Bao Hong Shen

Fault-Tolerance in Sensor Networks: A New Evaluation Metric

A new metric for measuring the fault-tolerance capability of a multihop wireless sensor network is introduced in this paper. Most of the studies on fault-tolerance in sensor networks use connectivity as the metric of fault-tolerance. If the underlying network is k-connected, it can tolerate up to k − 1 failures. In measuring fault tolerance in terms of connectivity, no assumption regarding the locations of the failed sensor nodes is made they may be very close to each other or very far from each other. In other words, the connectivity metric fails to capture any notion of locality of faults. However, in sensor networks, it is highly likely that the faults will be localized. This is particularly true in military applications, where an enemy bomb may inflict massive but localized damage to the sensor network. To capture the notion of locality in fault-tolerance, we introduce the notion of region-based connectivity. The region-based connectivity of a network may be informally defined to be the minimum number of nodes within a region whose failure will disconnect the network. Obviously, the notion of region-based connectivity is tied to the notion of a region. A region may be defined in several different ways and they are discussed in detail in the paper. The attractive feature of the region-based connectivity as the fault-tolerance metric is that it can achieve the same level of fault-tolerance as the traditional metric connectivity, but requires much lower transmission power for the sensor nodes. We provide both analytical as well as extensive simulation results to support our claim.

Survivable routing in WDM networks - logical ring in arbitrary physical topology

We consider the problem of routing the lightpaths of a logical topology of a WDM network on an arbitrary physical topology, such that the logical topology remains,connected even after the failure of a physical link. We focus our attention on the ring interconnection as the logical topology because it is widely used in many protection schemes. We first establish the necessary and sufficient condition for a ring logical topology to withstand failure of a single physical link. Next we show that the testing of this necessary and sufficient condition is an NP-complete problem. Finally, we give an algorithm for testing the necessary and sufficient condition and demonstrate the execution of the algorithm with the help of an example.

Optimal routing for fast transfer of bulk data files in time-varying networks

Efficient transfer of bulk data requires routing that minimizes the net transfer time instead of providing flow level bandwidth guarantees. In this work, we consider a realistic scenario where available bandwidth for each link in a given network is time varying. In such a network and for a given file size, we provide an optimal algorithm that minimizes the total time required to transfer the file from source to destination. We further consider the problem where path shifting is allowed to increase the net throughput for bulk data transfer. For this problem, we provide a dynamic programming based optimal algorithm that minimizes the number of path shifts required to transfer the file in the specified time. Solution to the above problems addresses both the performance and scalability issues that arise in large bulk data transfer for long duration of time.

On multipath routing using widest pair of disjoint paths

Multipath and disjoint path routing schemes have received a considerable amount of attention in the networking research community in recent years due to many of advantages offered by such schemes. Delay, delay-jitter, bandwidth, are often used as parameters to specify quality-of-service. One of the most important parameters is the bandwidth of the route used for data transmission. A path from a source node s to a destination node t is known as the widest path if the bandwidth of this path is the largest among all paths between s to t. Algorithms for computing a widest path between a source-destination node pair is well-known. In this paper, we consider two problems where the notions of multipath, disjoint path and widest path are combined. In one version, we address the problem of finding a pair of disjoint paths between a source-destination node pair, such that the combined bandwidth of this path-pair is the maximum over all such pairs. In the other version, we want to find a pair of disjoint paths, such that the bandwidth of the first path is at least X/sub 1/ and the bandwidth of the second path is at least X/sub 2/, for some pre-specified values X/sub 1/ and X/sub 2/. We prove that both versions of the problem are NP-complete. We provide exact and approximate solutions for both versions of the problem. We show that our approximate solutions provide near-optimal solutions for almost all the instances of the problem.

On a preemptive multi-class routing scheme with protection paths for WDM networks

A large optical network may carry multiple traffic classes with different priorities and fault-tolerance requirement. Higher priority traffic may require having a back-up path so that the traffic can be switched quickly to this path in case of a failure in the primary path. The lower priority traffic classes may not have any such requirement. In the path protection schemes currently in use for the WDM networks, a backup path is computed for all traffic, whenever a primary path is established between a source-destination pair. The resource needed for communication using the backup path are reserved (or set aside) for data communication between a source-destination pair, and are utilized only when the primary path is unavailable due to a failure in the network. The traffic carrying capacity of a network can be increased, if the resources set aside for the backup paths are utilized for data communication. In this paper we propose a path protection scheme for networks with multiple classes of traffic. The key features of our scheme are (i) not all traffic classes have backup paths - only higher priority classes have backup paths, (ii) primary paths of lower priority share wavelengths with secondary paths of higher priority traffic, and (iii) lower priority traffic can be preempted by higher priority traffic in case of a failure. The sharing of a wavelength between a primary path of a lower priority communication with the secondary path of a higher priority communication allows the network to satisfy more call requests, thereby reducing the call blocking probability. We provide a mathematical programming formulation for computing the primary and backup paths for call requests in a dynamic environment. We also compute the call blocking probability of our scheme and compare it with the call blocking probability of the conventional scheme through simulation. Our experimental results show significant gain by the proposed scheme over the conventional scheme.

On topological design of service overlay networks

The notion of service overlay network (SON) was proposed recently to alleviate difficulties encountered in providing end-to-end quality of service (QoS) guarantees in the current Internet architecture. The SONs are able to provide QoS guarantees by purchasing bandwidth from individual network domains and building a logical end-to-end data delivery infrastructure on top of existing Internet. In this paper, we consider a generalized framework for SON, which is categorized based on three different characteristics: a) single-homed/multi-homed end-system b) usage-based/leased cost model and c) capacitated/uncapacitated network. We focus on the algorithmic analysis of the topology design problem for the above generalized SON. We prove that for certain case, polynomial-time optimal algorithm exists, while for other cases, the topology design problem is NP-complete. For the NP-complete cases, we provide approximation algorithms and experimental results.

Coverage Problem for Sensors Embedded in Temperature Sensitive Environments

The coverage and connectivity problem in sensor networks has received significant attention of the research community in the recent years. In this paper, we study this problem for sensors deployed in temperature sensitive environments. This paper is motivated by the issues encountered during deployment of bio-sensors in a human/animal body. Radio transmitters during operation dissipate energy and raise the temperature of its surroundings. A temperature sensitive environment like the human body can tolerate such increase in temperature only up to a certain threshold value, beyond which serious injury may occur. To avoid such injuries, the sensor placement must be carried out in a way that ensures the surrounding temperature to remain within the threshold. Using a thermal model for heat distribution from multiple heat sources (radio transmitters), we observed that if the sensor nodes are placed sufficiently apart from each other, then the temperature of the surrounding area does not exceed the threshold. This minimum separation distance constraint gives rise to a new version of the sensor coverage problem that has not been studied earlier. We prove that both the optimization version and the feasibility version of the new problem are NP-complete. We further show that an epsiv-approximation algorithm for the problem cannot exist unless P = NP. We provide two heuristic solutions for the problem and evaluate the efficacy of these solutions by comparing their performances against the optimal solution. The simulation results show that our heuristic solutions almost always find near optimal solution in a fraction of the time needed to find the optimal solution. Finally, an algorithm for forming a connected sensor network with minimum transmission power in such a scenario is provided.

On multipath routing with transit hubs

Empirical studies report frequent occurrences of path failure in the Internet. In providing resilience to such failures, we propose the computation of alternate backup end-to-end path that is disjoint to the default IP path. This disjoint path is created using transit hubs that can be located at diverse points in the Internet. Transit hubs provide better utilization of network resources. Assuming an IP layer routing between any two nodes, we show that the problem of computing such a disjoint path is NP-complete. We present an exact and a heuristic solution for the problem. Using routing data obtained from PlanetLab, we evaluate the efficacy of our heuristic solution.

A new metric for fault-tolerance in sensor networks

Faults in some sensor networks are likely to be localized. The conventional metric of fault-tolerance - connectivity of the network graph - fails to capture any notion of locality. This research introduces a new metric - <i><b>region-based connectivity</b></i> - that incorporates the notion of locality.

On maximum available bandwidth through disjoint paths

The use of multiple paths to route data between two end points provides various advantages by efficiently utilizing the underlying network resources. Prior research work has focused on the use of multiple paths with the objective of maximizing the end-to-end QoS such as bandwidth. Another category of work has proposed solutions for computing disjoint paths for fault-tolerance. In this work, we investigate the problem of maximizing the total bandwidth from disjoint paths. The solution to this problem combines the two main advantage of exploiting multiple paths: end-to-end QoS and end-to-end reliability. The solution has practical importance in providing fault-tolerance to bandwidth-intensive applications such as high quality video streaming. We prove that above problem if NP-complete and present its ILP formulation. We describe a heuristic algorithm along and evaluate its performance.