Showing papers by "Andrzej Pelc published in 1997"

Power Consumption in Packet Radio Networks (Extended Abstract)

Abstract: In this paper we study the problem of assigning transmission ranges to the nodes of a multi-hop packet radio network so as to minimize the total power consumed under the constraint that adequate power is provided to the nodes to ensure that the network is strongly connected (i.e., each node can communicate along some path in the network to every other node). Such assignment of transmission ranges is called complete. We also consider the problem of achieving strongly connected bounded diameter networks.

Hop-Congestion Trade-Offs for High-Speed Networks

Abstract: Message transmission in ATM networks is via virtual paths. Packets are routed along virtual paths by maintaining a routing field whose subfields determine the intermediate destinations of the packet. In such a network it is important to construct path layouts that minimize the hop number (i.e. the number of virtual paths used to travel between any two nodes) as a function of edge-congestion (i.e. the number of virtual paths passing through a link). In this paper we construct asymptotically optimal virtual path layouts for chains and meshes.

Globally optimal diagnosis in systems with random faults

Abstract: We consider probabilistic diagnosis in multiprocessor systems. Processors can test one another; fault-free processors give correct test results, while faulty testers are unpredictable. Processors fail independently with constant probability p<1/2 and the goal is to identify correctly the status of all processors, based on the set of test results. A diagnosis algorithm is globally optimal if it has the highest probability of correctness among all (deterministic) diagnosis algorithms. We give fast globally optimal diagnosis algorithms for a class of test assignments including complete directed graphs and directed acyclic graphs. This is the first time that globally optimal diagnosis is given in a probabilistic model without any assumptions on the behavior of faulty processors.

Optimal Adaptive Broadcasting with a Bounded Fraction of Faulty Nodes (Extended Abstract)

Abstract: We consider broadcasting among n processors, f of which can be faulty. A fault-free processor, called the source, holds a piece of information which has to be transmitted to all other fault-free processors. We assume that the fraction f /n of faulty processors is bounded by a constant ,γ < 1. Transmissions are fault free. Faults are assumed of crash, type: faulty processors do not send or receive messages. We use the whispering model: pairs of processors communicating in one round must form a matching. A fault-free processor sending a message to an other processor becomes aware of whether this processor is faulty or fault free and can adapt future transmissions accordingly. The main result of the paper is a broadcasting algorithm working in O(log n) rounds and using O(n) messages of logarithmic size, in the worst case. This is an improvement of the result from [10] where O((log n)2) rounds were used. Our method also gives the first algorithm for adaptive distributed fault diagnosis in O (log n) rounds.

An Optimal Algorithm for Broadcasting Multiple Messages in Trees.

Abstract: We consider multiple message broadcasting in tree networks. The source (considered as the root of the tree) has k messages which have to be broadcast to all nodes of the tree. In every time unit each node can send one of its already obtained messages to one of its children. A k-message broadcasting scheme prescribes in which time unit a given node should send a message to which child. It is k-optimal if it achieves the smallest possible time for broadcasting k messages from the source to all nodes. We give an algorithm to construct a k-optimal broadcasting scheme for an arbitrary n-node tree. The time complexity of our algorithm is O(nk), i.e., the best possible.

Optimal Fault-Tolerant Broadcasting in Trees (Extended Abstract)

Abstract: We consider broadcasting a message from one node of a tree to all other nodes. In the presence of up to k link failures the tree becomes disconnected, and only nodes in the connected component C containing the source can be informed. The maximum ratio between the time used by a broadcasting scheme B to inform C and the optimal time to inform C, taken over all components C yielded by configurations of at most k faults, is the k-vulnerability of B. This is the maximum slowdown incurred by B due to the lack of a priori knowledge of fault location, for at most k faults. This measure of fault-tolerance is similar to the competitive factor of on-line algorithms: in both cases, the performance of an algorithm lacking some crucial information is compared to the performance of an “off-line” algorithm, one that is given this information as input. It is also the first known tool to measure and compare fault-tolerance of broadcasting schemes in trees.

Transition-optimal token distribution

Abstract: There is given a graph, that models a communication network of a multiprocessor system, and there are tokens (jobs) allocated to nodes of the graph. The task is to distribute the tokens evenly, subject to the constraint that they may be moved only along the edges of the graph. The cost of a distribution strategy is measured as the total number of operations of moving a token along an edge. An algorithm for general graphs is developed, by reduction to a maximum-flow minimum-cost problem, that finds a cost-optimal distribution strategy, given a graph and an initial token allocation. The main result is an algorithm for graphs that are lines of nodes; it finds the distribution strategy in time O(n), for a line of n nodes.

Universally fault-tolerant broadcasting in trees

Abstract: We consider broadcasting a message from one node of a tree to all other nodes. In the presence of up to k link failures the tree becomes disconnected, and only nodes in the connected component C containing the source can be informed. The maximum ratio between the time used by a broadcasting scheme B to inform C and the optimal time to inform C, taken over all components C yielded by configurations of at most k faults, is the k-vulnerability of B. This is the maximum slowdown incurred by B due to the lack of a priori knowledge of fault location, for at most k faults. Since the upper bound k on the number of faults is not always known, it is important to design broadcasting schemes that behave well under any possible number of faults. It turns out that achieving the lowest possible k-vulnerability for all k simultaneously is impossible for some trees. Hence a natural goal is to seek, for any tree T, a broadcasting scheme that simultaneously approximates the lowest possible k-vulnerability for every k, up to a given constant factor c (independent of L). We describe a polynomial algorithm which decides if such a "universally fault-tolerant" broadcasting scheme exists for given T and c, and constructs such a scheme if it exists.