Scheduling of Sporadic Events in Wireless Sensor Networks

E lectronic Sensors have been around for decades. With the miniaturization of electronic circuits and large scale integration, it has become possible to combine the sensors with the digital circuitry to fabricate devices of small size. Such devices with combined sensing, processing and communicating capabilities are known as sensor nodes. These sensor nodes have a huge number of applications scaling from home automation, health care, agriculture, disaster management and military applications to industrial automation and control. Many of these applications require real-time data propagation. Health care to industrial control are the areas where these sensor nodes find their applications. The basic requirement of realtime applications is that data sensing, processing, communication and response generation must be accomplished within a certain time limit. In this regard, the IEEE 802.15. 4 standard has become defacto standard for low rate WPAN for wireless sensor networks. The contention free period of the beacon enabled mode of the IEEE 802.15. 4 standard makes the IEEE 802.15. 4 standard widely acceptable (due to its real-time communication support) for industrial control applications. The said mode requires that nodes are arranged in a star topology to form a cluster with a cluster head. The cluster head reserves bandwidth for each node of the cluster, where sensing event is expected to occur. In case of periodic events, the advanced knowledge is available in the form of time of occurrence, period, execution time and deadline of the occurred period event. This knowledge makes the offline bandwidth planning relatively easy. In case of sporadic events, however, the time of occurrence of sporadic events is not known before the occurrence of sporadic event. Therefore it is difficult to plan bandwidth for sporadic events offline. Wireless Sensor Networks (WSNs) find applications in the industrial automation where periodic and sporadic events occur. The combined propagation of information generated by periodic and sporadic events from a sensor node to an actuator node is challenging due to the random nature of sporadic events, particularly if missing the deadlines is disastrous. The IEEE 802.15. 4 standard provides the basis for a real-time communication mechanism between neighboring nodes of the WSN at the Media Access Control layer. However, the standard does not address realtime communications over multiple hops. To support the industrial applications with real-time requirements, this work proposes an offline bandwidth estimation technique and two novel on-line control protocols, which exploit the basis provided by the IEEE 802.15. 4 standard for on-line bandwidth allocation for sporadic data propagation. A control protocol is also proposed which ensures that a given off-line sporadic schedule can be adapted on-line in a timely manner such that the static periodic schedule has not been disturbed and the IEEE 802.15. 4 standard compliance remains intact. The proposed protocol is simulated in Opnet. The simulation results are collected in the form of time stamps of activities occurring at different nodes of the network. The compliance of the information is discussed to prove the correctness of the proposed protocol regarding efficient real-time sporadic event delivery along with the periodic events. Moreover, efficient energy dissipation is also discussed after collecting the energy dissipation, and presenting it in the form of bar graphs and analyzing these graphs.