Showing papers by "Giuseppe Prencipe published in 2006"

Searching for a black hole in arbitrary networks: optimal mobile agents protocols

Abstract: Consider a networked environment, supporting mobile agents, where there is a black hole: a harmful host that disposes of visiting agents upon their arrival, leaving no observable trace of such a destruction. The black hole search problem is the one of assembling a team of asynchronous mobile agents, executing the same protocol and communicating by means of whiteboards, to successfully identify the location of the black hole; we are concerned with solutions that are generic (i.e., topology-independent). We establish tight bounds on the size of the team (i.e., the number of agents), and the cost (i.e., the number of moves) of a size-optimal solution protocol. These bounds depend on the a priori knowledge the agents have about the network, and on the consistency of the local labelings. In particular, we prove that: with topological ignorance Δ+1 agents are needed and suffice, and the cost is Θ(n 2), where Δ is the maximal degree of a node and n is the number of nodes in the network; with topological ignorance but in presence of sense of direction only two agents suffice and the cost is Θ(n 2); and with complete topological knowledge only two agents suffice and the cost is Θ(n log n). All the upper-bound proofs are constructive.

Black hole search in common interconnection networks

Abstract: Mobile agents operating in networked environments face threats from other agents as well as from the hosts (i.e., network sites) they visit. A black hole is a harmful host that destroys incoming agents without leaving any trace. To determine the location of such a harmful host is a dangerous but crucial task, called black hole search. The most important parameter for a solution strategy is the number of agents it requires (the size); the other parameter of interest is the total number of moves performed by the agents (the cost). It is known that at least two agents are needed; furthermore, with full topological knowledge, Ω(n log n) moves are required in arbitrary networks. The natural question is whether, in specific networks, it is possible to obtain (topology-dependent but) more cost efficient solutions. It is known that this is not the case for rings. In this article, we show that this negative result does not generalizes. In fact, we present a general strategy that allows two agents to locate the black hole with O(n) moves in common interconnection networks: hypercubes, cube-connected cycles, star graphs, wrapped butterflies, chordal rings, as well as in multidimensional meshes and tori of restricted diameter. These results hold even if the networks are anonymous. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 47(2), 61–71 2006

Distributed Algorithms for Autonomous Mobile Robots

Abstract: The distributed coordination and control of a team of autonomous mobile robots is a problem widely studied in a variety of fields, such as engineering, artificial intelligence, artificial life, robotics. Generally, in these areas, the problem is studied mostly from an empirical point of view. Recently, a significant research effort has been and continues to be spent on understanding the fundamental algorithmic limitations on what a set of autonomous mobile robots can achieve. In particular, the focus is to identify the minimal robot capabilities (sensorial, motorial, computational) that allow a problem to be solvable and a task to be performed. In this paper we describe the current investigations on the interplay between robots capabilities, computability, and algorithmic solutions of coordination problems by autonomous mobile robots.

Point-of-Failure Shortest-Path Rerouting: Computing the Optimal Swap Edges Distributively

Abstract: We consider the problem of computing the optimal swap edges of a shortest-path tree. This problem arises in designing systems that offer point-of-failure shortest-path rerouting service in presence of a single link failure: if the shortest path is not affected by the failed link, then the message will be delivered through that path; otherwise, the system will guarantee that, when the message reaches the node where the failure has occurred, the message will then be re-routed through the shortest detour to its destination. There exist highly efficient serial solutions for the problem, but unfortunately because of the structures they use, there is no known (nor foreseeable) efficient distributed implementation for them. A distributed protocol exists only for finding swap edges, not necessarily optimal ones. We present two simple and efficient distributed algorithms for computing the optimal swap edges of a shortest-path tree. One algorithm uses messages containing a constant amount of information, while the other is tailored for systems that allow long messages. The amount of data transferred by the protocols is the same and depends on the structure of the shortest-path spanning-tree; it is no more, and sometimes significantly less, than the cost of constructing the shortest-path tree.

Self-deployment algorithms for mobile sensors on a ring

Abstract: We consider the self-deployment problem in a ring for a network of identical sensors: starting from some initial random placement in the ring, the sensors in the network must move, in a purely decentralized and distributed fashion, so to reach in finite time a state of static equilibrium in which they evenly cover the ring. A self-deployment algorithm is exact if within finite time the sensors reach a static uniform configuration: the distance between any two consecutive sensors along the ring is the same, d; the self-deployment algorithm is e-approximate if the distance between two consecutive sensors is between d − e and d + e. We prove that exact self-deployment is impossible if the sensors do not share a common orientation of the ring. We then consider the problem in an oriented ring. We prove that if the sensors know the desired final distance d, then exact self-deployment is possible. Otherwise, we present another protocol based on a very simple strategy and prove that it is e -approximate for any chosen e> 0. Our results show that a shared orientation of the ring is an important computational and complexity factor for a network of mobile sensors operating in a ring.

Self-deployment Algorithms for Mobile Sensors on a Ring

Abstract: We consider the self-deployment problem in a ring for a network of identical sensors: starting from some initial random placement in the ring, the sensors in the network must move, in a purely decentralized and distributed fashion, so to reach in finite time a state of static equilibrium in which they evenly cover the ring. A self-deployment algorithm is exact if within finite time the sensors reach a static uniform configuration: the distance between any two consecutive sensors along the ring is the same, d; the self-deployment algorithm is e-approximate if the distance between two consecutive sensors is between d - ∈ and d + e. ∈. We prove that exact self-deployment is impossible if the sensors do not share a common orientation of the ring. We then consider the problem in an oriented ring. We prove that if the sensors know the desired final distance d, then exact self-deployment is possible. Otherwise, we present another protocol based on a very simple strategy and prove that it is e-approximate for any chosen ∈ > 0. Our results show that. a shared orientation of the ring is an important computational and complexity factor for a network of mobile sensors operating in a ring.

Pattern Formation by Autonomous Mobile Robots

Abstract: A group of mobile autonomous robots, each with very limited capabilities, can form (complex) patterns in the space it occupies. These patterns can be used to program the robots to accomplish high-level tasks (e.g., surrounding and removal of a mine). The basic research questions address which patterns can be formed, and how they can be formed. These questions have been studied mostly from an empirical point of view. Most solutions do not have any guarantee of correctness; actually many solutions never terminate and never form the desired pattern. On the contrary, we are interested in (provably correct) solutions which always form the pattern within finite time. With this goal, we have been studying what patterns can be formed and how; in this paper we describe the results of our investigations.

Distributed algorithms for autonomous mobile robots

Abstract: The distributed coordination and control of a team of autonomous mobile robots is a problem widely studied in a variety of fields, such as engineering, artificial intelligence, artificial life, robotics. Generally, in these areas, the problem is studied mostly from an empirical point of view. Recently, a significant research effort has been and continues to be spent on understanding the fundamental algorithmic limitations on what a set of autonomous mobile robots can achieve. In particular, the focus is to identify the minimal robot capabilities (sensorial, motorial, computational) that allow a problem to be solvable and a task to be performed. In this paper we describe the current investigations on the interplay between robots capabilities, computability, and algorithmic solutions of coordination problems by autonomous mobile robots.