On Multihop Broadcast over Adaptively Duty-Cycled Wireless Sensor Networks Shouwen Lai 1 and Binoy Ravindran 1 Virginia Tech, ECE Department, Blacksburg, VA 24060, USA, {swlai,binoy}@vt.edu Abstract. We consider the problem of multihop broadcast over adap- tively duty-cycled wireless sensor networks (WSNs) where neighborhood nodes are not simultaneously awake. We present Hybrid-cast, an asyn- chronous and multihop broadcasting protocol, which can be applied to low duty-cycling or quorum-based duty-cycling schedule where nodes send out a beacon message at the beginning of wakeup slots. Hybrid- cast achieves better tradeoff between broadcast latency and broadcast count compared to previous broadcast solutions. It adopts opportunis- tic data delivery in order to reduce the broadcast latency. Meanwhile, it reduces redundant transmission via delivery deferring and online for- warder selection. We establish the upper bound of broadcast count and the broadcast latency for a given duty-cycling schedule. We evaluate Hybrid-cast through extensive simulations. The results validate the ef- fectiveness and efficiency of our design. 1 Introduction Multihop broadcast [17] is an important network service in WSNs, especially for applications such as code update, remote network configuration, route discovery, etc. Although the problem of broadcast has been well studied in always-on net- works [12,22] such as wireless ad hoc networks where neighbor connectivity is not a problem, broadcast is more difficult in duty-cycled WSNs where each node stays awake only for a fraction of time slots and neighborhood nodes are not simultaneously awake for receiving data. The problem becomes more difficult in asynchronous [24] and heterogenous duty-cycling [9] scenarios. To support broadcast, synchronization of wakeup schedules is one promising approach adopted by many duty-cycling MAC protocols, such as S-MAC [23] and T-MAC [4]. Such protocols simplify broadcast communication by letting neighborhood nodes stay awake simultaneously. However, this approach results in high overhead for periodic clock synchronization when compared to the low frequency of broadcast service in WSNs. Since energy is critical to WSNs, energy- efficient asynchronous MAC protocols have become increasingly attractive for data communication, as proposed in B-MAC [14], RI-MAC [18], Disco [5], and quorum-based wakeup scheduling [24, 10]. However, previous asynchronous MAC protocols for duty-cycled WSNs mostly focus on unicast communication, and do not work well for broadcasting. One