We consider the distributed setting of N autonomous mobile robots that operate in Look-Compute-Move (LCM) cycles and communicate with other robots using colored lights (the robots with lights model). We study the fundamental problem of repositioning N autonomous robots on a plane sothat each robot is visible to all others (the Complete Visibility problem) on this model; a robot cannot see another robot if a third robot is positioned between them on the straight line connecting them. There exists an O(1) time, O(1) color algorithm for this problem in the semi-synchronous setting. In this paper, we provide the first O(log N) time, O(1) color algorithm for this problem in the asynchronous setting. This is a significant improvement over an O(N)-time translation of the semi-synchronous algorithm to the asynchronous setting. The proposed algorithm is collision-free - robots do not share positions and their paths do not cross.

We consider the problem of repositioning N autonomous robots on a plane so that each robot is visible to all others (the Complete Visibility problem), a robot cannot see another robot if there is a third robot positioned between them on the straight line joining them. Robots communicate using collared lights. The computation is synchronous and each robot performs a look-compute-move during a round. Specifically, during a round a robot is permitted to observe the light and position of every robot visible to it. It may also perform an internal computation based on the observed lights and positions(including deciding on a new-collar for its own light), and possibly moving to a new position at the end of the round. The challenge posed by this model of computation stems from the fact that each robot has only a constant number of colours for its lights(symbols for communication) and no memory(except for the persistence of lights) between rounds. In this paper we first show that the best previously known algorithm for the complete visibility problem on this model runs in linear time in the worst case. We then present the first logarithmic time complexity algorithm for Complete Visibility. The model we assume use sterility, is fully-synchronous and allows robot paths to cross.

This paper addresses an NP-hard problem, referred to as Network Topology Design with minimum Cost subject to a Reliability constraint (NTD-CR), to design a minimal-cost communication network topology that satisfies a pre-defined reliability constraint. The paper describes a dynamic programming (DP) scheme to solve the NTD-CR problem, and proposes a DP approach, called Dynamic Programming Algorithm to solve NTD-CR (DPCR-ST), to generate the topology using a selected sequence of spanning trees of the network, STXmin. The paper shows that our DPCR-ST approach always provides a feasible solution, and produces an optimal topology given an optimal order of spanning trees. The paper proves that the problem of optimally ordering the spanning trees is NP-complete, and proposes three greedy heuristics to generate and order only k spanning trees of the network. Each heuristic allows the DPCR-ST approach to generate STXminusing only k spanning trees, which improves the time complexity while producing a near optimal topology. Simulations based on fully connected networks that contain up to 2.3×109spanning trees show the merits of using the ordering methods and the effectiveness of our algorithm vis-à-vis to four existing state-of-the-art techniques. Our DPCR-ST approach is able to generate 81.5% optimal results, while using only 0.77% of the spanning trees contained in networks. Further, for a typical 2 × 100 grid network that contains up to 1.899102spanning trees, DPCR-ST approach requires only k=1214 spanning trees to generate a topology with a reliability no larger than 5.05% off from optimal.

This paper addresses an NP-hard problem to design a network topology with maximum all-terminal reliability subject to a cost constraint, given the locations of the various computer centers (nodes), their connecting links, each link's reliability and cost, and the maximum budget cost to install the links. Because cost is always a major focus in network design, this problem is practical for critical applications requiring maximized reliability. This paper first formulates a Dynamic Programming (DP) scheme to solve the problem. A DP approach, called DPA-1, generates the topology using all spanning trees of the network (STG). The paper shows that DPA-1 is optimal if the spanning trees are optimally ordered. Further, the paper describes an alternative DP algorithm, called DPA-2, that uses only k spanning trees ( k ≤ n, where n=|STG|) sorted in increasing weight and lexicographic order to improve the time efficiency of DPA-1 while producing similar results. Extensive simulations using hundreds of benchmark networks that contain up to 1.899102spanning trees show the merits of using the sorting method, and the effectiveness of our algorithms. DPA-2 is able to generate 85% optimal results, while using only a small number of k spanning trees, and up to 16.83 CPU seconds. Furthermore, the non-optimal results are only up to 3.4% off from optimal for the simulated examples.

This paper addresses an NP-complete problem, called NTD-CR, to design a minimal-cost communication network topology that satisfies a pre-defined two terminal reliability constraint, given the locations of the various computer centers (nodes), their connecting links, each link's reliability and cost, and the required reliability for the network to operate. This paper formulates a dynamic programming (DP) scheme to solve the NTD-CR problem. DP approach, called DPCR-P, generates the topology using a selected set of paths of the network. We propose two different greedy heuristics to generate and order only k≤n paths, where n is the total number of paths in the network. Each heuristic allows DPCR-P to enumerate the selected paths using only k paths, which improves the time complexity while producing near optimal topology. Extensive simulations using benchmark networks with various sizes show the merits of path-orders, and the effectiveness of our approach. DPCR-P is able to generate 91% optimal results on the networks using only 8.89% to 27.5% of all paths in the networks. Further, its non-optimal results are no more than 10.97% off from optimal.

Multipath routing protocols (MRP) help improve the network quality of service (QoS), including load balancing, fault tolerance (reliability), aggregate bandwidth and delay. While multipaths communication provides better failure-tolerance, their resilience only holds if the multiple paths are selected carefully. Note that selecting an optimal path set is a NP-complete problem. Recently, several algorithms have appeared in the literature to help construct multiple node-disjoint paths or multiple edge-disjoint paths. This paper presents two algorithms, discussing the later issue. The first algorithm, called clique-based-approach (CBA), finds the edge-disjoint-path-set with the optimal reliability. The second algorithm, greedy-CBA (CBA-G), is a heuristic that reduces the computational complexity of CBA. Results show that CBA-G improves the efficiency of CBA without negatively affecting its effectiveness. We also provide an explanation as to why CBA-G is able to produce edge-disjoint-path-sets with reliabilities equal or better than a benchmark protocol, DPSP.

Wideband source localization using acoustic sensors has been drawing a lot of research interest recently in wireless communication applications, such as cellular handset localization, global positioning systems (GPS), and land navigation technologies, etc. The maximum-likelihood is the predominant objective which leads to a variety of source localization approaches. However, the appropriate optimization (search) algorithms are still in pursuit by researchers since different aspects about the effectiveness of such algorithms have to be addressed on different circumstances. In this paper, we focus on the two popular source localization methods for wideband acoustic signals, namely the alternating projection (AP) algorithm and the expectation maximization (EM) algorithm. We explore the respective limitations of these two methods and design a new hybrid approach thereupon. Through Monte Carlo simulations, we demonstrate that the trade-off can be achieved between the computational complexity and the localization accuracy using our newly proposed scheme.

Dynamic IP router table schemes, proposed in the literature, perform an IP lookup or an on-line prefix update in O(log n) memory accesses. In term of lookup time, they are still slower than FEC (CNHA/CWA) scheme, which requires exactly (at most) three memory accesses for each lookup, irrespective of the number of prefixes n. The prefix update in FEC (CNHA/CWA) has a drawback: Off-line solutions need structure reconstruction, or implementing on-line prefix updates is difficult. This paper solves this problem. We propose the use of lexicographic ordered prefixes to reduce off-line construction time. Simulations on several real routing databases, run on the same platform, show that our approach constructs FEC (CNHA/CWA) tables in 2.56 to 7.74 (1.56 to 2.7) times faster than that from previous techniques. Our on-line update scheme uses an updatable-address-set and selectively decompresses the FEC and CNHA/CWA structures to modify only the next-hops of the addresses in the set. Recompressing the updated structures, the resulting tables are identical to what would have been obtained by structure reconstructions, but at much lower computational cost. Our on-line update for FEC (CNHA/CWA) scheme takes at most 10.1 (7.21) -Ýs, which is of same order as achieved by the recently proposed schemes.

Researchers use random graph models to analyze complex networks that have no centralized control such as the Internet, peer-to-peer systems, and mobile ad hoc networks. These models explain phenomena like phase changes, clustering, and scaling. It is necessary to understand these phenomena when designing systems where exact node configurations cannot be known in advance. This paper presents a method for analyzing random graph models that combine discrete mathematics and probability theory. A graph connectivity matrix is constructed where each matrix element is the Bernoulli probability that an edge exists between two given nodes. We show how to construct these matrices for many graph classes, and use linear algebra to analyze the connectivity matrix. We present an application that uses this approach to analyze network cluster self-organization for sensor network security. We conclude by discussing the use of these concepts in mobile systems design.

This paper presents an efficient cutset approach to compute the reliability of a large communication network having heterogeneous link capacities. The reliability measure has been defined as capacity related reliability (CRR). The proposed method, subset cut technique (SCT), requires the cutset information of the network. For each minimal cut C/sub i/, and a given minimum bandwidth requirement W/sub min/, the method enumerates all nonredundant subset cut (SC), where each SC relates link capacities & minimum bandwidth requirements. Note that if all links in an SC fail, the capacity of the induced cut C/sub i/ will be less than W/sub min/. Given the failure probability of each link, and the nonredundant SC, any Boolean technique for generating mutually disjoint terms can be utilized to obtain a capacity related unreliability (CRU) of the network. Thus, the CRU for an (s,t) node pair is the probability that the network has a capacity of less than W/sub min/ for the given node pair. Note that CRR=1-CRU. Two SCT algorithms are proposed: Algorithm-1, and Algorithm-2; and the suitability of using either algorithm is also discussed. Examples are given to illustrate the techniques. The time complexity, and the proof of correctness for the proposed algorithms are also included. It is shown empirically that the time complexity of generating the nonredundant SC of a network is polynomial in the order of the number of cuts of the network. The proposed SCT algorithms have been implemented in C. We have utilized the SCT to generate the CRR of some large communication networks with various W/sub min/ values.

This paper presents modifications to the Core Based Tree (CBT) multicast routing protocol for selecting a set of optimal routers in a domain as core routers for a multicast group. Currently, there is no mechanism in CBT to locate core routers in an automatic fashion, adapting to network changes. Our proposed modifications help configure an optimal router as core based on three heuristics: Center of Members (CBT-CoM), Center of Sources (CBT-CoS), and Center of Sources and Members (CBT-CoSM). We next evaluate and compare the performance of these heuristics using network dynamics. For this, an implementation based on the simulation package NSv2 and a topology generation package GT-ITM is considered.

This paper proposes a cut-based technique to compute bounds on the full access probability of an extra stage shuffle exchange network (ESEN) and a wrap-around inverse banyan network (WIBN). Note that the problem of finding an exact full access probability is known to be NP-hard. Our results obtain tighter bounds as compared to those using existing techniques. For a small size multistage interconnection network, it deviates less from the exact value. We also notice that our proposed lower bound is conservative. Further, the lower bound is important as it suggests that a network is at least this much reliable.

In this paper, we investigate the issue of how multiparty-to-multiparty communication should be realized. In order to quantify the different techniques, we examine the network-layer (bandwidth and buffer) allocations of four models for realizing multiparty-to-multiparty communication. An example application considered in this paper, where such communication will be needed, is video conferencing based on MPEG video streams. Analytical comparisons are carried out based on actual data obtained from empirical measurements of the source characteristics of MPEG video (determined to be a 40 state Markov modulated fluid source). Our results show that in order to minimize bandwidth and buffer (network-layer resources) allocations, we must exploit the effect of statistical multiplexing by using shared buffers for such multiparty-to-multiparty communication. Using these results we identify the best technique for multiparty-to-multiparty communications.

This article provides an overview of doubly linked networks. We first classify the doubly linked network on the basis of the number of tuples used to represent each node address in the network. Then, each subclass of network is further partitioned into loop or nonloop categories. Various routing algorithms are studied and grouped into static or adaptive routing type, based on the number of paths between nodes used in the point-to-point message transmission. The useful protocols of the token ring and the register insertion ring and their relative advantages are also described. Some issues, such as multiple destinations routing, graceful degradation on cluster faults, and network behavior under unbalanced traffic conditions, in a doubly linked network are still open. There seems to be a need for new protocols when optical fiber is used as the medium for message transmission, which necessitates high-quality and large bandwidth data transmission. © 1996 John Wiley & Sons, Inc.

This paper considers the problem of applying neural network for logic circuit testing and proposes an efficient method based on hyperneural network (HNN). The HNN uses an energy function that not only considers binary relations but also captures all higher order relations among N neurons. We illustrate the hyperneural concept using two formulations. First, a constraint energy function is defined and the gate model is obtained. Second, the Hopfield network is reformulated to generate the gate level hyperneural model. The gate level HNN are used to give a mathematical form to the digital circuit that, in turn, requires optimization techniques to solve the test generation problem. We have used ISCAS'85 benchmark circuits to illustrate the method. Results are compared with those obtained from PODEM, MODEM, and FAN.

This paper modifies the tree RAM (TRAM) architecture (Jarwala and Pradhan, 1988) of multimegabit dynamic random access memories using a tree-star (TS) interconnection topology. The modified TS-RAM design offers a reduced access time and an improved yield for the proposed TS-RAM architecture. We also propose an improved built-in self test (BIST) approach for the architecture.

Abstract Several algorithms have been developed to solve the reliability problem for nonseries‐parallel networks using thesum of disjoint products (SDP)approach. This paper provides a general framework for most of these techniques. It reviews methods that help improve computer time and memory requirements in reliability computation. These parameters are generally used to compare SDP algorithms. We also overview three multiple variable inversion algorithms that result in sum of disjoint products expressions with fewer terms than that of algorithms that use only a single‐variable inversion. One common network is solved for two‐terminal network reliability using each of these algorithms. Finally, we have provided a comparison among these techniques.

In order to analyze dependability measures in a fault tolerant system, we generally consider a nonstate space or a state space type model. A fault tree with repeated events (FTRE's) presents an important strategy for the nonstate space model. The paper deals with a conservative assessment to complex fault tree models, henceforth called CRAFT, to obtain an approximate analysis of the FTRE's. It is a noncutset, direct, bottom-up approach. It uses failure probability or failure rate as input and determines a bound on the probability of occurrence of the TOP event. CRAFT generalizes the concept of a cutting heuristic that obtains the signal probabilities for testability measurement in logic circuits. The method is efficient and solves coherent and noncoherent FTRE's having AND, OR, XOR, and NOT gates. In addition, CRAFT considers M/N priority AND, and two types of functional dependency, namely OR and AND types. Examples such as the Cm* architecture and a fault-tolerant software based on recovery block concept are used to illustrate the approach. The paper also provides a comparison with approaches such as SHARPE, HARP, and FTC.>

The processor allocation problem requires recognizing and locating a free subcube that can accommodate a request for a subcube of a specified size for an incoming task. Methods reported in the literature fall into two strategies: bottom-up or bit mapped technique (BMT) and top-downer available cube technique (ACT). Our algorithm that solves the allocation problem in faulty hypercubes falls into the category of ACT's which offer the advantage over BMT's of quickly recognizing whether or not a requested subcube is available in the list of fault-free subcubes. We introduce new algebraic functions and the concept of separation factor to select a subcube for allocation. The notion of overlap-syndrome, defined in the text, quantifies the overlap among free subcubes. Our technique has full subcube recognition ability and thus recognizes more subcubes as compared to bit mapped techniques: Buddy, Gray code and its variants. The advantages of our approach over some of the existing ACT's in terms of fragmentation and overall completion time are described in the text and in simulation results.>

The hypercube topology, also known as the Boolean n-cube, has recently been used for multiprocessing systems. The paper considers two structural-reliability models, namely, terminal reliability (TR) and network reliability (NR), for the hypercube. Terminal (network) reliability is defined as the probability that there exists a working path connecting two (all) nodes. There are no known polynomial time algorithms for exact computation of TR or NR for the hypercube. Thus, lower-bound computation is a better alternative, because it is more efficient computationally, and the system will be at least as reliable as the bound. The paper presents algorithms to compute lower bounds on TR and NR for the hypercube considering node and/or link failures. These algorithms provide tighter bounds for both TR and NR than known results and run in time polynomial in the cube dimension n, specifically, within time O(n/sup 2/).>

Abstract A multistage interconnection network (MIN) uses switching elements for interconnecting processors to memory or processors. This paper first considers a stochastic model of a shuffle exchange network (of sizeN) with an extra stage and develops terminal, broadcast, andK‐terminal reliability evaluation algorithms that run in timeO(log logN),O(logN), andO(klogN), respectively, wherek= |K|. Second, a deterministic model for the MIN is considered and decision problems in which one is interested in evaluating whether a network can effect a specified set of connections with a given set of failures are solved. We develop a set of approaches for the SENE for the terminal decision, broadcast decision, and network decision problems and for the generalS, Tdecision problem for an input setSand output setT. Each algorithm runs within time polynomial in the size of the SENE and the number of faults and, in some cases, logarithmic in the size of the SENE and polynomial in the number of faults. These approaches are based on either testing for the existence of an appropriate pathset or testing for the nonexistence of an appropriate cutset. ©1993 by John Wiley & Sons, Inc.

Experimental results are presented showing the number of disjoint products and computer time involved in generating sum of disjoint product (SDP) terms. To help obtain the results, the authors have considered 19 benchmark networks containing paths (cuts) varying from 4 (4) to 780 (7376). Several SDP techniques are reviewed and are generalized into three propositions to find their inherent merits and demerits. An efficient SDP technique is, then, utilized to run input files of paths/cuts preprocessed using (1) cardinality, (2) lexicographic, and (3) Hamming distance ordering methods and their combinations. The experimental evaluation has been performed on an FPS 500 system. Results are analyzed, and it is shown that the preprocessing based on cardinality or its combinations with (2) and/or (3) performs better.>

An efficient method to compute the terminal reliability (the probability of communication between a pair of nodes) of a distributed computing system (DCS) is presented. It is assumed that the graph model G(V,E) for DCS is given and that the path and/or cut information for the network G(V,E) is available. Boolean algebraic concepts are used to define four operators: compare, reduce, combine, and generate. The proposed method, called CAREL, uses the four operators to generate exclusive and mutually disjoint events. CAREL has been implemented using bit vector representation on an Encore MULTIMAX 320 system. It is shown that CAREL solves large DCS networks (having a pathset on the order of 780 and a cutset on the order of 7300 or more) with a reasonable memory requirement. A comparison with other algorithms reveals the computational efficiency of the method. The proof of correctness of CAREL is included.>

The authors present a group of multidimensional regular topologies with two incoming and two outgoing link per node. The use of a mixed-radix numbering system in defining the network topology leads to many choices for the jump distance, which provides better performance when the total number of nodes is large. They introduce a performance parameter named spanning radius (SPR) which determines the addition traversal required to use all different choices when the number of choices exceeds two. The impact of the number of choices and its SPR on the network performance is also studied. The results show that a multidimensional network performs better in terms of diameter and reliability when the penalty of the extra link traversal is offset by the saving due to the additional jump distance choices used.>

The authors introduce two techniques for computing the reliability of a distributed computing system (DCS). The first scheme uses two steps and requires enumeration of multiterminal connections which, in turn, leads to the reliability expression. The second technique, called FARE (Fast Algorithm for Reliability Evaluation), does not require an a priori knowledge of multiterminal connections for computing the reliability expression. A performance parameter called the communication cost index (CCI) is defined. The authors compare algorithms with an existing method in terms of computer time and memory requirement. The results show that FARE outperforms two-step methods.>

The authors propose a technique to compute software system reliability (SSR). The method, called FARE (fast algorithm for reliability evaluation), does not require a priori knowledge of multiterminal connections (MCs) for computing the reliability expression. An effort is made to sole the problem of N-version programming by using the FARE approach. Owing to its short execution time and low memory requirement, FARE can be used to calculate the SSR of fairly large distributed systems.>