## 1 Introduction
However, the change in topology causes the reconstruction of a CDS or the update of the random number table used for slot number assignment. In Reference [10], the authors use the opportunistic routing method, in which if a certain node happens to receive a broadcast message from one of its two-hop upstream nodes in the broadcast forwarding path and has higher connectivity than other nodes, it can forward the message earlier. A node can know whether or not its upstream node has received the message by using an acknowledgement (ACK) message. Meanwhile, in Reference [11], they propose a dynamic delegation-based efficient broadcast protocol (DDEB) in which if a node has any neighbor(s) who is uncovered (or failed to receive its broadcast message), it selects one of its covered neighbors based on link quality and asks it to cover the uncovered one. A broadcast node goes to sleep if all of its neighbors can be covered either directly or by one of its delegates. This approach tries to increase reliability by considering link quality. These approaches focus on reducing the number of rebroadcast nodes. Some tree-based broadcasting methods have been proposed for energy saving and collision reduction. In Reference [12], they proposed a distributed heuristic algorithm to construct an energy-optimized tree under the delay bound. In Reference [13], they presented the delay-aware energy-optimized flooding (DEF) algorithm, in which each node reports the maximum delivery delay of its downstream nodes to the sink. The sink adjusts a flooding tree globally for energy efficiency under the delay bound. Another approach in this category is the reliable slotted broadcast protocol (RSBP) [14] that removes internal interference completely by allocating a distinct broadcast slot to each node except for leaf nodes. This approach optimizes energy consumption such that every node remains active only for its parent's broadcast slot to receive a message and its own broadcast slot to rebroadcast it. However, it degrades reliability due to the complete removal of redundancy and has to reschedule the slots if the topology changes. Thus, these tree-based approaches may not be appropriate for dynamic WSNs. In this paper, we improve reliability in message delivery and energy consumption by devising a slotted broadcasting method and applying it to the original SSMA broadcast (SSMAb) protocol [16] that was previously presented at a conference. We use the same protocol name, SSMAb. In SSMAb, one distinct broadcast sharable slot (BSS) is allocated to each tree level, and only the nodes at the same level compete to rebroadcast a message within the BSS, resulting in reduced competition. Every node can generate the schedule of BSSs based on tree levels, independently of the change in topology. Then, a node wakes up at the beginning of its parent's BSS and goes to sleep immediately after receiving a message, and also wakes up at the beginning of its own BSS and goes to sleep immediately after rebroadcasting a message. In this way, a node can have multiple chances of receiving an identical message from different nodes at its parent level. However, collisions can still occur if two or more nodes broadcast a message simultaneously. To alleviate the possibility of collision, the protocol employs the concept of the slotted Aloha protocol [17]. A BSS is further divided into a number of broadcast slots (BSs), each BS being of a size sufficient to broadcast one message. Then, we present a BS scheduling algorithm with which a node can assign BSs to its children _on-the-fly_ before rebroadcasting. The SSMAb protocol was evaluated using the commercial simulator, QualNet 5.02, and was also experimented on in a testbed that consisted of one sink and 25 sensor nodes. It achieved a high reliability or high-packet delivery ratio comparable to the flooding scheme, and low-energy consumption comparable to the energy-optimized RSBP protocol against different degrees of interference and changes in topology.