Anatolij Zubow

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.

Automated and transparent model fragmentation for persisting large models

Existing model persistence frameworks either store models as a whole or object by object. Since most modeling tasks work with larger aggregates of a model, existing persistence frameworks either load too many objects or access many objects individually. We propose to persist a model broken into larger fragments. First, we assess the size of large models and describe typical usage patterns to show that most applications work with aggregates of model objects. Secondly, we provide an analytical framework to assess execution time gains for partially loading models fragmented with different granularity. Thirdly, we propose meta-model-based fragmentation that we implemented in an EMF based framework. Fourthly, we analyze our approach in comparison to other persistence frameworks based on four common modeling tasks: create/modify, traverse, query, and partial loads. We show that there is no generally optimal fragmentation, that fragmentation can be achieved automatically and transparently, and that fragmentation provides considerable performance gains.

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.

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.

Adjacent channel interference in IEEE 802.11n

In this paper we analyze the adverse effects of Adjacent Channel Interference (ACI) on 802.11 with a focus on new 802.11n standard. ACI is causing problems that are related to the carrier sensing mechanism in 802.11. On the one hand, the carrier sensing is sometimes too restrictive thus preventing concurrent transmissions which leads to a variant of the exposed terminal problem. On the other hand, the carrier sensing is sometimes too optimistic thus causing packet collisions which is a form of the hidden node problem. Both problems are especially severe in multi-radio systems, where the radios are very closely spaced. Such problems already investigated in 802.11a/b/g still remain with 802.11n. Our results show that the number of available orthogonal channels in IEEE 802.11n depends on the spatial spacing between the radios, channel width (20MHz vs. 40 MHz), RF band (2.4 vs. 5GHz) and traffic pattern. In a multi-radio system the situation is worst, e.g. in the 2.4 GHz we were not able to find 2 orthogonal channels. The adverse effect of ACI can be reduced in two ways. First, by increasing the spatial separation between the radios; a spacing of less than 1 meter already improves the situation significantly, e.g. 40 cm are sufficient to get 2-3 orthogonal 20 MHz channels in the 2.4 GHz band with reduced transmission power. Furthermore, a distance of 90 cm is also sufficient so that a 40 and a 20MHz channel can be used simultaneously without any interference. However, in a multi-radio system the spatial spacing between the radios cannot be increased due to space limitations. The only option to overcome ACI related problems is to reduce the transmit power making power control essential. Finally, our analysis revealed that 802.11 is an inappropriate protocol for multi-channel MAC/routing protocols based on multi-radio systems where an explicit MAC layer link-scheduling is more promising.

ClickWatch — An experimentation framework for communication network test-beds

It is hard to experiment with test-beds for communication networks: data produced in the network has to be retrieved and analyzed, networks must be reconfigured before and between experiments, data is often little structured (log-files) and analysis methods and tools are generic. Even though many problems of experimentation are the same for all experiments, re-use is sparse and even simple experiments require large efforts. We present a framework that attempts to solve these problems: we define a set of requirements for experimenting with network test-beds, we describe the principles and inner workings of our framework, demonstrate it with a typical example experiment, and present measurement results that illustrate the feasibility and scalability of our approach. Some qualitative and quantitative aspects of ClickWatch are compared to the commonly used logfile based approach to experimentation.

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.

A low-cost MIMO mesh testbed based on 802.11n

In this paper, we present a configurable and inexpensive MIMO mesh network research platform based on IEEE 802.11n and open source software. There are two requirements for such a research testbed. First configurability: the researcher needs the ability to modify hard- and software on each ISO/OSI layer. Secondly, the testbed has to comprise hundreds of nodes to make sound conclusions on network performance. Therefore, a single mesh node has to be cheap. Both requirements have contradicting implications and a tradeoff is necessary. We propose a solution based on off-the-shelf 802.11n hardware with Atheros WiFi chips, the open source WiFi driver ath9k, and the Click Router API. This represents a good tradeoff. Procurement costs for a single network node are below 100

HWL — A high performance wireless sensor research network

. The solution yet allows a variety of adjustments based on the open-source driver and a highly configurable WiFi hardware. We evaluated our platform and present measurement results.