Abhijit Bhattacharya

A shortest path tree based algorithm for relay placement in a wireless sensor network and its performance analysis

In this paper, we study a problem of designing a multi-hop wireless network for interconnecting sensors (hereafter called source nodes) to a Base Station (BS), by deploying a minimum number of relay nodes at a subset of given potential locations, while meeting a quality of service (QoS) objective specified as a hop count bound for paths from the sources to the BS. The hop count bound suffices to ensure a certain probability of the data being delivered to the BS within a given maximum delay under a light traffic model. We observe that the problem is NP-Hard. For this problem, we propose a polynomial time approximation algorithm based on iteratively constructing shortest path trees and heuristically pruning away the relay nodes used until the hop count bound is violated. Results show that the algorithm performs efficiently in various randomly generated network scenarios; in over 90% of the tested scenarios, it gave solutions that were either optimal or were worse than optimal by just one relay. We then use random graph techniques to obtain, under a certain stochastic setting, an upper bound on the average case approximation ratio of a class of algorithms (including the proposed algorithm) for this problem as a function of the number of source nodes, and the hop count bound. To the best of our knowledge, the average case analysis is the first of its kind in the relay placement literature. Since the design is based on a light traffic model, we also provide simulation results (using models for the IEEE 802.15.4 physical layer and medium access control) to assess the traffic levels up to which the QoS objectives continue to be met.

Delay constrained optimal relay placement for planned wireless sensor networks

In this paper, we study the problem of wireless sensor network design by deploying a minimum number of additional relay nodes (to minimize network design cost) at a subset of given potential relay locations in order to convey the data from already existing sensor nodes (hereafter called source nodes) to a Base Station within a certain specified mean delay bound. We formulate this problem in two different ways, and show that the problem is NP-Hard. For a problem in which the number of existing sensor nodes and potential relay locations is n, we propose an O(n) approximation algorithm of polynomial time complexity. Results show that the algorithm performs efficiently (in over 90% of the tested scenarios, it gave solutions that were either optimal or exceeding optimal just by one relay) in various randomly generated network scenarios.

SmartConnect: A system for the design and deployment of wireless sensor networks

We have developed SmartConnect, a tool that addresses the growing need for the design and deployment of multihop wireless relay networks for connecting sensors to a control center. Given the locations of the sensors, the traffic that each sensor generates, the quality of service (QoS) requirements, and the potential locations at which relays can be placed, Smart-Connect helps design and deploy a low-cost wireless multihop relay network. SmartConnect adopts a field interactive, iterative approach, with model based network design, field evaluation and relay augmentation performed iteratively until the desired QoS is met. The design process is based on approximate combinatorial optimization algorithms. In the paper, we provide the design choices made in SmartConnect and describe the experimental work that led to these choices. Finally, we provide results from some experimental deployments.

QoS constrained optimal sink and relay placement in planned wireless sensor networks

We are given a set of sensors at given locations, a set of potential locations for placing base stations (BSs, or sinks), and another set of potential locations for placing wireless relay nodes. There is a cost for placing a BS and a cost for placing a relay. The problem we consider is to select a set of BS locations, a set of relay locations, and an association of sensor nodes with the selected BS locations, so that the number of hops in the path from each sensor to its BS is bounded by hmax, and among all such feasible networks, the cost of the selected network is the minimum. The hop count bound suffices to ensure a certain probability of the data being delivered to the BS within a given maximum delay under a light traffic model. We observe that the problem is NP-Hard, and is hard to even approximate within a constant factor. For this problem, we propose a polynomial time approximation algorithm (SmartSelect) based on a relay placement algorithm proposed in our earlier work, along with a modification of the greedy algorithm for weighted set cover. We have analyzed the worst case approximation guarantee for this algorithm. We have also proposed a polynomial time heuristic to improve upon the solution provided by SmartSelect. Our numerical results demonstrate that the algorithms provide good quality solutions using very little computation time in various randomly generated network scenarios.

Challenges in Developing and Deploying a PIR Sensor-Based Intrusion Classification System for an Outdoor Environment

The aim of the current paper is to describe the challenges faced during the development and deployment of a PIR sensor-based intrusion classification system in an outdoor environment. Some potential solutions to overcome these challenges are also presented. The challenges are of three types: (a) challenges in designing a sensor tower platform that is physically aligned to the accuracies demanded by the application, (b) challenges involved in implementing high-accuracy algorithms on a mote and, finally and perhaps most importantly, (c) challenges faced in attempting to operate a PIR-based sensor tower platform at temperatures close to that of the human body. A test deployment over a 15-week period, beginning Feb 6, 2016, was carried out within the campus of the Indian Institute of Science, Bengaluru. This period of deployment coincided with the peak summer season at which time the ambient temperature in Bengaluru ranged from 15 degree C to 38 degree C. This made it possible to study the behavior of the PIR-based sensor tower platform in settings where the ambient temperature is close to that of the human body.

An approximation to the QoS aware throughput region of a tree network under IEEE 802.15.4 CSMA/CA with application to wireless sensor network design

In the context of wireless sensor networks, we are motivated by the design of a tree network spanning a set of source nodes that generate packets, a set of additional relay nodes that only forward packets from the sources, and a data sink. We assume that the paths from the sources to the sink have bounded hop count, that the nodes use the IEEE 802.15.4 CSMA/CA for medium access control, and that there are no hidden terminals. In this setting, starting with a set of simple fixed point equations, we derive explicit conditions on the packet generation rates at the sources, so that the tree network approximately provides certain quality of service (QoS) such as end-to-end delivery probability and mean delay. The structures of our conditions provide insight on the dependence of the network performance on the arrival rate vector, and the topological properties of the tree network. Our numerical experiments suggest that our approximations are able to capture a significant part of the QoS aware throughput region (of a tree network), that is adequate for many sensor network applications. Furthermore, for the special case of equal arrival rates, default backoff parameters, and for a range of values of target QoS, we show that among all path-length-bounded trees (spanning a given set of sources and the data sink) that meet the conditions derived in the paper, a shortest path tree achieves the maximum throughput.

Analytical Modeling of IEEE 802.11-Type CSMA/CA Networks With Short Term Unfairness

We consider single-hop topologies with saturated transmitting nodes, using carrier-sense multiple access with collision avoidance (CSMA/CA) for medium access, as standardized under the IEEE 802.11 distributed coordination function. We study systems where one or more backoff parameters of the CSMA/CA protocol (the initial backoff, the backoff multiplier, and the number of retries) are different from the standard. It is known that, for several classes of these protocol parameters, such systems exhibit a certain performance anomaly known as short term unfairness. We also find that the phenomenon of short term unfairness is observed in systems where the propagation delays among the participating nodes are not negligible compared with the duration of a backoff slot, even when the nodes use the default backoff parameters of the standard. It also turns out that the standard fixed point analysis technique (and its simple extensions) does not predict the system behavior well in such cases. For systems with large propagation delays, we observe that, as propagation delay increases, the collision probability of a node initially increases, but then flattens out, contrary to what is predicted by the standard fixed point approximation. Our study of several example systems reveals some interesting connections between the protocol parameters, the number of nodes, the propagation delay, and the degree of unfairness. This paper reveals that the inability of the standard fixed point model to capture the performance in such cases is due to its state-independent attempt rate assumption. In this paper, we develop a novel approximate, but accurate, analysis that uses state-dependent attempt rates with a parsimonious state representation for computational tractability. The analytical method is also able to quantify the extent of short term unfairness in the system, something not possible with existing analytical techniques, and can, therefore, be used to tune the protocol parameters to achieve desired throughput and fairness objectives.

Mean delay analysis of cache assisted file transfers over the Internet

We address the problem of analyzing the mean delay experienced by end-users in a Content Distribution Network (CDN) consisting of a set of surrogate (cache) servers connected via persistent TCP connections over the Internet cloud to a set of remotely located origin servers. We first consider the simplest scenario of a single cache server and a single origin server. Taking into account several factors such as cache hit probability, pending interest table, number of concurrent sessions, background load, bottleneck link speed etc., we derive an exact expression for the mean file transfer delay for a wide range of service disciplines at the origin server, under the restriction of equal rates of request arrivals for all the files. The derivation involves establishing the ergodicity of the underlying stochastic process, and is of interest on its own. The expression for the mean delay involves the second moment of the file sojourn time in the origin server. This expression enables us to compare, using an adaptation of certain standard sample path arguments, several service disciplines at the origin server in terms of their mean delay performance. Among a wide range of commonly used service disciplines, FCFS achieves the best mean delay performance. We discuss an implication of this result for system design. Finally, we provide an approximate mean delay analysis for the general scenario of multiple caches and origin servers, which is accurate when the number of files is large compared to the number of requests in service at any point in time.

Modeling, performance analysis, and optimization of single hop IEEE 802.11 networks with large propagation delays: Challenges and solutions

We consider single-hop topologies with saturated transmitting nodes, using IEEE 802.11 DCF for medium access, where the propagation delays among the nodes are not negligible compared to the backoff slot duration. In this situation, we find that there is misaligned sensing of channel idleness, and also short-term unfairness in access to the medium. We demonstrate that existing analysis techniques (or, extensions thereof) are unable to account for these features, resulting in inaccurate prediction of the performance. Focusing on the case in which transmitters are equidistant from one another, and also each receiver is equidistant from all the transmitters, we provide a detailed stochastic model that accurately captures the system evolution. Since an exact analysis of this model is computationally intractable, we develop a novel approximate, but accurate, analysis that uses a parsimonious state representation for computational tractability. Numerical experiments show, the approximate analysis predicts the system throughput to a relative accuracy of 2-3%, and collision probabilities to a relative accuracy of 3-8% compared to simulations. Interestingly, we observe that as propagation delay increases, the collision probability of a node initially increases, but then flattens out, contrary to what one might intuitively expect. Finally, we also demonstrate how to optimize slot duration using the approximate analysis for maximizing system throughput.