Jens-Peter Redlich

Cooperative Opportunistic Routing Using Transmit Diversity in Wireless Mesh Networks

We propose a new high performance forwarding scheme, denoted transmit diversity based cooperative opportunistic routing (TDiCOR), that efficiently exploits multiuser and transmit diversity to improve the overall throughput in wireless multi-hop networks. Neighboring nodes cooperate through simultaneous transmissions in a MISO fashion in order to overcome the destructive effects of fading. TDiCOR uses distributed transmit diversity to increase the robustness of acknowledgements as well as data transmissions while preserving the opportunistic nature by using multiple candidates for packet relaying. We present measurements from a wireless testbed which suggest that cooperation using transmit diversity is feasible even with todays off-the-shelf hardware. At the instance of TDiCOR, we demonstrate how to realize the idea of cooperative opportunistic routing as an operational protocol and present very promising simulation results. TDiCOR outperforms traditional routing (i.e. DSR) in typical outdoor scenarios in terms of throughput by 30% and by 50% in indoor scenarios with high shadow fading, without consuming additional bandwidth or additional hardware resources.

Distributed object technology for networking

Distributed object systems have become a foundation technology and operational platform for large, complex communications networks. This article explains what distributed object technology is and what it does for networks and communication services. It describes the current key technologies in this field, the Common Object Request Broker Architecture (CORBA) and the Distributed Component Object Model (DCOM), and gives examples of networking architectures, such as TINA and TMN, that utilize these technologies. Finally, this article introduces a simplified model for a CORBA-based networking architecture that facilitates open network interfaces and provides a generic platform for services of the future.

A wireless mesh sensing network for early warning

Earthquake early warning systems should provide reliable warnings as quickly as possible with a minimum number of false and missed alarms. Wireless meshed networks, coupled with low-cost seismometers for monitoring, evaluation, and information about seismic vibrations in space and time are introducing a new generation of warning infrastructures for mega-cities. The use of a cooperative method for signal analysis makes it possible to distinguish earthquakes (with a certain minimal magnitude) from other ground shaking in a city. The paper gives a short overview of our approach for developing decentralized early warning systems and an evaluation based on experiences gained from model investigations, testbeds in Berlin, and prototype installations in Istanbul.

An Opportunistic Cross-Layer Protocol for Multi-Channel Wireless Networks

In this paper, we propose a new multi-hop forwarding scheme for wireless multi-hop networks, denoted Multi-Channel Extremely Opportunistic Routing (MCExOR), that efficiently exploits radio channel characteristics and makes use of multiple RF channels. MCExOR reduces the overall number of transmissions in wireless multi-hop networks by opportunistically skipping nodes in a packets forwarding path. The use of multiple non overlapping RF channels contributes to the reduction of overall interference. In contrast to other approaches MCExOR only needs one RF transceiver per device. We present algorithms for route discovery and packet forwarding. In contrast to other multi-channel protocols the packet forwarding is decoupled from route discovery. Therefore, both protocols have low complexity and can be addressed separately. With the help of simulations we show that MCExOR significantly outperforms traditional protocols like AODV through the simultaneous use of multiple RF channels and its opportunistic behaviour. With the increasing number of RF channels the overall throughput increases superproportionally. MCExOR with 2 RF channels surpasses AODV by an average of 140%. Unlike other multi channel approaches even a single packet flow can benefit from the existence of multiple channels.

Heat Waves, Urban Climate and Human Health

The European heat wave of 2003 was an outstanding weather event. The months of June and August have been nearly everywhere in Germany the warmest months since the start of registrations in 1901. The registered mean air summer temperature was 19, 6 ◦C, that is 3,4 K higher than the mean value. At August 9 and August 13, 2006, the highest maximum temperatures ever registered in Germany, 40, 2 ◦C, have been measured in Karlsruhe and in Freiburg. This extreme weather was caused by a blocking action of the westerly circulation due to a stationary wave forming a so-called Omegaweather type. High pressure systems with cloudless sky conditions permitted extreme sun radiation and caused repeatedly record temperatures. In southern Germany 53 hot days with maximum temperatures higher than 30 ◦C have been registered. This heat wave concerned not only Germany, but large regions of Western Europe with France and Great Britain. Other European countries like Switzerland, Spain, Portugal and Italy have been concerned, too. The questions are (1), if this extreme event is in relation with the human impact on the climate system via the emission of greenhouse gases and (2), if such events will be more frequent in the future Based on a climate change simulation the distribution of the maximum temperatures of the summer 2003 indicate that this extreme summer is expected to be a normal one by the end of this century in Central Europe (Beniston 2004, Schar et al. 2004, Meehl & Tebaldi 2004; Fig. 1)! This is in good accordance with the IPCC (2001) statements that in the future extreme weather events are very likely and that more hot days over nearly all land areas are very likely, too.

Self-Organization in Community Mesh Networks The Berlin RoofNet

A community network must be usable for inexperienced end users; thus self-organization is essential. On the one hand, we propose an approach for self-organization in ad-hoc wireless multi-hop mesh networks, where the client is fully freed from such mundane tasks as IP configuration, etc. On the other hand, the community mesh network itself is fully self-organized thus no operator or provider is required. We present the architecture of the Berlin RoofNet (BRN) and a distributed realization of services like DHCP, ARP and Internet gateway discovery and selection. In addition, results of a detailed simulation and experimental evaluation comparing our distributed hash table based approach to traditional methods are presented. We show that our approach is more reliable, efficient and responsive

Considerations on forwarder selection for opportunistic protocols in wireless networks

Opportunistic Routing gained lots of attention as a way to improve the performance of wireless multi-hop relay mesh networks. The key characteristic is its ability to take advantage of the numerous, yet unreliable wireless links in the network. The most important part of every opportunistic routing protocol is the forwarder selection algorithm. Most of the currently known protocols assume that signal paths from a sender to all candidates are independent to each other and therefore resulting in independent packet error rates. However, this assumption does not hold for spatially close candidates which are not so uncommon in indoor networks. In this paper, we present empirical measurements from our 802.11 indoor test-bed which reveal that signal paths to spatially close nodes are correlated. We believe that the loss in the radio propagation due to shadow fading is similar to spatially close nodes. For our setup we find out that a spatial correlation exists when the nodes are closer than 2 m to each other. We present a candidate set selection algorithm which is able to calculate the packet error rate of a candidate set even when the individual packet error rates are correlated, e.g. due to spatial correlation. Therefore only a simple modification to the existing ordinary link probing is required. Finally, we present modifications we made to our packet-level simulator to respect spatial correlation as well as simulation results.

Secure candidate access router discovery

The candidate access router (CAR) discovery protocol is designed for use in wireless IP networks to dynamically collect information about neighboring access routers and their capabilities. This capability enables mobile nodes to dynamically execute low-latency hand-offs and to intelligently select a target access router. This paper presents possible approaches for CAR discovery, a security analysis of those approaches, and our novel security CAR discovery mechanisms.

Virtual networks in the Internet

Selective treatments are needed for different types of traffic and different user groups, even on the Internet. Virtual networks provide capabilities for custom routings, priorities, and processing that are not available in either integrated services or differentiated services. This paper relates virtual networks to public network concepts of traffic grooming, virtual private networks and virtual paths, and shows how these and broader capabilities may be realized on the Internet. It describes a networking strategy incorporating intelligent routers that can decouple application QoS needs from core network resource allocations. These routers can be programmed to provide virtual networks, which can satisfy a wide range of application requirements.

Multi-Channel Opportunistic Routing in Multi-Hop Wireless Networks

We propose and investigate Multi-Channel Extremely Opportunistic Routing (MCExOR) which is a protocol that extends Extremely Opportunistic Routing by utilizing multiple RF channels in multi-hop wireless networks. Large numbers of transmissions per end-to-end delivery combined with interference are the main reasons for the low capacity of wireless multi-hop networks. MCExOR reduces the overall number of transmissions in wireless multi-hop networks by opportunistically skipping nodes in a packet’s forwarding path. The use of multiple non overlapping RF channels contributes to the reduction of overall interference. In contrast to other approaches MCExOR only needs one RF transceiver per device. We present algorithms for route discovery and packet forwarding. A significant benefit of MCExOR is that the selection of RF channels is independent of the routing function. Finally, with the help of simulations we show that MCExOR outperforms traditional protocols like ad-hoc on-demand distance vector routing through the simultaneous use of multiple RF channels. In combination with realistic radio propagation models an increase in the throughput is observed due to the opportunistic feature of MCExOR. With the increasing number of RF channels the overall throughput increases superproportionally. Unlike other multi channel approaches even a single packet flow can benefit from the existence of multiple channels.