Fichera, 1996; Finsterwald & Ziegler, 2007; Lindner & Lukesch, 1994). Die Darstellung von traditionellen Rollenbildern sowie die Unterrepräsentation von weiblichen im Vergleich zu männlichen Charakteren gaben dabei Anlass zur Kritik, weil Kindern dadurch nur eingeschränkte Identifikationsmöglichkeiten zur Verfügung stehen (z.B. Diekman & Murnen, 2004). Seit den ersten Studien zu geschlechtergerechten Darstellungen in Büchern hat sich die gesellschaftliche und soziale Situation von Frauen und Männern und der gesellschaftliche Diskurs über Geschlecht sehr verändert (Wilde & Diekman, 2005). Deshalb stellt sich im vorliegenden Beitrag die Frage, ob diese Veränderungen in neuen, in Deutschland zugelassenen Schulbüchern auch gefunden werden. Mittels Inhaltsanalyse (Mayring, 2003) wurden jeweils neun Schulbücher für die Fächer Deutsch und Mathematik der Klassenstufen eins, drei und fünf, die nach dem Inkrafttreten des Bundesgleichstellungsgesetzes (2001) herausgegeben wurden, analysiert und verglichen. Mittels Zufallsauswahl wurden jeweils 10% der Gesamtanzahl an Seiten eines jeden Buches untersucht. Der Fokus der Analyse lag dabei auf den beiden am meisten kritisierten Aspekten vergangener Studien: (1) den Häufigkeiten der Darstellung von weiblichen und männlichen Charakteren sowie (2) deren Aktivitäten und Rollen. Darüber hinaus schließt die Analyse auch ein in den vergangenen Jahren kaum untersuchtes subtileres Merkmal geschlechter(un)gerechter Darstellungen mit ein: (3) die Verwendung von geschlechter(un)gerechter Sprache (Moser, Hannover, & Becker, 2013). Wie eine Reihe von Studien zeigte, führt die Verwendung von generisch männlichen Personenbezeichnungen dazu, dass sich Rezipierende weniger weibliche Personen vorstellen, als wenn geschlechtergerechte Sprachformen verwendet werden (z.B. Stahlberg, Braun, Irmen, & Sczesny, 2007). Gerade im Schulkontext sollte jedoch die Verwendung von geschlechtergerechter Sprache von besonderer Bedeutung sein, um stereotypische Vorstellungen abzubauen. Die Ergebnisse unserer Studie (Moser & Hannover, 2014) zeigen, dass auch in neuen Mathematikbüchern männliche Charaktere häufiger vertreten sind als weibliche. Für die Deutschbücher fand sich dieses Ergebnis nur für die Darstellung erwachsener Personen. Für beide Schulfächer zeigte sich, dass insgesamt weibliche und männliche Charaktere gleich oft vielen verschiedenen und nicht nur stereotypischen Aktivitäten nachgingen. Anlass zur Kritik geben aber, wie bereits in früheren Studien, die familiären und beruflichen Rollen erwachsener Charaktere. Männer wurden im Vergleich zu Frauen häufiger in beruflichen Tätigkeiten dargestellt, während Frauen im Vergleich zu Männern häufiger in ihrer familiären Rolle als Mutter oder Großmutter dargestellt waren. Geschlechtergerechte Sprache wurde in den Büchern zu beiden Schulfächern angewendet, jedoch wurde die Anwendung nicht konsequent umgesetzt. Zusammengefasst zeigen die Ergebnisse, dass neue Schulbücher im Vergleich zu früheren geschlechtergerechter geworden sind, sie aber nach wie vor auf subtile Weise zur sozialen Konstruktion der Ungleichheit der Geschlechter beitragen. Diekman & Murnen, 2004). Seit den ersten Studien zu geschlechtergerechten Darstellungen in Büchern hat sich die gesellschaftliche und soziale Situation von Frauen und Männern und der gesellschaftliche Diskurs über Geschlecht sehr verändert (Wilde & Diekman, 2005). Deshalb stellt sich im vorliegenden Beitrag die Frage, ob diese Veränderungen in neuen, in Deutschland zugelassenen Schulbüchern auch gefunden werden. Mittels Inhaltsanalyse (Mayring, 2003) wurden jeweils neun Schulbücher für die Fächer Deutsch und Mathematik der Klassenstufen eins, drei und fünf, die nach dem Inkrafttreten des Bundesgleichstellungsgesetzes (2001) herausgegeben wurden, analysiert und verglichen. Mittels Zufallsauswahl wurden jeweils 10% der Gesamtanzahl an Seiten eines jeden Buches untersucht. Der Fokus der Analyse lag dabei auf den beiden am meisten kritisierten Aspekten vergangener Studien: (1) den Häufigkeiten der Darstellung von weiblichen und männlichen Charakteren sowie (2) deren Aktivitäten und Rollen. Darüber hinaus schließt die Analyse auch ein in den vergangenen Jahren kaum untersuchtes subtileres Merkmal geschlechter(un)gerechter Darstellungen mit ein: (3) die Verwendung von geschlechter(un)gerechter Sprache (Moser, Hannover, & Becker, 2013). Wie eine Reihe von Studien zeigte, führt die Verwendung von generisch männlichen Personenbezeichnungen dazu, dass sich Rezipierende weniger weibliche Personen vorstellen, als wenn geschlechtergerechte Sprachformen verwendet werden (z.B. Stahlberg, Braun, Irmen, & Sczesny, 2007). Gerade im Schulkontext sollte jedoch die Verwendung von geschlechtergerechter Sprache von besonderer Bedeutung sein, um stereotypische Vorstellungen abzubauen. Die Ergebnisse unserer Studie (Moser & Hannover, 2014) zeigen, dass auch in neuen Mathematikbüchern männliche Charaktere häufiger vertreten sind als weibliche. Für die Deutschbücher fand sich dieses Ergebnis nur für die Darstellung erwachsener Personen. Für beide Schulfächer zeigte sich, dass insgesamt weibliche und männliche Charaktere gleich oft vielen verschiedenen und nicht nur stereotypischen Aktivitäten nachgingen. Anlass zur Kritik geben aber, wie bereits in früheren Studien, die familiären und beruflichen Rollen erwachsener Charaktere. Männer wurden im Vergleich zu Frauen häufiger in beruflichen Tätigkeiten dargestellt, während Frauen im Vergleich zu Männern häufiger in ihrer familiären Rolle als Mutter oder Großmutter dargestellt waren. Geschlechtergerechte Sprache wurde in den Büchern zu beiden Schulfächern angewendet, jedoch wurde die Anwendung nicht konsequent umgesetzt. Zusammengefasst zeigen die Ergebnisse, dass neue Schulbücher im Vergleich zu früheren geschlechtergerechter geworden sind, sie aber nach wie vor auf subtile Weise zur sozialen Konstruktion der Ungleichheit der Geschlechter beitragen.

Enhanced Caching Strategies At the Edge of LTE Mobile Networks

With a boom in the usage of mobile devices for traffic-heavy applications, mobile networks struggle to deliver good performance while saving resources to support more users and save on costs. In this paper, we propose enhanced strategies for the preemptive migration of content stored in Information-Centric Networking caches at the edge of LTE mobile networks. With such strategies, the concept of content following the users interested in it becomes a reality and content within caches is more optimized towards the requests of nearby users. Results show that the strategies are feasible, efficient and, when compared to default caching strategies, ensure that content is delivered faster to end users while using bandwidth and storage resources more efficiently at the core of the network.

Fine-grained Indoor Tracking by Fusing Inertial Sensor and Physical Layer Information in WLANs

Indoor positioning has become an emerging research area because of huge commercial demands for location-based services in indoor environments. Channel State Information (CSI) as fine-grained physical layer information has been recently proposed to achieve high positioning accuracy by using range based methods, e.g., trilateration. In this work, we propose to fuse the CSI-based ranging and velocity estimated from inertial sensors by an enhanced particle filter to achieve highly accurate tracking. The algorithm relies on some enhanced ranging methods and further mitigates the remaining ranging errors by a weighting technique. Additionally, we provide an efficient method to estimate the velocity based on inertial sensors. The algorithms are designed in a network-based system, which uses rather cheap commercial devices as anchor nodes. We evaluate our system in a complex environment along three different moving paths. Our proposed tracking method can achieve 1.3m for mean accuracy and 2.2m for 90% accuracy, which is more accurate and stable than pedestrian dead reckoning and range-based positioning.

Trends in Human-Centric Multimedia Networking Scenarios

The evolution of wireless access technologies and mobile devices, together with the constant demand for video services, has created new Human-Centric Multimedia Networking (HCMN) scenarios. However, HCMN poses several challenges for content creators and network providers to deliver multimedia data with an acceptable quality level based on the user experience. Moreover, human experience and context, as well as network information play an important role in adapting and optimizing video dissemination. In this paper, we discuss trends to provide video dissemination with Quality of Experience (QoE) support by integrating HCMN with cloud computing approaches. We identified five trends coming from such integration, namely Participatory Sensor Networks, Mobile Cloud Computing formation, QoE assessment, QoE management, and video or network adaptation.

Information-centric communication in mobile and wireless networks

Information-centric networking (ICN) is a new communication paradigm that has been proposed to cope with drawbacks of host-based communication protocols, namely scalability and security. In this thesis, we base our work on Named Data Networking (NDN), which is a popular ICN architecture, and investigate NDN in the context of wireless and mobile ad hoc networks. In a first part, we focus on NDN efficiency (and potential improvements) in wireless environments by investigating NDN in wireless one-hop communication, i.e., without any routing protocols. A basic requirement to initiate informationcentric communication is the knowledge of existing and available content names. Therefore, we develop three opportunistic content discovery algorithms and evaluate them in diverse scenarios for different node densities and content distributions. After content names are known, requesters can retrieve content opportunistically from any neighbor node that provides the content. However, in case of short contact times to content sources, content retrieval may be disrupted. Therefore, we develop a requester application that keeps meta information of disrupted content retrievals and enables resume operations when a new content source has been found. Besides message efficiency, we also evaluate power consumption of information-centric broadcast and unicast communication. Based on our findings, we develop two mechanisms to increase efficiency of information-centric wireless one-hop communication. The first approach called Dynamic Unicast (DU) avoids broadcast communication whenever possible since broadcast transmissions result in more duplicate Data transmissions, lower data rates and higher energy consumption on mobile nodes, which are not interested in overheard Data, compared to unicast communication. Hence, DU uses broadcast communication only until a content source has been found and then retrieves content directly via unicast from the same source. The second approach called RC-NDN targets efficiency of wireless broadcast communication by reducing the number of duplicate Data transmissions. In particular, RC-NDN is a Data encoding scheme for content sources that increases diversity in wireless broadcast transmissions such that multiple concurrent requesters can profit from each others’ (overheard) message transmissions. If requesters and content sources are not in one-hop distance to each other, requests need to be forwarded via multi-hop routing. Therefore, in a second part of this thesis, we investigate information-centric wireless multi-hop communication. First, we consider multi-hop broadcast communication in the context of rather static community networks. We introduce the concept of preferred forwarders, which relay Interest messages slightly faster than non-preferred forwarders to reduce redundant duplicate message transmissions. While this approach works well in static networks, the performance may degrade in mobile networks if preferred forwarders may regularly move away. Thus, to enable routing in mobile ad hoc networks, we extend DU for multi-hop communication. Compared to one-hop communication, multi-hop DU requires efficient path update mechanisms (since multi-hop paths may expire quickly) and new forwarding strategies to maintain NDN benefits (request aggregation and caching) such that only a few messages need to be transmitted over the entire end-to-end path even in case of multiple concurrent requesters. To perform quick retransmission in case of collisions or other transmission errors, we implement and evaluate retransmission timers from related work and compare them to CCNTimer, which is a new algorithm that enables shorter content retrieval times in information-centric wireless multi-hop communication. Yet, in case of intermittent connectivity between requesters and content sources, multi-hop routing protocols may not work because they require continuous end-to-end paths. Therefore, we present agent-based content retrieval (ACR) for delay-tolerant networks. In ACR, requester nodes can delegate content retrieval to mobile agent nodes, which move closer to content sources, can retrieve content and return it to requesters. Thus, ACR exploits the mobility of agent nodes to retrieve content from remote locations. To enable delay-tolerant communication via agents, retrieved content needs to be stored persistently such that requesters can verify its authenticity via original publisher signatures. To achieve this, we develop a persistent caching concept that maintains received popular content in repositories and deletes unpopular content if free space is required. Since our persistent caching concept can complement regular short-term caching in the content store, it can also be used for network caching to store popular delay-tolerant content at edge routers (to reduce network traffic and improve network performance) while real-time traffic can still be maintained and served from the content store.