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Broadcast/multicast is a key service for mobile ad hoc networks. A great number of applications rely on a reliable and efficient MAC layer broadcast. The IEEE 802.11 broadcast protocol, which is based on carrier sense multiple access with collision avoidance (CSMA/CA), does not offer any MAC layer recovery on broadcast frames. Consequently, the increasing probability of lost frames may deteriorate the quality of broadcast/multicast services offered at upper layers. In this paper, we first formulate the broadcast problem as an optimization problem and show that it is NP-hard, even if the upper layer service is periodical beacons. An approximation algorithm with a guaranteed approximation ratio is also suggested. Then, a reliable and efficient MAC layer broadcast protocol, named broadcast protocol with busy tone (BPBT), is proposed. BPBT applies a busy tone to solve the hidden terminal problem. Finally, BPBT is compared with previous protocols for performance evaluation by simulation.