List of notations.
Abstract
Wireless sensor network is a collection of autonomous devices called sensor nodes which sense the environmental factors such as temperature, pressure, humidity, moisture, etc. The nodes sense the data, process it and transmit to the other nodes within their transmission range through radio propagation. Energy minimization in wireless sensor networks is a significant problem since the nodes are powered by a small battery of limited capacity. In case of networks with several thousand nodes, the simulation of algorithms can be very slow. The parallel computing model provides significantly faster simulation time for larger networks. Parallel processing involves executing the program instructions by dividing them among multiple processors with the objective of reducing the running time. So, we propose algorithms for the range assignment problem in wireless sensor networks using the parallel processing techniques. We also discuss the complexity of the proposed algorithms and significance of the parallel processing techniques in detail. The proposed techniques will be useful for implementing the distributed algorithms in WSNs.
Keywords
 range assignment
 parallel processing
 minimum spanning tree
 wireless sensor networks
 algorithm
 complexity
1. Introduction
In this section, preliminaries of Wireless Sensor Networks (WSNs) and parallel processing are discussed in detail.
1.1 Wireless sensor networks
Wireless Sensor Network (WSN) is a group of spatially dispersed autonomous devices called sensor nodes equipped with a radio transceiver along with an antenna. The devices are responsible for sending and receiving radio signals. Transmission range is assigned to the nodes to facilitate the communication between the nodes of a network. The sensor nodes sense the physical conditions of the environment such as temperature, pressure, sound, heat, light and humidity, etc., at various locations, process the data and transmit to the other nodes that lie within their transmission range through radio propagation. WSN monitors a physical system in real world and has applications in several fields such as environmental monitoring, biological detection, military and health care, etc. [1]. Because of the power constraints for the nodes using batteries with limited capacity, minimization of energy consumption is a significant problem in WSNs [2].
Constructing an efficient topology by assigning transmission range to the nodes of a network to minimize the total energy consumption has become a major challenge in WSNs. In general, the transmission range assigned to a node can be tuned so that the required connectivity constraint for the resultant network is satisfied and the total energy is minimized; this class of problems is categorized as topology control problems [3]. Some of the specified constraints include simple connectivity, biconnectivity,
Transmission range of a node is the range within which the data sent by other node is received properly [5]. Two nodes in a WSN can communicate if and only if one node is in the transmission range of the other. Transmission range of a node
Let
In reality, some problems use mathematical models that describe the problem in a systematic way and enable us to solve the problem efficiently. Graphs can be used to model many relations and represent many physical problems in the real world. A WSN is modeled as an undirected graph to solve the energy minimization problems. In this model, a vertex of the undirected graph represents a node and the edge joining two vertices represents the communication link between the nodes. The nodes are deployed on a Euclidean plane and each pair of two nodes is associated to the Euclidean distance between the nodes. Each node is located by its
Figure 2
illustrates the graph theoretic modeling of a WSN with four sensor nodes
Throughout this chapter, the considered topology is bidirectional. Therefore, we use an undirected graph along with the weight function
The total range of the graph which is the sum of the ranges of all the vertices of
Minimum range assignment problem in a WSN is to assign transmission range to the nodes of the network such that the resultant subgraph
Chen et al. [9] studied the problem of strong connectivity in multi hop packet radio networks and proposed a 2approximation algorithm which initially considers an undirected Minimum Spanning Tree (MST) and later establishes the bidirectional connectivity. Strong Minimum Energy Topology problem (SMET) in a WSN is studied by Cheng et al. [10], in which the objective is to assign transmission range to the nodes of the network such that the total range of the network is minimized and the reduced topology is connected. The authors have proved that the SMET problem is NPcomplete and proposed two algorithms namely: Minimum Spanning Tree (MST) based Heuristic which is of 2approximation ratio and an Incremental Power Greedy Heuristic and performed the simulation to show that the Incremental Power Greedy Heuristic performs better than the MST Heuristic. Formulation of the SMET problem is as follows:
1.2 Parallel processing
Large scale WSNs such as environmental monitoring systems produce large amount of data in the order of Peta bytes. The challenges in this case are storage and computation as the sensors are constrained by their resources which are scarcely available [11]. If the large amount of data is processed centrally, all data needs to be transmitted to a central server using multi hop transmissions which leads to high computational cost. One of the best ways to avoid such problem is to exploit the advantages of the distributed storage and parallel processing. Instead of transmitting data to a central server, data is stored and processed in network which reduces the computational cost. In this process, the computational task is decomposed into many small tasks, and the computation is executed in parallel by the distributed sensors.
Sullivan et al. [12] proposed a set of new parallel algorithms for a tree decompositionbased approach to solve the maximum weighted independent set problem. Independent set problem for a given graph is to find a subset of vertices with maximum cardinality such that no two vertices in the graph are adjacent to each other. Maximum Weighted independent set problem in which, the vertices are assigned weights, is to find an independent set with maximum weight [13]. Zhu et al. [14] proposed parallel algorithm for the hierarchicalbased protocol for WSNs based on the LowEnergy Adaptive Clustering Hierarchy (LEACH) algorithm in order to improve the routing efficiency of the networks. Authors have implemented the algorithms using the parallel processing technique and showed that this technique has increased the overall performance. Lounis et al. [15] presented a new model for parallel computing of energy consumption in WSNs, and implemented on GPU architecture. Through simulation, authors showed that the proposed model provides simulation times significantly faster than that of the sequential model for large networks and for long simulations.
Andoni et al. [16] gave algorithms for geometric graph problems in the modern parallel models. The authors found a
In this chapter, we discuss the parallel processing techniques for the range assignment problem for simple connectivity. We propose algorithms for this problem using parallel processing techniques and determine the complexity. The proposed algorithms initially find an MST and assign range to each vertex of the network based on its range in the Euclidean MST. We find the Euclidean MST using well known algorithms: Prim’s and Kruskal’s and later we assign the range using parallel processing technique for both the cases. We also discuss the complexity of the proposed algorithms in detail for both the algorithms.
Simulation is an act of imitating operation of real world process or system over time [19]. The simulation is performed on a model which represents the system. It is known that simulations closely reflect accurate behavior of the system. Simulations help in evaluating the algorithm before investing on the physical hardware.
2. Related work
Finding a Euclidean MST is an important problem in Graph Theory and there are several algorithms in the literature [20]. MST problems have significant applications in computer and communication network design, as well as indirect applications in fields such as computer vision and cluster analysis [21]. Parallelization of Prim’s algorithm is studied by Deo and Yoo [22] and the proposed algorithms used shared memory computers. Gonina et al. [23] studied the parallel implementation of the Prims’s algorithm for finding the MST of dense graphs. Authors presented the simulation results which demonstrate the advantage of proposed algorithms.
Vladimir et al. [24] studied serial variants of Prim’s and Kruskal’s algorithm and presented their parallelization, targeting message passing parallel machine with distributed memory. Authors have considered large graphs and presented experimental results which show that Prim’s algorithm is a good choice for dense graphs while Kruskal’s algorithm is better for sparse graphs. Authors have proposed algorithms based on the sequential algorithms of Prim and Kruskal, targeting message passing parallel machine with distributed memory. The proposed algorithm which uses the Prim’s algorithm has a running time of
The considered input for the parallelization of both Prim’s and Kruskal’s algorithm by Vladimir et al. [24] is an undirected simple graph with weights assigned to the edges. For our research the input considered is a complete graph along with edge weights, where the edge weights are the Euclidean distance between the nodes.
2.1 Parallelization of Prim’s algorithm by Loncar et al.
Prim’s algorithm starts from an arbitrary vertex and then grows the subtree by choosing a new vertex from the set of unvisited vertices and adding it to the subtree in each iteration. The Prim’s algorithm runs until the number of edges becomes
The weights are represented using the adjacency matrix which is defined as follows:
Prim’s algorithm is implemented using the auxiliary array
2.2 Parallelization of Kruskal’s algorithm by Loncar et al.
Kruskal’s algorithm finds a minimum weighted edge which connects two components in the forest in each iteration, i.e., it grows multiple trees in parallel. Initially, Kruskal’s algorithm creates a forest considering each vertex as a tree and next it sorts all the edges. Each time a minimum weighted edge is chosen and added to the forest if it connects two different trees else it is discarded. This process is repeated until the forest becomes a spanning tree. The complexity of the proposed algorithm is
Similar to the Prim’s algorithm, we have adjacency matrix which stores the weights between the vertices, and each processor is assigned a subset of vertices. In parallelization of Kruskal’s algorithm, each processor sorts the edges of its vertices according to their weights. At each process, a local MST is found using Kruskal’s algorithm and finally, all the local MST’s are merged. The complexity of the proposed algorithm is
3. Parallelization of range assignment problem
In this section, we present the proposed algorithms for range assignment problem using parallel processing techniques. The proposed algorithms adopt the algorithms for parallelization of both Prim’s and Kruskal’s algorithms by Loncar et al. [24]. In this environment, the nodes are deployed on a Euclidean plane and each pair is associated with the Euclidean distances between those nodes. Since a WSN is modeled as an undirected graph along with a weight function, the input for these algorithms is a complete graph
Objective of this problem is to find a spanning tree such that the total range of the spanning tree is minimized. In the proposed algorithms, we initially find the spanning tree of the given complete graph and assign the range to the vertices based on their range in the Euclidean MST using parallel processing. We propose two algorithms: one using the Prim’s algorithm and the other using the Kruskal’s algorithm. We also compute the complexity of the proposed algorithm in both the cases. The list of notations used in this chapter is given in Table 1 .
Notation  Explanation 


Set of vertices 

Set of edges 

Number of nodes 

Euclidean distance between


Range of vertex


Auxiliary array 

The set of processors 



Number of processors 

Resultant spanning tree 
3.1 Range assignment using Prim’s algorithm
Let
In the proposed algorithm, we consider a complete graph and the number of processors as input and compute a spanning tree which minimizes the total range. In this algorithm, we partition the vertices into
Each process finds a minimum weighted edge which connects the MST with vertices of the set
Next, we assign range to the vertices in the following way: for a vertex, each process finds the farthest vertex to it in the MST and sends the distance between them to the leader processor using alltoone reduction. Later, leader processor selects one with maximum edge weight among all the received edge weights and that weight is assigned as range to the vertex
Theorem 1.1 [24] The parallelization of Prim’s algorithm takes
Theorem 1.2 The algorithm PARAPrim’s takes
3.2 Range assignment using Kruskal’s algorithm
Similar to the Prim’s algorithm in this algorithm, we have
The sequence of steps for the above explained procedure is presented in Algorithm 2 named PARAKruskal’s.
Theorem 1.3 [24] The parallelization of Kruskal’s algorithm takes
Theorem 1.4 The algorithm PARAKruskal’s takes
4. Comparison with the state of arts
Solving the range assignment problem by computing Euclidean MST using Prim’s algorithm and Kruskal’s algorithm takes running time of
5. Conclusions
In this research, we have studied the range assignment problem by employing parallel processing techniques of both Prim’s and Kruskal’s algorithms. The complexity of the proposed two algorithms is discussed in detail and it is shown that complexity can be improved by using parallel processing techniques for larger networks. It is an interesting problem to study variations of range assignment problems using parallel processing techniques. The implementation of the proposed algorithms can be realized using CUDA programming which supports programming GPU with multiple cores.
Abbreviations
WSN  wireless sensor network 
MST  minimum spanning tree 
MSF  minimum spanning forest 
PARA  parallel algorithm for range assignment 
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