Markovian evolving graphs are dynamic-graph models where the links among a fixed set of nodes change during time according to an arbitrary Markovian rule. They are extremely general and they can well describe important dynamic-network scenarios.We study the speed of information spreading in the stationary phase by analyzing the completion time of the flooding mechanism. We prove a general theorem that establishes an upper bound on flooding time in any stationary Markovian evolving graph in terms of its node-expansion properties.We apply our theorem in two natural and relevant cases of such dynamic graphs. Geometric Markovian evolving graphs where the Markovian behaviour is yielded by n mobile radio stations, with fixed transmission radius, that perform independent random walks over a square region of the plane. Edge-Markovian evolving graphs where the probability of existence of any edge at time t depends on the existence (or not) of the same edge at time t − 1.In both cases, the obtained upper bounds hold with high probability and they are nearly tight. In fact, they turn out to be tight for a large range of the values of the input parameters. As for geometric Markovian evolving graphs, our result represents the first analytical upper bound for flooding time on a class of concrete mobile networks. * A preliminary version of this work was presented at the 24th IEEE IPDPS 2009

We introduce stochastic time-dependency in evolving graphs: starting from in arbitrary, initial edge probability distribution, at every time step! every edge changes it's state (existing or not) according to a two-state Markovian process with probabilities 1) (edge birth-rate) and q (edge death-rate). If all edge exists at time t then, at time t+1 it dies with probability q. If instead the edge does not exist at time 1, then it will come into existence at time t + 1 with Probability 1). Such evolving graph model is a. wide generalization of time-independent dynamic random graphs [6] and will be called edge-Markovian. dynamic graphs. We investigate the speed of information dissemination in such dynamic graphs. We provide nearly tight; bounds (which in fact turn out to be tight for a wide range of probabilities p and q) oil the completion Chile of the flooding mechanism aiming to broadcast a piece of information from a source node to all nodes. In particular, we provide: i) A tight characterization of the class of edge-Markovian dynamic graphs where flooding time is constant and. thus, it does not asymptotically depend oil the initial probability distribution. ii) A flight characterization of the class of edge-Markovian dynamic graphs where flooding time does not asymptotically depend oil the edge death-rate q

A multi-hop synchronous radio network is said to be unknown if the nodes have no knowledge of the topology. A basic task in radio network is that of broadcasting a message (created by a fixed source node) to all nodes of the network. Typical operations in real-life radio networks is the multi-broadcast that consists in performing a set of r independent broadcasts. The study of broadcast operations on unknown radio network is started by the seminal paper of Bar-Yehuda et al. [J. Comput. System Sci. 45 (1992) 104] and has been the subject of several recent works. In this paper, we study the completion and the termination time of distributed protocols for both the (single) broadcast and the multi-broadcast operations on unknown networks as functions of the number of nodes n, the maximum eccentricity D, the maximum in-degree Delta, and the congestion c of the networks. We establish new connections between these operations and some combinatorial concepts, such as selective families, strongly selective families (also known as superimposed codes), and pairwise r-different families. Such connections, combined with a set of new lower and upper bounds on the size of the above families, allow us to derive new lower bounds and new distributed protocols for the broadcast and multi-broadcast operations. In particular, our upper bounds are almost tight and strongly improve over the previous bounds for a large class of networks. (C) 2002 Elsevier Science B.V. All rights reserved

We introduce stochastic time-dependency in evolving graphs: starting from an initial graph, at every time step, every edge changes its state (existing or not) according to a two-state Markovian process with probabilities $p$ (edge birth-rate) and $q$ (edge death-rate). If an edge exists at time $t$, then, at time $t+1$, it dies with probability $q$. If instead the edge does not exist at time $t$, then it will come into existence at time $t+1$ with probability $p$. Such an evolving graph model is a wide generalization of time-independent dynamic random graphs [A. E. F. Clementi, A. Monti, F. Pasquale, and R. Silvestri, J. Comput. System Sci., 75 (2009), pp. 213–220] and will be called edge-Markovian evolving graphs. We investigate the speed of information spreading in such evolving graphs. We provide nearly tight bounds (which in fact turn out to be tight for a wide range of probabilities $p$ and $q$) on the completion time of the flooding mechanism aiming to broadcast a piece of information from a source node to all nodes. In particular, we provide i) a tight characterization of the class of edge-Markovian evolving graphs where flooding time is constant and, thus, it does not asymptotically depend on the initial graph; ii) a tight characterization of the class of edge-Markovian evolving graphs where flooding time does not asymptotically depend on the edge death-rate $q$. An interesting consequence of our results is that information spreading can be fast even if the graph, at every time step, is very sparse and disconnected. Furthermore, our bounds imply that the flooding time can be exponentially shorter than the mixing time of the edge-Markovian graph

We consider the following pebble motion problem. We are given a tree T with n vertices and two arrangements R and S of k < n distinct pebbles numbered 1, . . . , k on distinct vertices of the tree. Pebbles can move along edges of T provided that at any given time at most one pebble is traveling along an edge and each vertex of T contains at most one pebble. We are asked the following question:Is arrangement S reachable from R?We present an algorithm that, on input two arrangements of k pebbles on a tree with n vertices, decides in time O(n) whether the two arrangements are reachable from one another. We also give an algorithm that, on input two reachable configurations, returns a sequence of moves that transforms one configuration into the other.The pebble motion problem on trees has various applications including memory management in distributed systems, robot motion planning, and deflection routing.

It is reasonable to claim that almost all major questions related to radio broadcasting can be considered closed as far as static networks are considered: the network never changes during the entire protocol's execution. On the other hand, theoretical results on communication protocols in any scenario where the network topology may change during protocol's execution (i.e. a dynamic radio network) are very few. In this paper, we present a theoretical study of broadcasting in radio networks having dynamic unknown topology. The dynamic network is modeled by means of adversaries: we consider two of them. We first analyze an oblivious, memoryless random adversary that can be seen as the dynamic version of the average-case study presented by Elsässer and Gasieniec in JCSS, 2006. We then consider the deterministic worst-case adversary that, at each time slot, can make any network change (thus the strongest adversary). This is the dynamic version of the worst-case study provided by Bar-Yehuda, Goldreich and Itai in JCSS, 1992. In both cases we provide tight bounds on the completion time of randomized broadcast protocols. © 2008 Elsevier Inc. All rights reserved

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We study the completion time of distributed broadcast protocols in dynamic radio networks. The dynamic network is modelled by means of adversaries: we consider two of them that somewhat are the extremal cases. We first analyze the weakest one, i.e., an oblivious, memoryless random adversary. At each time slot t, a graph Gt is selected according to the well-known random graph model Gn,p. We derive a randomized protocol that, on input nand p, completes broadcasting in O(logn) time. Then, we prove that any randomized protocol has (logn)completion time. When p is unknown, we present an oblivious homogeneous version of the Bar Yehuda-Goldreich-Itai's randomized protocol having O(log2n) completion time and we prove a lower bound (log 2 n/loglogn) that holds for any randomized oblivious homogeneous protocol. We emphasize that the above (poly-)logarithmic upper bounds also hold when random graphs are sparse and disconnected, i.e., for p=o(lnn/n). We then consider the deterministic worst-case adversary that, at each time slot, can make any network change (thus the strongest adversary). Up to now, it is not even known whether finite expected completion time is achievable against this adversary. We present a simple randomized protocol that works in O(n 2/log n) completion time. This bound is then shown to be optimal. Copyright © 2007 ACM