Ozan K. Tonguz

Integrated cellular and ad hoc relaying systems: iCAR

Integrated cellular and ad hoc relaying systems (iCAR) is a new wireless system architecture based on the integration of cellular and modern ad hoc relaying technologies. It addresses the congestion problem due to unbalanced traffic in a cellular system and provides interoperability for heterogeneous networks. The iCAR system can efficiently balance traffic loads between cells by using ad hoc relaying stations (ARS) to relay traffic from one cell to another dynamically. This not only increases the systems capacity cost effectively, but also reduces the transmission power for mobile hosts and extends system coverage. We compare the performance of the iCAR system with conventional cellular systems in terms of the call blocking/dropping probability, throughput, and signaling overhead via analysis and simulation. Our results show that with a limited number of ARSs and some increase in the signaling overhead (as well as hardware complexity), the call blocking/dropping probability in a congested cell and the overall system can be reduced.

Broadcast storm mitigation techniques in vehicular ad hoc networks

Several multihop applications developed for vehicular ad hoc networks use broadcast as a means to either discover nearby neighbors or propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem, a scenario in which there is a high level of contention and collisions at the link layer due to an excessive number of broadcast packets. While this is a well-known problem in mobile ad hoc wireless networks, only a few studies have addressed this issue in the VANET context, where mobile hosts move along the roads in a certain limited set of directions as opposed to randomly moving in arbitrary directions within a bounded area. Unlike other existing works, we quantify the impact of broadcast storms in VANETs in terms of message delay and packet loss rate in addition to conventional metrics such as message reachability and overhead. Given that VANET applications are currently confined to using the DSRC protocol at the data link layer, we propose three probabilistic and timer-based broadcast suppression techniques: weighted p-persistence, slotted 1-persistence, and slotted p-persistence schemes, to be used at the network layer. Our simulation results show that the proposed schemes can significantly reduce contention at the MAC layer by achieving up to 70 percent reduction in packet loss rate while keeping end-to-end delay at acceptable levels for most VANET applications.

Routing in Sparse Vehicular Ad Hoc Wireless Networks

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.

Broadcasting in VANET

In this paper, we report the first complete version of a multi-hop broadcast protocol for vehicular ad hoc networks (VANET). Our results clearly show that broadcasting in VANET is very different from routing in mobile ad hoc networks (MANET) due to several reasons such as network topology, mobility patterns, demographics, traffic patterns at different times of the day, etc. These differences imply that conventional ad hoc routing protocols such as DSR and AODV will not be appropriate in VANETs for most vehicular broadcast applications. We identify three very different regimes that a vehicular broadcast protocol needs to work in: i) dense traffic regime; ii) sparse traffic regime; and iii) regular traffic regime. We build upon our previously proposed routing solutions for each regime and we show that the broadcast message can be disseminate efficiently. The proposed design of the distributed vehicular broadcast (DV-CAST) protocol integrates the use of various routing solutions we have previously proposed.

DV-CAST: A distributed vehicular broadcast protocol for vehicular ad hoc networks

The potential of infrastructureless vehicular ad hoc networks for providing safety and nonsafety applications is quite significant. The topology of VANETs in urban, suburban, and rural areas can exhibit fully connected, fully disconnected, or sparsely connected behavior, depending on the time of day or the market penetration rate of wireless communication devices. In this article we focus on highway scenarios, and present the design and implementation of a new distributed vehicular multihop broadcast protocol, that can operate in all traffic regimes, including extreme scenarios such as dense and sparse traffic regimes. DV-CAST is a distributed broadcast protocol that relies only on local topology information for handling broadcast messages in VANETs. It is shown that the performance of the proposed DV-CAST protocol in terms of reliability, efficiency, and scalability is excellent.

Impact of Vehicles as Obstacles in Vehicular Ad Hoc Networks

A thorough understanding of the communications channel between vehicles is essential for realistic modeling of Vehicular Ad Hoc Networks (VANETs) and the development of related technology and applications. The impact of vehicles as obstacles on vehicle-to-vehicle (V2V) communication has been largely neglected in VANET research, especially in simulations. Useful models accounting for vehicles as obstacles must satisfy a number of requirements, most notably accurate positioning, realistic mobility patterns, realistic propagation characteristics, and manageable complexity. We present a model that satisfies all of these requirements. Vehicles are modeled as physical obstacles affecting the V2V communication. The proposed model accounts for vehicles as three-dimensional obstacles and takes into account their impact on the LOS obstruction, received signal power, and the packet reception rate. We utilize two real world highway datasets collected via stereoscopic aerial photography to test our proposed model, and we confirm the importance of modeling the effects of obstructing vehicles through experimental measurements. Our results show considerable obstruction of LOS due to vehicles. By obstructing the LOS, vehicles induce significant attenuation and packet loss. The algorithm behind the proposed model allows for computationally efficient implementation in VANET simulators. It is also shown that by modeling the vehicles as obstacles, significant realism can be added to existing simulators with clear implications on the design of upper layer protocols.

On the Broadcast Storm Problem in Ad hoc Wireless Networks

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem. In this paper, we explore how serious the broadcast storm problem is in both MANET and VANET by examining how broadcast packets propagate in a 2-dimensional open area and on a straight road or highway scenarios. In addition, we propose three novel distributed broadcast suppression techniques; i.e., weighted p-persistence, slotted 1-persistence, and slotted p- persistence schemes. Our simulation results show that the proposed schemes can achieve up to 90% reduction in packet loss rate while keeping the end-to-end delay at acceptable levels for most VANET applications. They can also be used together with the route discovery process to guide the routing protocols to select routes with fewer hop counts.

Ad hoc wireless networks : a communication-theoretic perspective

Preface. List of Acronyms. 1 Related Work and Preliminary Considerations. 1.1 Introduction. 1.2 Related Work. 1.3 A New Perspective for the Design of Ad Hoc Wireless Networks. 1.4 Overview of the Underlying Assumptions in the Following Chapters. 1.5 The Main Philosophy Behind the Book. 2 A Communication-Theoretic Framework for Multi-hop Ad Hoc Wireless Networks: Ideal Scenario. 2.1 Introduction. 2.2 Preliminaries. 2.3 Communication-Theoretic Basics. 2.4 BER Performance Analysis. 2.5 Network Behaviour. 2.6 Concluding Remarks. 3 A Communication-Theoretic Framework for Multi-hop Ad Hoc Wireless Networks: Realistic Scenario. 3.1 Introduction. 3.2 Preliminaries. 3.3 Communication-Theoretic Basics. 3.4 Inter-node Interference. 3.5 RESGOMAC Protocol. 3.6 RESLIGOMAC Protocol. 3.7 Network Behavior. 3.8 Conclusions. 4 Connectivity in Ad Hoc Wireless Networks: A Physical Layer Perspective. 4.1 Introduction. 4.2 Quasi-regular Topology. 4.3 Random Topology. 4.4 Concluding Remarks and Discussion. 5 Effective Transport Capacity in Ad Hoc Wireless Networks. 5.1 Introduction. 5.2 Modeland Assumptions. 5.3 Preliminaries. 5.4 Single-Route Effective Transport Capacity. 5.5 Aggregate Effective Transport Capacity. 5.6 Comparison of the RESGO and RESLIGOMAC Protocols. 5.7 Spread-RESGO: Improved RESGOMAC Protocol with Per-route Spreading Codes. 5.8 Discussion. 5.9 Concluding Remarks. 6 Impact of Mobility on the Performance of Multi-hop Ad Hoc Wireless Networks. 6.1 Introduction. 6.2 Preliminaries. 6.3 Switching Models. 6.4 Mobility Models. 6.5 Numerical Results. 6.6 Conclusions. 7 Route Reservation in Ad Hoc Wireless Networks. 7.1 Introduction. 7.2 Related Work. 7.3 Network Models and Assumptions. 7.4 The Two Switching Schemes. 7.5 Analysis of the Two Switching Techniques. 7.6 Results and Discussion. 7.7 Concluding Remarks. 8 Optimal Common Transmit Power for Ad Hoc Wireless Networks. 8.1 Introduction. 8.2 Modeland Assumptions. 8.3 Connectivity. 8.4 BER at the End of a Multi-hop Route. 8.5 Optimal Common Transmit Power. 8.6 Performance Metrics. 8.7 Results and Discussion. 8.8 Related Work. 8.9 Conclusions. 9 Routing Problem in Ad Hoc Wireless Networks: A Cross-Layer Perspective. 9.1 Introduction. 9.2 Experimental Evidence. 9.3 Preliminaries: Analytical Models and Assumptions. 9.4 Route Selection: Simulation Study. 9.5 Network Performance Evaluation. 9.6 Discussion. 9.7 Related Work. 9.8 Conclusions. 10 Concluding Remarks. 10.1 Introduction. 10.2 Extensions of the Theoretical Framework: Open Problems. 10.3 Network Architectures. 10.4 Network Application Architectures. 10.5 Standards. 10.6 Applications. 10.7 Conclusions. Appendix A. Appendix B. Appendix C. Appendix D. Appendix E. References. Index.

Optimal Transmit Power in Wireless Sensor Networks

Power conservation is one of the most important issues in wireless ad hoc and sensor networks, where nodes are likely to rely on limited battery power. Transmitting at unnecessarily high power not only reduces the lifetime of the nodes and the network, but also introduces excessive interference. It is in the network designers best interest to have each node transmit at the lowest possible power while preserving network connectivity. In this paper, we investigate the optimal common transmit power, defined as the minimum transmit power used by all nodes necessary to guarantee network connectivity. This is desirable in sensor networks where nodes are relatively simple and it is difficult to modify the transmit power after deployment. The optimal transmit power derived in this paper is subject to the specific routing and medium access control (MAC) protocols considered; however, the approach can be extended to other routing and MAC protocols as well. In deriving the optimal transmit power, we distinguish ourselves from a conventional graph-theoretic approach by taking realistic physical layer characteristics into consideration. In fact, connectivity in this paper is defined in terms of a quality of service (QoS) constraint given by the maximum tolerable bit error rate (BER) at the end of a multihop route with an average number of hops

Failure location algorithm for transparent optical networks

Fault and attack management has become a very important issue for network operators that are interested to offer a secure and resilient network capable to prevent and localize, as accurately as possible, any failure (fault or attack) that may occur. Hence, an efficient failure location method is needed. To locate failures in opaque optical networks, existing methods which allow monitoring of the optical signal at every regeneration site can be used. However, to the best of our knowledge, no method exists today that performs failure location for transparent optical networks. Such networks are more vulnerable to failures than opaque networks since failures propagate without being isolated due to optoelectronic conversions. In this paper, we present a failure location algorithm that aims to locate single and multiple failures in transparent optical networks. The failure location algorithm developed in this paper can cope with ideal scenarios (i.e., no false and/or lost alarms), as well as with nonideal scenarios having false and/or lost alarms.