Francisco J. Ros

Acknowledgment-Based Broadcast Protocol for Reliable and Efficient Data Dissemination in Vehicular Ad Hoc Networks

We propose a broadcast algorithm suitable for a wide range of vehicular scenarios, which only employs local information acquired via periodic beacon messages, containing acknowledgments of the circulated broadcast messages. Each vehicle decides whether it belongs to a connected dominating set (CDS). Vehicles in the CDS use a shorter waiting period before possible retransmission. At time-out expiration, a vehicle retransmits if it is aware of at least one neighbor in need of the message. To address intermittent connectivity and appearance of new neighbors, the evaluation timer can be restarted. Our algorithm resolves propagation at road intersections without any need to even recognize intersections. It is inherently adaptable to different mobility regimes, without the need to classify network or vehicle speeds. In a thorough simulation-based performance evaluation, our algorithm is shown to provide higher reliability and message efficiency than existing approaches for nonsafety applications.

Reliable and Efficient Broadcasting in Vehicular Ad Hoc Networks

Most of the envisioned services over vehicular networks need to deliver information to all vehicles inside a certain region. Several such broadcasting protocols have been reported so far, but surprisingly only one of them addresses the issue of intermittent connectivity. In this paper, we present a broadcast protocol which is suitable for a wide range of vehicular scenarios and traffic conditions. The protocol employs local position information acquired via periodic beacon messages. Beacons are used by cars to decide whether or not they belong to a connected dominating set (CDS). Vehicles in the CDS use shorter waiting period before possible retransmissions. Identifiers of circulated broadcast messages are added to beacons as piggybacked acknowledgements. When waiting timeout expires, vehicle retransmits if it has at least one neighbor which did not acknowledge circulated message with the last beacon, and sets a new waiting period. Our algorithm does not depend on any parameter or threshold which varies its operation. Despite its simplicity, the protocol is shown to provide high reliability and efficiency by means of a simulation-based performance evaluation. It also greatly outperforms the only competing algorithm we found in the literature which explicitly considers different mobility scenarios.

Cluster-based OLSR extensions to reduce control overhead in mobile ad hoc networks

Proactive routing protocols for Mobile Ad Hoc Networks (MANETs) traditionally fail to scale up to large networks, since they generate a big amount of routing overhead. Based on OLSR, a proactive solution specifically designed for dense ad hoc networks, we develop a low overhead protocol called Clustered OLSR (C-OLSR). C-OLSR assumes that somehow the network is partitioned into clusters, and restricts the propagation of topology control messages inside every cluster. The generation and forwarding of inter-cluster topology information is based on the use of Multipoint Relays (MPRs) at the level of clusters. Through a simulation study, we show that C-OLSR outperforms OLSR both in terms of overhead generation and achievable throughput.

Internet connectivity for mobile ad hoc networks: solutions and challenges

The interconnection of mobile ad hoc networks to fixed IP networks is one of the topics receiving more attention within the MANET working group of the IETF as well as in many research projects funded by the European Union. Several solutions have recently been proposed, but at this time it is unclear which ones offer the best performance compared to the others. In addition to introducing the main challenges and design options that need to be considered, we perform a simulation-based evaluation aiming at providing some insight on the performance of these approaches. These simulation results have proven themselves valuable by showing that some of the most eye-catching features of the proposed approaches have practical performance issues which need to be enhanced.

Five nines of southbound reliability in software-defined networks

In order to deploy fault-tolerant Software-Defined Networks (SDN), the logically centralized controller must be physically distributed among different devices. In this paper, we present our initial work on determining how many controllers need to be instantiated, where they must be deployed, and what network nodes are under control of each of them, in order to achieve at least five nines reliability in the southbound interface between controllers and nodes. For this, we introduce the Fault Tolerant Controller Placement problem and develop a heuristic algorithm that computes placements with (at least) the required reliability. We run such algorithm on a set of 124 publicly available network topologies. We find that each node is required to connect to just 2 or 3 controllers, which typically provide more than five nines reliability. While the total number of controllers varies greatly and is more related to the network topology than to the network size, 10 controllers or less cover 75% of the most interesting cases. Therefore, fault tolerant SDNs are achievable by carefully determining the placement of controllers.

Simulation-Based Study of Common Issues in VANET Routing Protocols

Vehicular communications have been one of the hottest research topics for the last few years. Many routing protocols have been proposed for such kind of networks. Most of them try to exploit the information which may be available at the vehicle by the time that a routing decision must be made. In addition, some solutions are designed taking into account the particular, highly partitioned, network connectivity in vehicular settings. To do so, they embrace the store-carry- forward paradigm of delay-tolerant networks. Despite the great variety of approaches which have been proposed, we found that there is a set of issues which are common to many vehicular ad hoc routing protocols in the literature. In this paper, we perform a simulation-based analysis of five of those protocols, which are representative of the various categories of vehicular routing. We describe in detail every problem and show simulation results which support our reasonings. Moreover, solutions to solve every presented problem are outlined. The paper is concluded with some guidelines which may be helpful to prospective VANET routing protocol designers.

A survey on modeling and simulation of vehicular networks: Communications, mobility, and tools

Abstract Simulation is a key tool for the design and evaluation of Intelligent Transport Systems (ITS) that take advantage of communication-capable vehicles in order to provide valuable safety, traffic management, and infotainment services. It is widely recognized that simulation results are only significant when realistic models are considered within the simulation toolchain. However, quite often research works on the subject are based on simplistic models unable to capture the unique characteristics of vehicular communication networks. If the implications of the assumptions made by the chosen models are not well understood, incorrect interpretations of simulation results will follow. In this paper, we survey the most significant simulation models for wireless signal propagation, dedicated short-range communication technologies, and vehicular mobility. The support that different simulation tools offer for such models is discussed, as well as the steps that must be undertaken to fine-tune the model parameters in order to gather realistic results. Moreover, we provide handy hints and references to help determine the most appropriate tools and models. We hope this article to help prospective collaborative ITS researchers and promote best simulation practices in order to obtain accurate results.

On reliable controller placements in Software-Defined Networks

In order to deploy fault-tolerant Software-Defined Networks (SDNs), a logically centralized controller must be physically distributed among different devices. In this paper, we focus on determining how many controllers need to be instantiated, where they must be deployed, and what network nodes are under control of each of them, in order to achieve high reliability in the southbound interface between controllers and nodes. For this, we define the fault tolerant controller placement problem and develop a heuristic algorithm that computes placements with (at least) the required reliability. We run such algorithm on a set of 124 publicly available network topologies. The results are thoroughly analyzed and provide insight on the feasibility of achieving fault tolerant SDNs by carefully determining the placement of controllers.

BRAVE: Beacon-less routing algorithm for vehicular environments

We study the problem of multihop routing in vehicular ad hoc networks (VANET). IEEE 802.11p and other vehicular network standards advocate vehicles to issue periodic broadcast messages at regular intervals called beacons. Beacons include among other information geographic coordinates of the vehicle, heading, speed, etc. Thus, most VANET routing solutions in the literature use those beacons to know available neighbors and take position-based routing decisions. However, we argue that using that information to take routing decisions can result in inefficiencies such as temporal loops in the forwarding path, backward progress due to stale information and use of low-quality links. We propose a new protocol called BRAVE in which neighbor selection is done opportunistically in collaboration with neighbors. Our simulation results show that BRAVE is able to outperform existing solutions in a realistic urban scenario for a variety of network densities.

Minimum Broadcasting Structure for Optimal Data Dissemination in Vehicular Networks

In this paper, we model a vehicular multihop network as an evolving graph and formulate the problem of optimal data dissemination over the network in terms of minimum number of transmissions. We show that the problem belongs to the NP complexity class and provide an easy to implement polynomial-time 2 τH(Δ)-approximation algorithm, where τ is the number of different subgraphs that comprise the evolving graph, and H(Δ) is the harmonic number of the degree of the evolving graph. By means of applying our heuristic over a vehicular scenario generated by a microscopic road-traffic simulator, we provide some insight into the data dissemination issue. In addition, the proposed algorithm is employed to benchmark a state-of-the-art communication protocol. We hope this paper will inspire more efficient heuristics and data dissemination solutions.