A Novel Systematic Resource Transfer Method for Wireless Sensor Networks Winnie Louis Lee, Amitava Datta, and Rachel Cardell-Oliver School of Computer Science and Software Engineering The University of Western Australia Perth WA 6009 Australia Email: {winnie, datta, rachel}@csse.uwa.edu.au Abstract— The use of wireless sensor networks for gathering environmental and safety-critical data in real time is increasing at a rapid rate. Some of the main criteria in designing sensor network architectures are energy-efficiency, self-management and self-healing. However, most protocols for data gathering and routing in sensor networks implicitly assume a regular rate of data gathering by individual nodes. While this is sufficient for sensing parameters that change slowly over time, individual nodes in a small part of a network may need to increase the rate of data gathering considerably for reporting important data in real-time. Though CSMA protocols can report increasing amount of data in theory, increased contention for the wireless medium in a small part of the network usually results in increased message collision and retransmission. In this paper, we present a novel TDMA protocol for transferring time slots from one part of the network to another part to support non-uniform and reactive sensing in different parts of a network. We discuss the design of this protocol and show that it performs much better compared to CSMA protocols both in terms of energy-efficiency and supporting non- uniform sensing. I. I NTRODUCTION Wireless sensor networks (WSNs) consist of a large number of sensor nodes equipped with one or more sensing units that are used to gather information in diverse settings including nat- ural ecosystems, battlefields, and man made environments [1]. These sensor nodes work under severe resource constraints such as limited battery power, computing power, memory, wireless bandwidth, and communication capability. Therefore, every activity in wireless sensor networks must be run ef- ficiently without consuming too many resources. Typically, sensor nodes are deployed in remote and harsh conditions and so they need to be able to self-configure in the event of failures without prior knowledge of the network topology, and ideally, to self-manage without human interventions. Most protocols designed for data gathering and routing in WSNs implicitly assume a regular rate of data gathering by individual sensor nodes. While this is sufficient for sensing parameters that change slowly over time, individual nodes in a small part of a network may need to increase the rate of data gathering considerably for reporting important data in real-time. For example, in structural monitoring applications, sensor nodes are deployed to monitor vibration (e.g., wind and earthquakes) that could damage the structure of a building [2]. It is critical for sensor nodes to send their data more often to the base station when they detect event triggers such as sensor readings changing rapidly or exceeding user-specified thresh- olds. Though CSMA-based protocols can report increasing amount of data in theory, increased contention for the wireless medium in a small part of the network leads to increased message collisions and retransmissions. Recently, researches are employing TDMA-based protocols to achieve collision-free communication [3]. TDMA-based protocols separate nodes in time, which means that nodes are only active (transmit or receive) during their scheduled slots. The main challenge of this scheme is to allow nodes to adjust their schedule according to their current sensing requirement. TRAMA [4] is an example of a TDMA-based protocol that allows nodes to adjust their schedule according to traffic conditions through regular neighborhood information exchange. If a node has few packets to send, it may release its slot for the remainder of the frame to other nodes that have many packets to send. This scheme results in a high bandwidth utilization, however TRAMA does so at the ex- pense of high latency and high algorithmic complexity [3]. Furthermore, Miller and Vaidya [5] propose a two-radio based MAC protocol to allow senders to wake receivers using a second low power radio if a specified number of packets are buffered. Neither of these protocols support rescheduling based on nodes’ current sensing requirements. In this paper, we propose a systematic resource transfer (SRT) method that allows time slots (resources) from one part of the network to be transferred to another part. This allows non-uniform and reactive sensing in different parts of a network. We use a novel TDMA-protocol called Flexible- schedule-based MAC (FlexiMAC) [6], to support resource transfer among nodes in the network. FlexiMAC schedules node communication and continuously and efficiently collects and disseminates data, to and from sensor nodes in a data gathering tree. The problem of systematic resource transfer is interesting by itself and is an important property for most WSN applications. To the best of our knowledge, this problem has never been investigated before. There are three main contributions of our SRT method. First, it can reconfigure the traffic patterns of sub-networks in the network based on the analysis of sensor data by enabling a node to get extra slots from other nodes in the network. The requesting node can specify the number of extra slots required in a data gathering cycle and specify or change how