Catálogo de publicaciones - libros

Simulation tools are frequently used for performance evaluations of mobile ad-hoc networks. Currently the tools poorly support urban scenarios, since they do not take a spatial environment into account. In this paper, we describe a platform for the modeling of city scenarios. We extend ns-2 with corresponding mobility and wireless transmission models. By using its emulation facility, we integrate unmodified applications and real implementations of network protocols. We demonstrate the usefulness of the platform for performance evaluations by modeling a mobile application in a simulated environment of Stuttgart downtown. We show that it helps identifying application problems before deployment.

A novel approach to performing context discovery in ad-hoc networks based on the use of attenuated Bloom filters is proposed in this paper. In order to investigate the performance of this approach, a model has been developed. This document describes the model and its validation. The model has been implemented in Matlab, and some results are also shown in this document. Attenuated Bloom filters appear to be a very promising approach for context discovery in ad hoc networks.

This paper presents the Cluster-label-based mechanism for Backbones (CLaB) used in mobile ad hoc networks. The proposed mechanism provides a distributed topology control and consists of three parts: the part creating a backbone, the routing part, and the maintenance part on the backbone. The first part creates a clustered topology using a unique ID called a cluster label and establishes connections between neighboring clusters to create a backbone. The second part adapts existing routing protocols on the backbone. The third part maintains links on the backbone to minimize the influence of node movements, and needs no rerouting mechanism. The mechanism especially concentrates on maintenance by introducing constantly connected backbone elements based on cluster labels. A comparison with other backbone-based mechanisms is presented featuring different node densities and mobility levels. The results validate the effectiveness of the proposed mechanism.

The interaction between TCP flows and the IEEE 802.11 MAC protocol is analyzed in this paper. The goal is to provide comprehensive models capable of predicting the TCP performance (throughput) in WLAN Hot-Spot networks with persistent elastic flows. A model of the IEEE 802.11 MAC protocol is first introduced and validated through simulations. Using the MAC model as a basis, we then analyze several modeling strategies to catch the main interactions between TCP and MAC protocols. Results obtained from the models are compared among themselves and against simulations, and show the accuracy and the superior simplicity of the proposed models with respect to previously published work.

The deployment of high performance networks and the emergence of broadband wireless access technologies challenge the performance of standard TCP. Recent years have seen a few high performance and aggressive TCP variants being proposed. These TCP variants still suffer significant performance degradation from random/transient packet losses. In this paper, we introduce and evaluate a new method to improve high performance TCP in the presence of random packet losses. It is ‘blind’ in that our method does not attempt to differentiate between random and congestion-induced losses. Our method combines both TCP parallelisation and modification to the aggressiveness of studied AIMD algorithms. We show that our proposed method is no more aggressive than the studied TCP variants in the presence of congestion but more effectively utilises bandwidth in the presence of random packet losses.

Real-time streaming over satellite IP networks is challenging, since satellite links commonly exhibit long propagation delays and increased error rates, which impair TCP performance. In this context, we quantify the effects of satellite links on TCP efficiency and streaming video delivery. We investigate a solution-framework composed by TCP implementations which are expected to perform adequately in such environments. Furthermore, we study the supportive role of Selective Acknowledgments (SACK). Along with protocol performance, we also evaluate the impact of delayed acknowledgments. Our simulation results illustrate that most existing end-to-end solutions do not comply with the stringent QoS provisions of time-sensitive applications, resulting in inefficient bandwidth utilization and increased delays in data delivery. Finally, with the absence of a satellite-optimized TCP implementation for real-time streaming, we identify TCP Real as the most prominent solution, since it manages to alleviate most of the impairments induced by satellite links, sustaining a relatively smooth transmission rate.

With the widespread use of wireless Internet and wireless LANs, different wireless technologies, such as 3G cellular networks and WLAN, will cooperate to support more users and applications with higher data rates over wider areas. When a mobile node moves between hybrid networks, a need arises for seamless vertical handoffs between different wireless networks in order to provide high performance data transmission. When an application with a TCP connection in a mobile node performs a vertical handoff, TCP performance is degraded due to packet loss even while the previous TCP state information is maintained during handoff, as 3G and WLAN have different available bandwidths. In this paper, a new congestion control algorithm is proposed for a vertical handoff that improves TCP performance by measuring the rough end-to-end available bandwidth and calculating the slow-start threshold. By ns-2 simulation it is shown that, compared to previous algorithms, the proposed algorithm enhances TCP performance during vertical handoffs.

Many research work is going on in the domain of pure ad-hoc networking and even more issues are raised due to the missing central infrastructure to properly manage resources, guarantee fairness, and provide security features. On the other hand lot of research effort is spent to increase the performance of infrastructure-based access networks to cope with the steadily increasing demand for broadband data. When making a step back, the most promising evolution of heterogeneous networking is the integration of both paradigms. Taking advantage of the well controlled cellular environment and the high capacity of ad-hoc and direct node-to-node communication. The resulting hybrid networks are incorporating the best of both worlds. The concept of Cellular Assisted Heterogeneous Networking (CAHN) provides a framework to offer convenient and secure management of heterogeneous end-to-end sessions between nodes. The introduced separation of the signaling and the data plane allows to switch on power demanding broadband interfaces like GPRS, UMTS, or even WLAN only, if actually required, i.e., data has to be sent or received. The proposed out-of-band signaling enables furthermore the integration of ad-hoc links to offer best performance whenever nodes are within vicinity. Extensive simulations show that both, the integration of ad-hoc links and the selective activation of high power broadband interfaces, can significantly increase the performance of heterogeneous networks.

Classical micro-mobility solutions focus on accelerating local handovers and on minimizing signalling traffic. However, the micro-mobility components also slow down the service times in the user traffic domain. This is a critical drawback in terms of the hard delay requirements of future IP-based radio access networks. Network operators need algorithms to determine the optimal number and placement of anchor points within his network to balance both handover latency and user traffic delay. This paper presents a very fast and exact mathematical method for selecting the optimal number and placement of nodes supporting micro-mobility functionality for telecommunication networks. The method is in particular applicable for large networks, which is demonstrated by means of an optimization of a real wireless access network with more than 150 nodes. In order to ensure the applicability in real network scenarios we additionally provide a solution to estimate the stability of the optimal configuration. The presented method allows the autonomous and optimal configuration of the network’s micro-mobility functionality as described in concepts of zero-touch networks. Due to the optimization network operators can reduce the administration cost and maximize the quality of service as well.

IP multicast is an effective technology to distribute identical data simultaneously to multiple users. However, for technical and administrative reasons, IP multicast has not been globally deployed on the Internet. Another approach to multicast is application-level multicast (ALM). In ALM, multicast related features, such as group membership management, multicast routing and packet replication, are implemented at end-hosts instead of routers. Multicast delivery tree is constructed in the application layer, so all nodes in this tree are end-hosts. Packet transmission between end-hosts uses conventional IP unicast service. Therefore, ALM is a promising alternative to IP multicast. However, application-level multicast relying on end-hosts is more fragile than IP multicast relying on routers. Especially, when a mobile host forwards packets to downstream hosts, a handover causes performance degradations on downstream hosts. In this paper, to alleviate the impact of a handover, we propose a new tree building protocol which locates mobile hosts on leaves of multicast tree. A handover of the mobile host on a leaf does not affect other end-hosts. To investigate performance of our protocol, it is compared with existing application-level multicast protocols. Our simulation results show that our protocol outperforms existing protocols from the viewpoints of loss probability, throughput and delay performance.