Chessboard Clustering and Routing Protocol

Du and Xiao [16] propose a chessboard clustering and routing protocol for het-erogeneous WSNs to overcome the performance bottleneck and poor scalability of

homogeneous WSNs and to address, at same time, the problem of nonuniform energy consumption. A good observation is made that clustering alone does not solve the problem of nonuniform energy drainage; indeed, the center node in Figure 2.7 can as well be a cluster head rather than a base station.

Two types of nodes are assumed: a small number of high-end sensor nodes and a large number of low-end sensor nodes. Each node is assumed to be aware of its location. A cluster is formed around each high-end sensor node which serves as a cluster head. Low-end sensor nodes perform the basic sensing as well as the relaying of packets within the cluster. Given its powerful energy reserve and communication ability, each high-end node performs data fusion within its cluster, and it transmits the aggregated data to the sink via a single-hop link or a multihop path. In this way, the network is divided into multiple regions, with each region assuming a smaller burden of the communication due to the smaller number of sensor nodes within the cluster.

The network lifetime is therefore increased by transmitting fewer packets at low-end sensor nodes and utilizing the less power-constrained or non-power-constrained nodes as much as possible.

Figure 2.10 shows the sensor field divided into equal-sized cells with adjacent cells colored with different colors, resembling a chessboard. These nodes are assumed to be uniformly and randomly distributed in the sensor field. Since each node knows its location, it can determine if it is in a white cell or a black cell.

The basic idea is to use the underlying chessboard to define two clustered topolo-gies, with only one clustering in use at a given time. In a white clustering, all high-end sensor nodes in white cells are active while all high-high-end sensor nodes in black cells are inactive. In a black clustering, all high-end sensor nodes in black cells are active while all high-end sensor nodes in white cells are inactive. Low-end sensor nodes are all active, forming multihop clusters around the currently active high-end sensor nodes. The motivation for switching colors is as follows: sensor nodes that are critical nodes in a white clustering are likely to become non-critical nodes in a black clustering and vice versa. Since critical nodes consume more energy in packet

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Figure 2.10. Chessboard clustering scheme.

SCALABILITY AND SYSTEM LIFETIME 35

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“White” Cluster Head

“Black” Cluster Head Sensor Node

Figure 2.11. A black (left) and white (right) clustering of the heterogeneous WSN.

forwarding than do other sensor nodes, switching the color of the clustering balances the energy consumption among sensors, and prolongs the network lifetime.

Figure 2.11 shows an example of clustering based on black and on white cells, respectively. In the black clustering, sensor node 3 is a critical node forwarding packets on behalf of nodes 1 and 2. In the white clustering, nodes 1 and 2 become critical forwarding packets on behalf of other nodes in the cluster; in particular, node 2 now forwards the packets of node 3.

In order to form a black (white) clustering, each black (white) high-end sensor node broadcasts a hello packet, containing its identifier and its location. Low-end sensor nodes may receive hello packets from multiple black (white) high-end nodes. In a two-dimensional sensor field, each low-end sensor node selects the closest high-end sensor node as the cluster head; this leads to the formation of Voronoi cells where the cluster heads correspond to the nuclei of the cells [16].

The decision to switch the color of the clustering is based on the energy levels of the high-end nodes. Suppose the current clustering is black. Periodically, each black high-end sensor node exchanges packets with its neighboring white high-end nodes.

The packets contain the energy remaining in the node. If the remaining energy of the black high-end node drops below a threshold, its neighboring white high-end nodes become active and initiate cluster formation. As the network runs, the black high-end nodes drain their energy and become unavailable. Gradually, the white high-high-end nodes become active.

Both intra- and intercluster routing protocols are proposed [13, 16]. Routing within a cluster is achieved via a greedy geographic routing protocol. Each low-end sensor node simply forwards a packet to the neighbor closest to the cluster head.

In order to support intercluster routing, after the clusters are formed, each cluster head sends its location to the sink. The sink then broadcasts the locations of all clusters heads. For a cluster head to communicate with the sink, it draws a line between itself and the sink. The line intersects some number of Voronoi cells. The packet is forwarded from the source cluster head to the sink through the cluster heads in these relay cells. The chessboard routing protocol achieves a higher delivery ratio, lower total energy consumption, smaller end-to-end delay, and better throughput than two routing protocols for homogeneous WSNs. The details of the chessboard clustering

and routing protocols, as well as their performance evaluation in simulation, can be found in references 13 and 16.