PW-MAC: An Energy-Efficient Predictive-Wakeup MAC Protocol for Wireless Sensor Networks Lei Tang ∗ Yanjun Sun † Omer Gurewitz ‡ David B. Johnson ∗ ∗ Department of Computer Science, Rice University, Houston, TX, USA † Systems and Applications R&D Center, Texas Instruments, Dallas, TX, USA ‡ Department of Communication Systems Engineering, Ben Gurion University, Israel Abstract—This paper presents PW-MAC (Predictive-Wakeup MAC), a new energy-efficient MAC protocol based on asyn- chronous duty cycling. In PW-MAC, nodes each wake up to receive at randomized, asynchronous times. PW-MAC minimizes sensor node energy consumption by enabling senders to pre- dict receiver wakeup times; to enable accurate predictions, PW-MAC introduces an on-demand prediction error correction mechanism that effectively addresses timing challenges such as unpredictable hardware and operating system delays and clock drift. PW-MAC also introduces an efficient prediction- based retransmission mechanism to achieve high energy effi- ciency even when wireless collisions occur and packets must be retransmitted. We evaluate PW-MAC on a testbed of MICAz motes and compare it to X-MAC, WiseMAC, and RI-MAC, three previous energy-efficient MAC protocols, under multiple concurrent multihop traffic flows and under hidden-terminal scenarios and scenarios in which nodes have wakeup schedule conflicts. In all experiments, PW-MAC significantly outperformed these other protocols. For example, evaluated on scenarios with 15 concurrent transceivers in the network, the average sender duty cycle for X-MAC, WiseMAC, and RI-MAC were all over 66%, while PW-MAC’s average sender duty cycle was only 11%; the delivery latency for PW-MAC in these scenarios was less than 5% that for WiseMAC and X-MAC. In all experiments, PW-MAC maintained a delivery ratio of 100%. I. I NTRODUCTION The main sources of energy consumption in sensor nodes include listening to the wireless channel and transceiving packets. In recent years, many energy-efficient MAC protocols have been proposed to improve the lifetime of sensor networks by reducing the energy consumed by idle listening and overhearing. The idle listening problem [13] refers to a node listening to the channel even though there are no radio transmissions to receive. The overhearing problem refers to a node receiving a packet it is not intended to receive. An important mechanism for reducing energy consumption in sensor networks is duty cycling. The duty cycling technique saves energy by switching nodes between awake and sleeping states [1]. The average duty cycle measures the ratio of the time a node is awake to the total time. Existing duty cycling energy-efficient MAC protocols can be categorized into two types: synchronous and asynchronous. Synchronous duty-cycling MAC protocols (e.g., S-MAC [13], TRAMA [9], SCP [14], and DW-MAC [10]) reduce sensor energy consumption by synchronizing the sensors’ sleep and wakeup times. However, synchronous duty- cycling MAC protocols require multihop time synchronization. In addition, using fixed sleeping times and listening times [13] is inefficient in handling traffic with variable rates. In contrast, asynchronous duty-cycling MAC protocols do not require such synchronization. They may be either sender- initiated (e.g., B-MAC [8], X-MAC [1], and WiseMAC [3]) or receiver-initiated (e.g., RI-MAC [11]). With the sender- initiated approach, a sender transmits a preamble before a packet transmission to notify the receiver of the upcoming packet. WiseMAC pioneered predictive wakeup in sensor network MAC protocols by fixing the node wakeup interval, thereby enabling a sender to deduce future receiver wakeup times and send a shortened wakeup preamble shortly before the receiver wakes up. However, this fixed node wakeup inter- val may allow repeated node wakeup schedule collisions, par- ticularly in dense networks, thus degrading performance. With the receiver-initiated approach, in contrast, sender preambles are replaced with receiver wakeup beacons; as the beacon is substantially shorter than a preamble, wireless bandwidth usage and collisions are reduced [11]. In this paper, we present a new asynchronous duty cycling energy-efficient MAC protocol called PW-MAC (Predictive- Wakeup MAC). PW-MAC achieves near-optimal energy ef- ficiency both at receivers and at senders. In an optimally energy-efficient MAC protocol, when there is a packet to send, the sender and receiver wake up at the same time, transfer the packet reliably, and both then quickly go to sleep again. PW-MAC approaches this optimality in several ways. Specifically, PW-MAC is a receiver-initiated protocol but in- troduces use of an independently generated pseudo-random se- quence to control each node’s wakeup times, allowing senders to accurately predict the time at which a receiver will wake up. Thus, whereas previous receiver-initiated protocols (e.g., [11]) reduce the duty cycle only at receivers, PW-MAC reduces the duty cycle for receivers and for senders. In addition, to prevent senders from missing the wakeup of receivers due to factors such as hardware and operating system latency and clock drift, PW-MAC introduces a novel on-demand prediction-error cor- rection mechanism. Thus, unlike prior protocols using forms of wakeup prediction (e.g., [3]), PW-MAC is able to maintain accurate prediction; we have found in experiments on real sensor nodes that wakeup prediction without such correction can otherwise lead to significantly reduced performance. Furthermore, the traffic in wireless sensor networks can be bursty, and multiple sensors may attempt to transmit at the This paper was presented as part of the main technical program at IEEE INFOCOM 2011 978-1-4244-9921-2/11/$26.00 ©2011 IEEE 1305