Scheduling of Wireless Sensor Networks

Abstract

This chapter discusses the state of the art related to scheduling and network planning within wireless sensor networks (WSN). It first provides some background information concerning medium access control (MAC), communication protocol approaches, and scheduling mechanisms used by time-division multiple access protocols (TDMA protocols). Planning mechanisms used to plan wireless sensor networks with precise timing are then examined. Wireless Medium Access Control (MAC)One key issue in WSNs that influences whether the deployed system will be able to provide timing guarantees is the MAC protocol and its configurations. The MAC protocols for wireless sensor networks can be classified broadly into two categories: contention-and schedule-based. The contention-based protocols can easily adjust to the topology changes as new nodes may join and others may die after deployment. These protocols are based on Carrier Sense Multiple Access (CSMA) mechanisms and have higher costs due to potential interference, message collisions, overhearing, and idle listening than the schedule-based counterparts. Schedulebased protocols can avoid those problems by defining precise schedules, but they have strict time synchronization requirements. Contention-Based MAC ProtocolsIn contention-based MAC protocols, potential message receivers wake up periodically for a short time to sample the medium. When a sender has data, it transmits a series of short preamble packets, each containing the ID of the target node, until it either receives an acknowledgment (ACK) packet from the receiver or a maximum sleep time is exceeded. Following the transmission of each preamble packet, the transmitter node waits for a timeout. If the receiver is not the target, it returns to sleep immediately. If the receiver is the target, it sends an ACK during the pause between the preamble packets. Upon reception of the ACK, the sender transmits the data packet to the destination.These protocols are implemented using units of time called backoff periods. The expected number of times random backoff is repeated is a function of the probability of sensing the channel busy, which depends on the channel traffic. Since these do not provide a precise schedule to send data and use random backoff, these protocols have higher costs for message collisions, overhearing, and idle listening than schedule-based counterparts, and they are typically not used for applications requiring strict operation timing guarantees. On the other hand, they can easily adjust to topology changes, such as when new nodes join and others leave after deployment.Some protocols frequently used in WSNs, such as S-MAC, B-MAC, WiseMAC, and X-MAC, are contention-based. S-MAC [1] defines periodic frame structure divided into two parts, with nodes being active in the first fraction of the frame and asleep for the remaining duration. The length of each of the frame parts is fixed, according to the desired duty cycle. Virtual clustering permits that nodes adopt and propagate time schedules, but it leads to the exist...