Arun K. Somani

Distributed fault detection of wireless sensor networks

Wireless Sensor Networks (WSNs) have become a new information collection and monitoring solution for a variety of applications. Faults occurring to sensor nodes are common due to the sensor device itself and the harsh environment where the sensor nodes are deployed. In order to ensure the network quality of service it is necessary for the WSN to be able to detect the faults and take actions to avoid further degradation of the service. The goal of this paper is to locate the faulty sensors in the wireless sensor networks. We propose and evaluate a localized fault detection algorithm to identify the faulty sensors. The implementation complexity of the algorithm is low and the probability of correct diagnosis is very high even in the existence of large fault sets. Simulation results show the algorithm can clearly identify the faulty sensors with high accuracy.

Efficient algorithms for routing dependable connections in WDM optical networks

We consider the problem of establishing dependable connections in WDM networks with dynamic traffic demands. We call a connection with fault-tolerant requirements as a dependable connection (D-connection). We consider the single-link failure model in our study and recommend the use of a proactive approach, wherein a D-connection is identified with the establishment of the primary lightpath and a backup lightpath at the time of honoring the connection request. We develop algorithms to select routes and wavelengths to establish D-connections with improved blocking performance. The algorithms use the backup multiplexing technique to efficiently utilize the wavelength channels. To further improve channel utilization, we propose a new multiplexing technique called primary-backup multiplexing. Here, a connection may not have its backup lightpath readily available throughout its existence. We develop algorithms based on this technique to route D-connections with a specified restoration guarantee. We present an efficient and computationally simple heuristic to estimate the average number of connections per link that do not have backup lightpaths readily available upon a link failure. We conduct extensive simulation experiments on different networks to study the performance of the proposed algorithms.

Dynamic wavelength routing using congestion and neighborhood information

We present two dynamic routing algorithms based on path and neighborhood link congestion in all-optical networks. In such networks, a connection request encounters higher blocking probability than in circuit-switched networks because of the wavelength-continuity constraint. Much research has focused on the shortest-path routing and alternate shortest-path routing. We consider fixed-paths least-congestion (FPLC) routing in which the shortest path may not be preferred to use. We then extend the algorithm to develop a new routing method: dynamic routing using neighborhood information. It is shown by using both analysis and simulation methods that FPLC routing with the first-fit wavelength-assignment method performs much better than the alternate routing method in mesh-torus networks (regular topology) and in the NSFnet T1 backbone network (irregular topology). Routing using neighborhood information also achieves good performance when compared to alternate shortest-path routing.

A practical approach to operating survivable WDM networks

Several methods have been developed for joint working and spare capacity planning in survivable wavelength-division-multiplexing (WDM) networks. These methods have considered a static traffic demand and optimized the network cost assuming various cost models and survivability paradigms. Our interest primarily lies in network operation under dynamic traffic. We formulate various operational phases in survivable WDM networks as a single integer linear programming (ILP) optimization problem. This common framework avoids service disruption to the existing connections. However, the complexity of the optimization problem makes the formulation applicable only for network provisioning and offline reconfiguration. The direct use of this method for online reconfiguration remains limited to small networks with few tens of wavelengths. Our goal in this paper is to develop an algorithm for fast online reconfiguration. We propose a heuristic algorithm based on LP relaxation technique to solve this problem. Since the ILP variables are relaxed, we provide a way to derive a feasible solution from the relaxed problem. The algorithm consists of two steps. In the first step, the network topology is processed based on the demand set to be provisioned. This preprocessing step is done to ensure that the LP yields a feasible solution. The preprocessing step in our algorithm is based on: (a) the assumption that in a network, two routes between any given node pair are sufficient to provide effective fault tolerance and (b) an observation on the working of the ILP for such networks. In the second step, using the processed topology as input, we formulate and solve the LP problem. Interestingly, the LP relaxation heuristic yielded a feasible solution to the ILP in all our experiments. We provide insights into why the LP formulation yields a feasible solution to the ILP We demonstrate the use of our algorithm on practical size backbone networks with hundreds of wavelengths per link. The results indicate that the run time of our heuristic algorithm is fast enough (in order of seconds) to be used for online reconfiguration.

On optimal converter placement in wavelength-routed networks

Wavelength converters increase the traffic-carrying capacity of circuit-switched optical networks by relaxing the wavelength continuity constraints. We consider the problem of optimally placing a given number of wavelength converters on a path to minimize the call-blocking probability. Using a simple performance model, we first prove that uniform spacing of converters is optimal for the end-to-end performance when link loads are uniform and independent. We then show that significant gains are achievable with optimal placement compared to random placement. For nonuniform link loads, we provide a dynamic programming algorithm for the optimal placement and compare the performance with random and uniform placement. Optimal solutions for bus and ring topologies are also presented. Finally, we discuss the effect of the traffic model on the placement decision.

Computationally-efficient phased-mission reliability analysis for systems with variable configurations

Several techniques and a software tool for reliability analyses that generally apply to fault-tolerant systems operating in phased-missions are given. Efficient models, using Markov chains without an explosion of the state space, are provided for missions consisting of multiple phases, during which the system configuration or success criteria can change. In different phases, the failure rates and the fault and error handling models can also change, and the duration of any phase can use deterministic or random models. An efficient reconfiguration procedure that is computationally more efficient than any existing technique is developed. The technique is demonstrated using a numerical example to show the effects of mission phases on the system reliability. >

Soft error sensitivity characterization for microprocessor dependability enhancement strategy

This paper presents an empirical investigation on the soft error sensitivity (SES) of microprocessors, using the picoJava-II as an example, through software simulated fault injections in its RTL model. Soft errors are generated under a realistic fault model during program run-time. The SES of a processor logic block is defined as the probability that a soft error in the block causes the processor to behave erroneously or enter into an incorrect architectural state. The SES is measured at the functional block level. We have found that highly error-sensitive blocks are common for various workloads. At the same time soft errors in many other logic blocks rarely affect the computation integrity. Our results show that a reasonable prediction of the SES is possible by deduction from the processors microarchitecture. We also demonstrate that the sensitivity-based integrity checking strategy can be an efficient way to improve fault coverage per unit redundancy.

Routing dependable connections with specified failure restoration guarantees in WDM networks

This paper considers the problem of dynamically establishing dependable connections (D-connections) with specified failure restoration guarantees in wavelength-routed wavelength division multiplexed (WDM) networks. We call a connection with fault-tolerant requirements a D-connection. We recommend using a proactive approach to fault-tolerance wherein a D-connection is identified with the establishment of a primary and a backup lightpath at the time of honoring the connection request. However, the backup lightpath may not be available to a connection throughout its existence. Upon occurrence of a fault, a failed connection is likely to find its backup path available with a certain specified guarantee. We develop algorithms to select routes and wavelengths to establish D-connections with specified failure restoration guarantees. The algorithms are based on a technique called primary-backup multiplexing. We present an efficient and computationally simple method to estimate the average number of connections per link for which the backup paths are not readily available upon occurrence of a link failure. This measure is used for selecting suitable primary and backup lightpaths for a connection. We conduct extensive simulation experiments to evaluate the effectiveness of the proposed algorithms on different networks. The results show that the blocking performance gain is attractive enough to allow some reduction in guarantee. In particular, under the light load conditions, more than 90% performance gain is achieved at the expense of less than 10% guarantee reduction.

Understanding Fault Tolerance And Reliability

Most people who use computers regularly have encountered a failure, either in the form of a software crash, disk failure, power loss, or bus error. In some instances these failures are no more than annoyances; in others they result in significant losses. The latter result will probably become more common than the former, as society’s dependence on automated systems increases. The ideal system would be perfectly reliable and never fail. This, of course, is impossible to achieve in practice: System builders have finite resources to devote to reliability and consumers will only pay so much for this feature. Over the years, the industry has used various techniques to best approximate the ideal scenario. The discipline of fault-tolerant and reliable computing deals with numerous issues pertaining to different aspects of system development, use, and maintenance. The expression “disaster waiting to happen” is often used to describe causes of failure that are seemingly well known, but have not been adequately accounted for in the system design. In these cases we need to learn from experience how to avoid failure. Not all failures are avoidable, but in many cases the system or system operator could have taken corrective action that would have prevented or mitigated the failure. The main reason we don’t prevent failures is our inability to learn from our mistakes. It often takes more than one occurrence of the same failure before corrective action is taken.

A generalized framework for analyzing time-space switched optical networks

The advances in photonic switching have paved the way for realizing all-optical time switched networks. The current technology of wavelength division multiplexing (WDM) offers bandwidth granularity that matches peak electronic transmission speed by dividing the fiber bandwidth into multiple wavelengths. However, the bandwidth of a single wavelength is too large for certain traffic. Time division multiplexing (TDM) allows multiple traffic streams to share the bandwidth of a wavelength efficiently. While introducing wavelength converters and time slot interchangers to improve network blocking performance, it is often of interest to know the incremental benefits offered by every additional stage of switching. As all-optical networks in the future are expected to employ heterogeneous switching architectures, it is necessary to have a generalized network model that allows the study of such networks under a unified framework. A network model, called the trunk switched network (TSN), is proposed to facilitate the modeling and analysis of such networks. An analytical model for evaluating the blocking performance of a class of TSNs is also developed. With the proposed framework, it is shown that a significant performance improvement can be obtained with a time-space switch with no wavelength conversion in multiwavelength TDM switched networks. The framework is also extended to analyze the blocking performance of multicast tree establishment in optical networks. To the best of our knowledge, this is the first work that provides an analytical model for evaluating the blocking performance for tree establishment in an optical network. The analytical model allows a comparison between the performance of various multicast tree construction algorithms and the effects of different switch architectures.