Computer Networks, 69 : 51-65 Item record/more information http://hdl.handle.net/10197/7089 Abstract Mostly-o sensor network applications alternate between long periods of inactivity (ranging from minutes to hours) and short periods of activity (normally a few seconds). From an energy consumption point of view, it is desirable that the network switch o completely during application inactive periods and wake-up eciently at the start of application active periods. The fundamental problem preventing this is the inter-node clock skew arising from the network being o for a long period. Existing solutions maintain synchronization during the inactive period or use the radio excessively to enable asynchronous wake-up. Herein, we propose BailighPulse, a low duty cycle data gathering protocol for mostly-o WSN applications. BailighPulse incorporates a novel multihop wake-up scheme that allows for energy ecient recovery of network synchronization after long o periods. The scheme uses a staggered wake-up schedule and optimized channel polling during wake-up based on knowledge of the pre-dened application-level schedule. Herein, we provide an extensive assessment of the protocol's performance including an analytic model, simulations, and testbed results. We show that, for a homogeneous schedule with collection period greater than 2 minutes, BailighPulse reduces radio duty cycles by at least 30% and 90% compared to Dozer and B-MAC, respectively. We also show that BailighPulse is able to reduce radio duty cycle by to 68% for a heterogeneous schedule under similar conditions. 1 Introduction In data gathering applications Wireless Sensor Network nodes periodically sample their sensors and transmit the measurements back to a single node (the sink). In mostly-o WSN applications [1], the application alternates between long periods of inactivity (ranging from minutes to hours) and short periods of activity (normally a few seconds). Examples of these applications include predictive maintenance [2], utility metering networks [3], and environmental research [4]. Address(es) of author(s) should be given 2 Wojciech Bober and Chris J. Bleakley From an energy consumption point of view, it is desirable that a Wireless Sensor Network follows application characteristics and switches o completely during application inactive periods and switches on eciently at the start of application active periods. This mode of operation where a network is completely switched o between short on periodsis referred to as disconnected mode [5] . Clearly, to preserve energy, the time for which the sensor network is on should be as short as possible. Dutta et al. [5] showed that, in principle, a network using this mode of operation can operate at a duty cycle of 0.01%. In practice, however, achieving this goal has proven dicult due the fundamental problem of inter-node clock skew arising from the network being o for a long period [6] . Clock skew results from crystal oscillator instability which is mainly caused by temperature variation. In outdoor environments this can lead to clock drifts of 125-400 ms/h (35-110 ppm) [7] . Due to the resulting clock skew, nodes wake-up at dierent times and need to wait until the whole sensor network is ready before communicating. This increases the network on time, increasing the nodes' duty cycle. The problem can be addressed at the hardware or software level. At the hardware level, a temperature compensated crystal oscillator (TXCO), which has minimal clock drift can be used, so that nodes wake-up at almost the same time [6] . Another hardware approach uses an out-of-band wake-up radio that allows ecient asynchronous network wake-up [8] . Hardware approaches, however, are rarely used since they require additional electronics which increase sensor node cost and size. Hence most research focuses on the software level in the form of energy ecient data gathering protocols. Data gathering protocols can be categorized according to whether they use synchronous or asynchronous wake-up methods. In the case of synchronous wake-up, either explicit or implicit synchronization is used during the application inactive period. In the case of asynchronous wake-up, the sensor network is kept on so that the drift does not matter. Both approaches require energy which is wasted from the application point of view. As a result, no previously proposed protocol achieves the theoretical duty cycle target. Typically, in mostly-o sensor network applications the application schedule (the times of sensor sampling and data collection) is determined by a domain expert and xed a priori. An application schedule might, for example, set the the network to sample sensors less often during the day and more often during the night. In the protocol presented herein, we take advantage of this property to propose a dierent approach to dealing with the clock skew problem. In BailighPulse the network is switched o completely when the application is inactive. Fine grained network synchronization is then recovered at the beginning of the active phase using a novel multi-hop wake-up scheme. This wake-up scheme uses a staggered wake-up schedule and optimizes channel polling based on knowledge of the pre-dened application-level schedule. Using this approach energy consumption during wake-up is minimized. To the best of our knowledge, BailighPulse is the rst data gathering protocol for mostly-o sensor networks utilizing coarsely synchronized multi-hop wake-up.. BailighPulse is suitable for use with complex scheduling algorithms such as those described in [9, 10, 11, 12] . These algorithms derive sampling schedules based on criteria such as sensing converge and data correlation. For completeness we propose a compact notation for specifying the application level schedules. The notation allows for expression of application activity, i.e., sampling and collection, in a simple and compact way. The results of analytic and simulation based evaluations show that, for low data collection rates, the proposed protocol signicantly reduces the average radio duty cycle of the network. We show that in mostly-o applications BailighPulse outperforms Dozer, which is a state-of-art data gathering protocol. The reminder of the paper is organized as follows: In Section 2 we discuss related data gathering protocols. Section 3 denes assumptions and formulates the problem statement. Section 5 gives details of the protocol design. An analytic model of BailighPulse is given in Section 6. Performance evaluation of the protocol is presented in Section 7. A case study and testbed results are discussed in Section 8. We conclude the paper with Section 9. 2 Related Work A large number of techniques aiming to reduce energy consumption in WSNs have been proposed [13, 14] . The solutions most closely related to the proposal are data gathering protocols, i.e., protocols specically designed for collecting data from the entire network. Typically, data gathering protocols are cross-layer, integrating MAC layer, routing layer, and energy management mechanisms. From the point of view of mostly-o sensor network applications, these protocols can be divided according to whether they use synchronous or asynchronous wake-up. A simplied overview of both approaches is shown in Figure 1 . In the rst case (Figure 1(a) ), nodes are not synchronized and periodically poll (check) the wireless channel for a wake-up beacon. The polling period is typically in the range of 100-1000 ms. However, energy is wasted when the application does not require transmission of data. In the second case, nodes are synchronized (Figure 1(b) ) and time synchronization is maintained in the network. This means that clock drift is kept low. This allows the nodes to wake-up at scheduled times and send data immediately. Although less energy is wasted than in the case of asynchronous wake-up, energy is still wasted on maintaining time synchronization. Most proposals based on asynchronous wake-up are focused on the MAC layer. Protocols such as B-MAC [15], Wise-MAC [16], X-MAC [17] use periodic channel polling to reduce energy consumption. For data gathering
Bober and Bleakley proposed BailighPulse, an energy efficient data gathering protocol for mostly-off WSN applications REF .
BailighPulse: A low duty cycle data gathering protocol for mostly-off Wireless Sensor Networks
Comput. Networks
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