Contributions in Merlin

This fourth edition of the seminar entitled ""Mobile Systems IV"" discusses a number of selected topics in the area of mobile communication. The technical report has been prepared during the autumn term HS 2009 as the result of the Mobile Systems seminar. The report discusses driving topics in mobile communications technology, including investigations on challenges in mobile systems, support for mobile users, mobile network architectures and protocols, and applications in mobile environments.

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This master’s thesis relates to the field of requirements engineering for software product lines. Compared to single-system modeling, the creation of product line models is more challenging. This work bases on the ADORA language, a graphical modeling language for requirements and architecture that also supports the modeling of software product lines. Whereas this modeling language is relatively easy for single system models, the modeling of variability is more complicated, as sophisticated weaving semantics Mei09 have to be applied. Additionally, the creation of variants in ADORA is very unhandy, as a lot of klicking has to be done by the modeler. In this thesis we introduce a new approach that simplifies the creation of a product line model, called Feature Unweaving. This feature unweaving function allows the user to select model elements that should be extracted into a variant. According to the selection, the function is then able to create a variant with correct weaving semantics automatically. The automation concentrates on the model elements that define the weaving semantics of a variant. The development of this function was based on two real-world exemplars. Additionally, we have analyzed and extended the current weaving semantics to make sure that they were feasible to model such real-world software product lines. In a last step, we evaluated the feature unweaving function on the basis of the real-world product line exemplars. During this evaluation, we could conclude that the unweaving function was able to extract all the variants that occurred in the exemplars. Furthermore, the process of feature extraction in ADORA could be made more efficient, less effort is required to create variants. Additionally, modelers that don’t have detailed knowledge about ADORA’s weaving semantics yet, can introduce variability into an ADORA model more easily, this makes the ADORA language more accessible.

Despite all efforts in creating and disseminating requirements modeling languages, natural language is still the dominant language for writing requirements specifications in practice. Furthermore, when documenting early requirements, natural language (in combination with pictures) outperforms today’s requirements modeling languages. In this paper, we present a vision and research roadmap for an ultralightweight requirements modeling language which can be used as easily as natural language with pictures, but has a visual structure with some lightweight semantics that allow the visual expression of hierarchical structure, context, general relationship, flow, and influence, while all the details are specified in natural language, both form-based and free-text. Furthermore, it should be possible to evolve parts of such an ultralightweight model into classic models of structure and behavior with full-fledged semantics by incrementally adding more formal model elements and tightening the meaning of the already existing ones. We envisage that such a modeling language – when supported by appropriate tools – will (1) outperform natural language requirements specifications with respect to comprehensibility, changeability, analyzability and internal traceability, (2) be simpler and more straightforward to read and create than today’s heavyweight modeling languages, (3) provide an effi- cient and effective means for expressing requirements at an early stage.

An increasing number of Internet users and growing Internet bandwidth availability and its usage lead to the need for scalable systems, which scale with the increasing number of users and bandwidth. Client/server (C/S) systems have capacity limits and as soon as these limits are reached, the system infrastructure needs to be upgraded or replaced. Peer-to-peer (P2P) systems do not have such capacity limitations, because every participating peer is a client and a server, contributing its resources. Thus, the more users a P2P system has, the more resources are contributed. Compared to C/S systems, P2P systems shift infrastructure expenses from the server to its peers, reducing the expenses at the server-side. However, P2P systems require additional mechanisms to offer the same functionality as a centralized system. Large-scale C/S systems such as Wikipedia, Facebook, or YouTube need to keep extending their server infrastructure due to increasing end-user bandwidth and number of users. To cover theses infrastructure expenses, operators of such C/S systems need to generate sufficient revenues. Lack of revenues leads to reducing or shutting down services. Since P2P systems distribute its infrastructure expenses among its peers, operators could run such C/S systems at a lower cost if changing to a P2P system. The advantages for various P2P systems have been demonstrated by many real-world applications, such as file sharing applications, Voice over Internet Protocol applications, or media streaming applications. However, new types of P2P applications are still challenging as they require new distributed mechanisms and algorithms. The goal of this thesis is to present new distributed mechanisms and algorithms for a scalable, robust, and fault-tolerant, P2P document collaboration system. In a P2P document collaboration systems, users share the resources necessary to host and distribute documents. Supported tasks in such a system are searching, retrieving, creating, changing, and maintaining documents. This thesis presents the design, implementation and evaluation of the PeerCollaboration application and its three key features that have not been profoundly researched to date: (1) a new robust transitive tit-for-tat incentive mechanism to encourage peers to contribute resource and to discourage free-riders is presented. Each peer needs to contribute resources, such as providing documents to other peers, to make the system scalable; (2) a new efficient fast similarity search algorithm is presented to search for documents with misspelled text; (3) a new user-based voting mechanism is presented to maintain or improve the quality of user-generated documents and to provide decentralized administration mechanisms.

The popularity of video sharing over the Internet has increased significantly. High traffic generated by such applications at the source can be better distributed using a peer-to-peer overlay. LiveShift combines both live and on-demand video streaming -- while video is transmitted through the peer-to-peer network in a live fashion, all peers participate in distributed storage. This adds the ability to replay time-shifted streams from other peers in a distributed and scalable manner. This technical report describes an architecture, a mesh-pull protocol, and a set of policies that support the envisioned use case enable. User-focused evaluation results show its effectiveness and limits in terms of quality of experience.

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This technical report describes the discourse representation structures (DRS) derived from texts written in version 6.6 of Attempto Controlled English (ACE 6.6). The description is done by an exhaustive set of examples. Among other things, ACE 6.6 supports modal statements, negation as failure, and sentence subordination. These features require an extended form of discourse representation structures. The discourse representation structure itself uses a reified, or ‘flat’ notation, meaning that its atomic conditions are built from a small number of predefined predicates that take constants standing for words of the ACE text as their arguments.