Search

In many public transport networks, multiple providers cooperate to offer integrated services and, consequently, provide integrated fare collection. Thus, ticket revenues need to be redistributed so that each provider receives its respective share. Typically, the customers’ travel behavior is surveyed and the fares paid are aggregated over certain periods of time, and the revenue is periodically allocated based on this information. To avoid a centralized trusted third party or the exchange of sensitive information between providers, we present an approach that integrates revenue allocation directly into the payment process: The proposed approach is based on payment channel networks and utilizes multi-hop payments to perform revenue allocation. We thereby show how to make use of payment channel networks in this setting as well as the corresponding benefits.

When software systems become more complex, it can be advantageous to partition their code into multiple, separate components. In this work, we examine how multiple smart contracts can be coupled to work together. When coupling smart contracts, different design approaches are possible with their own advantages and disadvantages. As an example, we couple two smart contract applications on the Ethereum blockchain: Palinodia and DecentID. Palinodia can be used to ensure the integrity of downloaded executable binaries by checking their hashes against the hashes stored in the blockchain. To make sure that not everyone can modify the data stored on the blockchain, an identity management system is required. This task is fulfilled by DecentID, which provides decentralized identities stored as smart contracts on the blockchain. We evaluate approaches of coupling these two applications and discuss their benefits and drawbacks for this use case.

Interdigitated back contact (IBC) solar cells achieve the highest efficiencies of single-junction architectures, but complicated patterning of the rear fingers and spreading of dopants during processing inhibit their mainstream adoption due to concerns of shunting between the IBC fingers. One method of simplifying patterning at the rear is by using contact masks combined with plasma-enhanced chemical vapor deposition (PECVD) or ion implantation. However, the intrinsic isolation region becomes contaminated during high-temperature annealing by lateral diffusion of dopants and during masked PECVD by spreading of dopant radicals through region between the mask and the substrate. Despite this contamination, we show through scanning spreading resistance microscopy and Kelvin probe force microscopy that a ~20 µm wide compensating region exists with high enough resistivity to prevent shunting. We model this p-i-n poly-Si system using two simulation models: a simple resistor model considering only the capture of charge carriers by trap defects in poly-Si to reduce the conductivity, and a more refined 1-dimensional finite element model using Poisson’s equation, drift-diffusion equations, and recombination of carriers. Using this model, we show that high defect density significantly decreases the current across the region between the p- and n-type fingers, preventing shunting.

The performance of IBC solar cells depends on the ability of this region to electrically isolate the doped fingers so that shunting does not occur and reduce the cell fill factor. We report on simulations which model the region between the p- and n-type poly-Si fingers of interdigitated back contact (IBC) solar cells as a series of resistors extending from one doped finger to the other. We demonstrate that the existence of a well-compensated region between the doped fingers is enough to prevent shunting and loss of cell performance, despite contamination of dopants from the opposing dopant fingers. We show through these simulations that a net doping concentration below ~1018 cm−3 will enable a high resistivity and identify the conditions under which this highly resistive region forms despite imperfect finger edges. Additionally, we apply this analysis to tails measured by time-of-flight secondary ion mass spectrometry and confirm this hypothesis for our experimentally measured tails.

Poly-Si/SiO 2 passivating contacts have shown great potential for industry-relevant next-generation solar cells. Previous work has demonstrated an improved J sc from dry reactive ion etching in an SF 6 environment, using front metal grids as a self-aligned mask. Here, we show an effective method to measure the front poly-Si thickness on an alkaline textured surface using X-ray diffraction. The calculated thicknesses are compared with cross-section SEM analysis and Quokka simulation. We show that by thinning the front poly-Si from 200 nm to 60 nm, J sc increases by 2.4 mA/cm2, leading to an absolute efficiency gain of 1.73%. Finally, we discuss possible reasons for the premature loss of passivation before the removal of all poly-Si.

Protocol detection is the process of determining the application layer protocol in the context of network security monitoring, which requires a timely and precise decision to enable protocol-specific deep packet inspection. This task has proven to be complex, as isolated characteristics like port numbers are not sufficient to reliably determine the application layer protocol. Hence, more dynamic detection approaches have been developed. In this paper, we analyze the Dynamic Protocol Detection mechanisms employed by popular and widespread open-source network monitoring tools. We show on the example of HTTP that all analyzed detection mechanisms are vulnerable to evasion attacks, which pose a serious threat to real-world monitoring operations. We find that the underlying fundamental problem of protocol disambiguation is not adequately addressed in two of three monitoring systems that we analyzed. To enable adequate operational decisions, this paper highlights the inherent trade-offs within Dynamic Protocol Detection.

We investigate factors that are critical for the performance of interdigitated back contact (IBC) solar cells based on polycrystalline silicon (poly-Si) passivated contacts. During patterning of doped lines using direct plasma deposition through a shadow mask, we show that the intrinsic poly-Si gap becomes contaminated with dopants, leading to shunting. Possible contamination mechanisms during masked deposition and high- temperature annealing are tested. Strategies developed to mitigate the contamination, such as reactive ion etching after deposition and amorphous to poly-Si crystallization in oxygen, will lead to improved IBC fabrication methods.

Flooding Peer-to-Peer (P2P) networks form the basis of services such as the electronic currency system Bitcoin. The decentralized architecture enables robustness against failure. However, knowledge of the network's topology can allow adversaries to attack specific peers in order to, e.g., isolate certain peers or even partition the network. Knowledge of the topology might be gained by observing the flooding process, which is inherently possible in such networks,, performing a timing analysis on the observations. In this paper we present a timing analysis method that targets flooding P2P networks, show its theoretical, practical feasibility. A validation in the real-world Bitcoin network proves the possibility of inferring network links of actively participating peers with substantial precision, recall (both ~ 40%), potentially enabling attacks on the network. Additionally, we analyze the countermeasure of trickling, quantify the tradeoff between the effectiveness of the countermeasure, the expected performance penalty. The analysis shows that inappropriate parametrization can actually facilitate inference attacks.

Sharing data within a closed user group is the basis for lots of applications today in both personal and professional life. The risk that the storage provider or an attacker illegitimately inspects or tampers with the shared data can be mitigated by employing client-side cryptography. In this work, we estimate the resource consumption that secure data sharing based on client-side cryptography requires in terms of computation time and network traffic volume. We therefore go beyond asymptotical analyses and state the absolute resource consumption for different secure data sharing protocols and client devices under realistic sharing models. These sharing models are extracted from traces of real-world collaboration platforms running in production. To the best of our knowledge, this is the first time that such a characterization of sharing models is published. Furthermore, we provide a clearly defined resource consumption estimation model. Our results show that for users who just up-and download data, the execution time of required cryptographic operations is typically up to a few seconds. The results also show that group owners have to deal with significantly higher computation times and network traffic when a user is removed from a sharing group with a few hundred or more members, given that basic secure data sharing protocols are used as they are in place today. A further finding is that computation time and network traffic volume are considerably lowered by extending the secure data sharing protocol with group key management approaches, at the price of slightly raised computation times for smaller groups.

The emergence of decentralized crypto currencies such as Bitcoin and the success of the anonymizing network TOR lead to an increased interest in peer-to-peer based technologies lately -- not only due to the prevalent deployment of mass network surveillance technologies by authorities around the globe. While today's application services typically employ centralized client/server architectures that require the user to trust the service provider, new decentralized platforms that eliminate this need of trust are on their rise. In this paper we critically analyze a fully decentralized alternative to today's digital ecosystem -- MaidSafe -- that drops most of the commonly applied principles. The MaidSafe network implements a fully decentralized personal data storage platform on which user applications can be built. The network is made up by individual users who contribute storage, computing power and bandwidth. All communication between network nodes is encrypted, yet users only have to remember a username and password. To guarantee these objectives, MaidSafe combines mechanisms such as Self-Authentication, Self-Encryption, and a P2P-based public key infrastructure. This paper provides a condensed description of MaidSafe's key protocol mechanisms, derives the underlying identity and access management architecture, and evaluates it with respect to security and privacy aspects.

We present a simulation model of the Bitcoin peer-to-peer network, a widely deployed distributed electronic currency system. The model enables evaluations of the feasibility and cost of attacks on the Bitcoin network at full scale of 6,000 nodes. The simulation model is based on unmodified code from core segments of the Bitcoin reference implementation used by 99% of nodes. Parametrization of the model is performed based on large-scale measurements of the real-world network. We present preliminary validation results showing a reasonable correspondence of the propagation of messages in the Bitcoin network compared with simulation results. We apply the model to study the feasibility of a partitioning attack on the network and show that the attack is sensitive to the churn of the attacking nodes.

State-of-the-art denoising methods provide denoising results that can be considered close to optimal. The denoising methods usually have one or more parameters regulating denoising strength that can be adapted for a specific image. To obtain the optimal denoising result, the correct parameter setting is crucial. In this paper, we therefore propose a method that can automatically estimate the optimal parameter of a denoising algorithm. Our approach compares the gradient histogram of a denoised image to an estimated reference gradient histogram. The reference gradient histogram is estimated based on down- and upsampling of the noisy image, thus our method works without a reference and is image-adaptive. We evaluate our propsed down-/upsampling-based gradient histogram method (DUG) based on a subjective test with 20 participants. In the test data, we included images from both the Kodak data set and the more realistic ARRI data set and we used the state-of-the-art denoising method BM3D. Based on the test results we can show that the parameter estimated by our method is very close to the human perception. Despite being very fast and simple to implement, our method shows a lower error than all other suitable no-reference metrics we found.

Urban vehicle-to-vehicle radio propagation is commonly modeled using simplified path loss models which are either based on empirical measurements or analytical considerations. These models have in common that their parameterability is rather limited and it is thus only possible to consider, e.g., the distance between transmitter and receiver, antenna heights or street widths. Aspects, such as the shape of an intersection or to which degree vehicles or vegetation are present cannot explicitly be considered using these models. In contrast, recent work suggests that even effects, such as obstructed line-of-sight by vehicles on the road or destructive multipath propagation can drastically influence simulation results. Accordingly, adjusted models are suggested which, however, incorporate only one additional aspect at a time. In this paper, we pursue a different approach and take a first step in evaluating which aspects influence radio conditions in urban areas to which degree and how these individual aspects affect each other. We consequently consolidate existing works and evaluate in which situations a more detailed radio propagation modeling might yield substantially different results with respect to the requirements of IEEE 802.11p networks. In order to achieve comparable results and being able to model several aspects, we employed ray tracing with an accurate geographic scenario and individually modeled obstacles, such as post boxes or trees at the roadside. Our results show and explain that the presence of radio obstacles can indeed make a difference within certain ranges if considered individually, the overall path loss in a heterogeneous and crowded urban scenario, however, is still approximated well with currently employed path loss models.

Distributed Hash Tables (DHTs) provide information lookup within a Peer-to-Peer (P2P) network. A multitude of distributed applications leverages DHTs for offering more advanced services such as distributed file systems, web caches or distributed DNS. For such DHT-based applications, lookup performance is highly important. However, lookup performance is severely affected by network characteristics, i.e., churn and connectivity issues due to NAT routers. As those characteristics are heavily influenced by user behavior, changes are not only likely but also hard to predict. Although DHTs are known for their self-organization, current implementations often do not adapt optimally to variation in network characteristics. In this paper, we propose to dynamically optimize the client through tuning its parameters at run-time. For doing so, different configurations are tested and compared automatically. To reduce overhead, requests sent to other peers are recorded and replayed by a simulation engine, if the same peer is queried again using the same parameter. We evaluated our approach at two exemplary scenarios of the future state of the BitTorrent Mainline DHT (MDHT), one of the most widely used public DHTs. In these scenarios, the lookups of a client using static parameters were more than three times slower and had a 25% higher network overhead than those of an adaptive client. With only 4 additional UDP packets sent per second and a one-digit CPU load, the proposed approach also induces minimal overhead.

The suitability and performance of medium access protocols in vehicular environments is already being investigated over a long period of time. Carrier Sense Multiple Access (CSMA) has been shown to perform sufficiently well in most situations and being able to support safety and efficiency vehicular applications. Recently, Self-organizing Time-Division Multiple Access (STDMA) is being considered as an alternative and has been shown to coordinate the channel slightly better under certain situations. However, when comparing both protocols the precise details of radio and network conditions and parametrization of the protocols are decisive on which protocol takes a slight lead. Consequently, scenarios can be constructed quite easily in which one protocol is superior over the other one. The focus of this work is thus not to absolutely compare both protocols, but rather to understand the strengths and weaknesses of both protocols in certain situations. In particular, we consider i) to which degree hidden nodes influence the coordination ability, ii) how an extended carrier sensing range is beneficial and iii) how temporary fading influences the performance of both MAC protocols. Our results show that while an extended carrier sensing range is only beneficial for CSMA, the existence and severity of fading is far less detrimental for STDMA than for CSMA.

The purpose of Virtual Traffic Light (VTL) applications is to increase traffic efficiency without the use of conventional traffic light infrastructure. With VTL applications, vehicles self-organize for intersection crossing based on wireless communication. Although VTL applications must comply with high safety requirements, no verification of VTL's safety has been provided so far. We present a VTL protocol that is verified to be fail-safe using model checking as a verification approach. Performance evaluation through simulation showed that the verified fail-safe VTL protocol delivers-although far from optimum-decent results with respect to such traffic efficiency metrics like throughput and travel time. The investigated efficiency optimization substantially improved efficiency, yet compromised the safety of VTL. We quantify the tradeoff between efficiency and safety and show that increasing the safety level deteriorates efficiency only marginally. In particular, a safety level increase by a factor of 1,000 increases travel time by approximately 2% in the studied scenario. Therefore, high safety requirements can be met while maintaining high efficiency gains.

Recent studies suggest that Self-organizing Time-Division Multiple Access (STDMA) might be a better medium access strategy in inter-vehicle communication networks than Carrier Sense Multiple Access (CSMA), especially when considering safety focused applications. Although it is necessary to completely understand a protocol and the effect of its `turning knobs' on performance before adoption, STDMA has not yet been subjected to such rigorous treatment in the literature. In order to address this shortcoming we perform and present an in-depth analysis and evaluation of STDMA's fundamental principles. In particular, we contribute a detailed and complete description of the STDMA protocol, followed by the analysis and evaluation of two key questions: How can packet collisions occur in STDMA and whether packet collisions are `contagious'. We further perform a fair comparison with CSMA on the basis of which we provide recommendations on the configuration of STDMA. Our results show that STDMA coordinates multiple access effectively - even in highly congested situations - as long as all transmitted packets are decoded successfully. When non-decodable (but still carrier-sensible) transmissions are present, STDMA effectiveness drops below that achieved by CSMA due to the lack of control information. To ensure reproducibility and encourage further inquiry we release the STDMA implementation used in this paper to the wireless networks research community.

In order to interact with a Web site, humans must be able to distinguish and understand the purposes of different page blocks, e.g. header, navigation bar or content area. In case of navigational blocks, the block type determines the functionality of the hyperlinks it contains. For example, the hyperlinks in the main menu block represent the main topics of a site while the hyperlinks in a breadcrumb trail show the location in the content hierarchy. Hence, mining navigational blocks of specific types can provide valuable input for applications in the fields of crawling, ranking or presenting search results. However, analyzing visual features in order to identify specific navigational blocks as humans do is a difficult, resource-consuming task and a general solution does not exist yet. In this paper, we propose a novel approach to the problem and present the Graph-based block extraction method (GRABEX) that can be adapted to classify different types of navigational blocks. The fundamental concept is that a separate graph-based link-analysis is conducted for groups of blocks. Each block group consists of blocks from different pages that have similar CSS class attributes. This allows discovering navigational blocks of specific types, e.g. breadcrumb trails, without analyzing any presentational features. We apply our method to mine breadcrumb trails and are the first to describe an applicable solution to this problem. In an extensive evaluation including 700 different sites, the GRABEX-method performed with perfect precision and high recall.

Summary form only given. Not only the multicore dilemma massively reduces programmer productivity and the progress of energy-efficient performance — a critical issue for the long term overall affordability of computing. Because of the Tunnel Vision Syndrome the solutions coming from a few isolated areas, are by far too slow and massively imperfect. Systolic arrays (SA) have been introduced by a mathematician. His synthesis method was “of course” algebraic, supporting only a few applications and sequencing concepts were “not his job”. A decade later we transformed this SA draft into a general purpose machine paradigm which was presented at the 3rd and 8th through 11th ASAP. The acceptance of our other fundamental idea, Term Rewriting System (TRS) top-down use for microchip design EDA, was delayed by the TRS expert scene: by 30 years! The R&D landscape requires radically new solutions. We must avoid the reductionist philosophies of most specialized research areas and introduce connected thinking to bridge the gaps between different paradigms and between several abstraction levels. We must urgently rethink all basic assumptions and far-reaching cooperation patterns.

The main benefit of cooperative vehicular communication networks is the increased traffic safety that safety applications could introduce. Although the technological aspects have been extensively researched, the feasibility regions in which communication could support safety applications are yet not clear. This is, primarily, due to the lack of clearly defined application requirements. Moreover, reduction of false positives and false negatives are either not considered at all or only partially. In the current paper we investigate under which conditions vehicular communication can support safety applications. For this, we identified the requirements of safety applications, on the example of Forward Collision Warning, first from application perspective and pursuing zero false positive and zero false negative constraint. Afterwards, we quantify what is the maximum vehicle density for which application requirements can be supported under realistic communication conditions, as well as how much the idealistic application requirements have to be relaxed in order to achieve a balance between scalability and zero false rates.

Battery electric vehicles (BEV) are envisioned to play a significant role in the future of personal mobility. A key challenge in the transition from internal combustion engine (ICE) powered vehicles to BEV is the limited driving range of the latter, which makes an energy-efficient operation essential. In this work, we analyze how vehicle-to-x (V2X) communication, in particular traffic-light-to-vehicle communication (TLVC), can help the drivers of BEVs save energy and thus increase driving range. Furthermore, we analyze factors impacting energy consumption which are relevant to the design of V2X applications for BEV. Our results indicate that TLVC can significantly reduce the energy consumption of BEV, up to 20% in our setup. The actual result, however, is highly dependent on a combination of traffic situation, communication range, auxiliary consumer power demand, road gradient and minimum speed requirement.

It is most likely that upcoming vehicles will be equipped with inter-vehicle communication technologies that enable neighboring (and remote) vehicles to exchange information directly (or indirectly over multiple hops) as needed. Although those technologies obviously compete against cellular approaches, inter-vehicle communication will most likely contribute to applications such as cooperative sensing or traffic information systems. In order to evaluate the effectiveness and benefit of such applications — most importantly in very large network scenarios — it is essential to understand the time-delay characteristics of data dissemination processes and to model them efficiently. In this paper, we explore an information-centric modeling approach to describe the time delay of beaconing based data dissemination. Our approach is based on the analogy of a construction kit with elementary, connectable road shapes (or “blocks”, e.g. a straight road, an intersection, or similar). The proposed concept supports arbitrary road network layouts, is extensible, and allows to improve the accuracy (and statistical correctness) of blocks over time. We further provide two elementary blocks of the construction kit: a straight road of arbitrary length for highway, rural and city scenarios, as well as an intersection. Compared to previous work, our underlying model of these blocks addresses not only application and communication related aspects, but also mobility related aspects.

Federated identity management yields many advantages such as enhanced usability and improved quality of identity information. Web-based services are already successfully and widely federated using the Security Assertion Markup Language (SAML). In terms of usability and quality of identity information non web-based services benefit from being federated in a similar way web-based services do. However, up to this point no versatile approach that can be easily integrated has emerged to federate them. In this paper, we present FACIUS, an architecture that enables the integration of non web-based services into SAML-based federations. FACIUS aims at minimizing the integration effort in terms of both usability and necessary adjustments to existing service deployments. Furthermore, to prove the practicability of the proposed architecture, we present an implementation to federate SSH services.

For cloud-based outsourcing of confidential data, various techniques based on cryptography or data-fragmentation have been proposed, each with its own tradeoff between confidentiality, performance, and the set of supported queries. However, it is complex and error-prone to select appropriate techniques to individual scenarios manually. In this paper, we present a policy-based approach consisting of a domain specific language and a policy-transformator to automatically generate scenario-specific software adapters called mediators that set up data outsourcing and govern data access. Mediators combine state-of-the-art confidentiality techniques to ensure a user-specified level of confidentiality while still offering efficient data access. Thus, our approach simplifies data outsourcing by decoupling policy decisions from their technical implementation and realizes appropriate tradeoffs between confidentiality and efficiency.

Vehicle Safety Communications (VSC) is advancing rapidly towards product development and field testing. While a number of possible solutions have been proposed, the question remains open as how such a system will address the issue of scalability in its actual deployment. This paper presents a design methodology for congestion control in VSC as well as the description and evaluation of a resulting rate adaption oriented protocol named PULSAR. We start with a list of design principles reflecting the state of the art that define why and how vehicles should behave while responding to channel congestion in order to ensure fairness and support the needs of safety applications. From these principles, we derive protocol building blocks required to fulfill the defined objectives. Then, the actual protocol is described and assessed in detail, including a discussion on the intricate features of channel load assessment, rate adaptation and information sharing. A comparison with other state-of-the-art protocols shows that “details matter” with respect to the temporal and spatial dimensions of the protocol outcome.

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in Non-Line-of-Sight (NLOS) areas due to buildings at intersection corners. The periodic Cooperative Awareness Messages (CAM) are foreseen to be sent via 5.9GHz IEEE 802.11p. While reception rate measurements and validated propagation models for vehicular 5.9GHz NLOS conditions recently emerged, NLOS reception rates under competition on the channel are an open issue and require an in-depth evaluation to judge application feasibility. In this paper, we investigate NLOS reception on a 5.9GHz inter-vehicle communication channel by network simulation in ns-2. The simulator has been enhanced to support a state of the art vehicular NLOS propagation model. A traffic flow simulation is used to derive realistic simulation scenarios, reflecting cross-traffic assistance critical traffic situations. Results of the network simulations under different environment conditions show that reception rates in NLOS areas are on acceptable levels under competition on the channel by various transmitters.

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in Non-Line-of-Sight (NLOS) areas due to building at intersection corners. The periodic Cooperative Awareness Messages (CAM) are foreseen to be sent via 5.9 GHz IEEE 802.11p. While it is known that existing micro-cell models might not apply well, validated propagation models for vehicular 5.9 GHz NLOS conditions are still missing. In this paper, we develop a 5.9 GHz NLOS path-loss and fading model based on real-world measurements at a representative selection of intersections in the city of Munich. We show that a) the measurement data can very well be fitted to an analytical model, b) the model incorporates specific geometric aspects in closed-form as well as normally distributed fading in NLOS, and c) the model is of low complexity, thus, could be used in large-scale packet-level simulations. A comparison to existing micro-cell models shows that our model significantly differs.

Distributed Hash Tables (DHTs) prove valuable for distributed architectures by providing distributed information lookup, routing, and data storage while promising good scalability and robustness at the same time. From this point of view, a DHT could be seen as a basic service that can be used to build distributed applications. Whereas today no widely deployed and publicly accessible basic DHT service exists and thus DHT- based applications have to deploy their very own DHT networks, DHTs consisting of millions of peers are formed by file sharing clients. Although the interfaces of typical DHTs used for file sharing are too narrow, a basic DHT service could probably be created by bundling a suitable client implementation with file sharing software. In this paper, we evaluate whether a basic DHT service could suit the needs of DHT- based applications in terms of stability, number of participating peers, the peers'' session lengths, geographical distribution and peer connectivity when deployed similar to DHTs driven by file sharing. As these metrics mostly depend on end user behavior rather than on the DHT protocol, we report on measurement results gathered from monitoring of the BitTorrent Mainline client''s DHT over six months. We analyze which metrics would fit a basic DHT service and which could prove problematic. Furthermore, we discuss resulting technical requirements for an appropriate DHT protocol.

In recent years, a trend towards the usage of physical layer models with increased accuracy can be observed within the wireless network community. This trend has several reasons. The consideration of signals - instead of packets - as the smallest unit of a wireless network simulation enables the ability to reflect complex radio propagation characteristics properly, and to study novel PHY/MAC/NET cross-layer optimizations that were not directly possible before, e.g. cognitive radio networks and interference cancelation. Yet, there is a price to pay for the increase of accuracy, namely a significant decrease of runtime performance due to computationally expensive signal processing. In this paper we study whether this price can be reduced - or even eliminated - if GPU-based signal processing is employed. In particular, we present and discuss four different architectures that can be used to exploit GPU-based signal processing in discrete event-based simulations. Our evaluation shows that the runtime costs can not be cut down completely, but significant speedups can be expected compared to a non GPU-based solution.