Paolo Santi

Investigating upper bounds on network lifetime extension for cell-based energy conservation techniques in stationary ad hoc networks

Cooperative cell-based strategies have been recently proposed as a technique for extending the lifetime of wireless ad hoc networks, while only slightly impacting network performance. The effectiveness of this approach depends heavily on the node density: the higher it is, the more consistent energy savings can potentially be achieved. However, no general analyses of network lifetime have been done either for a base network (one without any energy conservation technique) or for one using cooperative energy conservation strategies. In this paper, we investigate the lifetime/density tradeoff under the hypothesis that nodes are distributed uniformly at random in a given region, and that the traffic is evenly distributed across the network. We also analyze the case where the node density is just sufficient to ensure that the network is connected with high probability. This analysis, which is supported by the results of extensive simulations, shows that even in this low density scenario, cell-based strategies can significantly extend network lifetime.

Crash faults identification in wireless sensor networks

In this paper we consider the problem of identifying faulty (crashed) nodes in a wireless sensor network. This problem is of fundamental importance in those applicative scenarios of wireless sensor networks in which battery replacement is feasible. The diagnostic information gathered by operational sensors can be used by an external operator for the sake of network reconfiguration and/or repair; thus extending network lifetime. A fault diagnosis protocol specifically designed for wireless sensor networks is introduced and analyzed. The protocol is proved to be optimal and energy efficient under certain assumptions.

Comparison-based system-level fault diagnosis in ad hoc networks

The problem of identifying faulty mobiles in ad-hoc networks is considered. Current diagnostic models were designed for wired networks, thus they do not take advantage of the shared nature of communication typical of ad-hoc networks. In this paper we introduce a new comparison-based diagnostic model based on the one-to-many communication paradigm. Two implementations of the model are presented. In the first implementation, we assume that the network topology does not change during diagnosis, and we show that both hard and soft faults can be easily, detected Based on this implementation, a diagnosis protocol is presented The evaluation of the communication and time complexity of the protocol indicates that efficient diagnosis protocols for ad-hoc networks based on our model can be designed In the second implementation we allow the system topology to change during diagnosis. As expected, the ability of diagnosing faults under this scenario is significantly reduced with respect to the stationary case.

Fair sharing of bandwidth in VANETs

We address the challenge of how to share the limited wireless channel capacity for the exchange of safety-related information in a fully deployed vehicular ad hoc network (VANET). In particular, we study the situation that arises when the number of nodes sending periodic safety messages is too high in a specific area. In order to achieve a good performance of safety-related protocols, we propose to limit the load sent to the channel using a strict fairness criterion among the nodes. A formal definition of this problem is presented in terms of a max-min optimization problem with an extra condition on per-node maximality. Furthermore, we propose FPAV, a power control algorithm which finds the optimum transmission range of every node, and formally prove its validity under idealistic conditions. Simulations are performed to visualize the result of FPAV in a couple of road situations. Finally, we discuss the issues that must be taken into account when implementing FPAV.

Distributed Fair Transmit Power Adjustment for Vehicular Ad Hoc Networks

Improving the safety of drivers and passengers by wirelessly exchanging information between vehicles represents a major driving force for the design of vehicular ad hoc networks. In a heavy loaded 802.11-based network, however, safety-related packets might collide frequently and cannot be decoded by a receiver, thus they might not be effective in increasing the safety level on the roads. In this paper, we propose to use transmit power control in order to reduce packet collisions, while taking into account the major design goal of vehicular ad hoc networks, i.e. increasing safety. While previous work has addressed the issue of power control primarily for optimizing network capacity and/or connectivity, the optimization criterion for improving safety has to be built upon the concept of fairness: a higher transmit power of a sender should not be selected at the expense of preventing other vehicles to send/receive their required amount of safety information. In this paper, we propose a fully distributed and localized algorithm called D-FPAV (distributed fair power adjustment for vehicular networks) for adaptive transmit power adjustment which is formally proven to achieve max-min fairness. Furthermore, we investigate the effectiveness and robustness of D-FPAV through extensive simulations based on a realistic highway scenario and different radio propagation models

Quantifying the benefits of vehicle pooling with shareability networks

Significance Recent advances in information technologies have increased our participation in “sharing economies,” where applications that allow networked, real-time data exchange facilitate the sharing of living spaces, equipment, or vehicles with others. However, the impact of large-scale sharing on sustainability is not clear, and a framework to assess its benefits quantitatively is missing. For this purpose, we propose the method of shareability networks, which translates spatio-temporal sharing problems into a graph-theoretic framework that provides efficient solutions. Applying this method to a dataset of 150 million taxi trips in New York City, our simulations reveal the vast potential of a new taxi system in which trips are routinely shareable while keeping passenger discomfort low in terms of prolonged travel time. Taxi services are a vital part of urban transportation, and a considerable contributor to traffic congestion and air pollution causing substantial adverse effects on human health. Sharing taxi trips is a possible way of reducing the negative impact of taxi services on cities, but this comes at the expense of passenger discomfort quantifiable in terms of a longer travel time. Due to computational challenges, taxi sharing has traditionally been approached on small scales, such as within airport perimeters, or with dynamical ad hoc heuristics. However, a mathematical framework for the systematic understanding of the tradeoff between collective benefits of sharing and individual passenger discomfort is lacking. Here we introduce the notion of shareability network, which allows us to model the collective benefits of sharing as a function of passenger inconvenience, and to efficiently compute optimal sharing strategies on massive datasets. We apply this framework to a dataset of millions of taxi trips taken in New York City, showing that with increasing but still relatively low passenger discomfort, cumulative trip length can be cut by 40% or more. This benefit comes with reductions in service cost, emissions, and with split fares, hinting toward a wide passenger acceptance of such a shared service. Simulation of a realistic online system demonstrates the feasibility of a shareable taxi service in New York City. Shareability as a function of trip density saturates fast, suggesting effectiveness of the taxi sharing system also in cities with much sparser taxi fleets or when willingness to share is low.

A probabilistic analysis for the range assignment problem in ad hoc networks

In this paper we consider the following problem for ad hoc networks: assume that n nodes are distributed in a d-dimensional region, with 1≤d≤3, and assume that all the nodes have the same transmitting range r; how large must r be to ensure that the resulting network is strongly connected? We study this problem by means of a probabilistic approach, and we establish lower and upper bounds on the probability of connectedness. For the one-dimensional case, these bounds allow us to determine a suitable magnitude of r for a given number of nodes and displacement region size. In an alternate formulation, the bounds allow us to calculate how many nodes must be distributed should the transmitting range be fixed. Finally, we investigate the required magnitude of r in the two- and three-dimensional cases through simulation. Based on the bounds provided and on the simulation analysis, we conclude that, as compared to the deterministic case, a probabilistic solution to this range assignment problem achieves substantial energy savings. A number of other potential uses for our analyses are discussed as well

On the Symmetric Range Assignment Problem in Wireless Ad Hoc Networks

In this paper we consider a constrained version of the range assignment problem for wireless ad hoc networks, where the value the node transmitting ranges must be assigned in such a way that the resulting communication graph is strongly connected and the energy cost is minimum. We impose the further requirement of symmetry on the resulting communication graph. We also consider a weaker notion of symmetry, in which only the existence of a set of symmetric edges that renders the communication graph connected is required. Our interest in these problems is motivated by the fact that a (weakly) symmetric range assignment can be more easily integrated with existing higher and lower-level protocols for ad hoc networks, which assume that all the nodes have the same transmitting range. We show that imposing symmetry does not change the complexity of the problem, which remains NP-hard in two and three-dimensional networks. We also show that a weakly symmetric range assignment can reduce the energy cost considerably with respect to the homogeneous case, in which all the nodes have the same transmitting range, and that no further (asymptotic) benefit is expected from the asymmetric range assignment. Hence, the results presented in this paper indicate that weak symmetry is a desirable property of the range assignment.

An evaluation of connectivity in mobile wireless ad hoc networks

We consider the following problem for wireless ad hoc networks: assume n nodes, each capable of communicating with nodes within a radius of r, are distributed in a d-dimensional region of side l; how large must the transmitting range r be to ensure that the resulting network is connected? We also consider the mobile version of the problem, in which nodes are allowed to move during a time interval and the value of r ensuring connectedness for a given fraction of the interval must be determined. For the stationary case, we give tight bounds on the relative magnitude of r, n and l yielding a connected graph with high probability in l-dimensional networks, thus solving an open problem. The mobile version of the problem when d=2 is investigated through extensive simulations, which give insight on how mobility affects connectivity and reveal a useful trade-off between communication capability and energy consumption.

An analysis of the node spatial distribution of the random waypoint mobility model for ad hoc networks

In this paper we analyze the node spatial distribution generated by nodes moving according to the random waypoint model, which is widely used in the simulation of mobile ad hoc networks. We extend an existing analysis for the case in which nodes are continuously moving (i.e., the pause time is 0) to the more general case in which nodes have arbitrary pause times between movements. We also generalize the mobility model, allowing the nodes to remain stationary for the entire simulation time with a given probabilit . Our analysis shows that the structure of the resulting as asymptotic spatial density is composed by three distinct components: the initial, the pause and the mobilit component. The relative values of these components depend on the mobilit parameters. We derive an explicit formula of the one-dimensional node spatial density, and an approximated formula for the two-dimensional case.The quality of this approximation is verified through experimentation, which shows that the accuracy heavily depends on the choice of the mobilit parameters.