# An Efficient SS-MAC Protocol for IEEE 802.15.4-Based WSNs of Cluster Tree Topology
## 1 Introduction
The technique effectively reduces the conflicts between nodes accessing the same channel, and is simple and flexible for low-load dynamic WSNs. However, multiple carrier sensing and ACK frames create additional overhead, which, in turn, reduces network throughput. Therefore, many researchers have established discrete-time Markov models to evaluate the performance of the CSMA/CA technique based on IEEE 802.15.4 and improve the technique [10; 11; 12; 13; 14; 15]. The throughput, delay, energy consumption and other performance of the slotted CSMA/CA technique of beacon-enabled IEEE 802.15.4 networks have been evaluated, and compared and analyzed against the performances of slotted and unslotted CSMA/CA techniques in [11]. References [12; 13] apply the MAC layer to perform CCA operation only once instead of twice to improve the transmission efficiency of the data frames. Furthermore, some researchers have sought to evaluate the impact of various factors, such as parameters setting [16], a hidden terminal [14] and interference from IEEE 802.11 [15], on the performance of the IEEE 802.15.4 network. The IEEE 802.15.4-2006 standard defines two address formats: a 64-bit extended address and a 16-bit short address. The former is a globally unique address that is assigned before the device accesses the network. Nodes in the same network can use assigned short addresses to communicate with each other. The BEST-MAC protocol proposed in [3] assigns each node an 8-bit shorter address to reduce the control overhead. Use of a channel-hopping mechanism is an effective method to reduce communication interference between nodes in wireless sensor networks. The IEEE 802.15.4-2006 standard divides the 2.4 GHz band into 16 channels. By setting the channel page field of the command frame format, nodes can implement communication in any of the channels. Mobile wireless sensor networks(MWSNs), which support the movement of nodes or sink in the deployment area, have eventual application in various areas, such as health care monitoring, wildlife monitoring and so on. In order to prolong the lifetime of MWSNs, researchers have developed many effective routing algorithms for various mobility patterns. In reference [17], the network was divided into different regions. The nodes in region 1 are closer to the sink and can communicate directly with the sink, whereas the nodes in region 2 need to forward their sensed data to the sink through routing nodes as described in [17]. Reference [18] proposed an Energy Constrained Mobile Routing (ECMR) algorithm for real-time applications and compared various performance with other existing routing protocols. In addition, many researchers enhance the performance of MWSNs by optimizing the routes of mobile sinks, such as [19; 20; 21]. They have proposed routing protocols based on expected area, target location algorithm and bipartite graph theory, respectively. In [22], the authors proposed a routing protocol called dynamic multi-hop low energy adaptive clustering hierarchy (DMH-LEACH), which combines multi-hop transmissions, dynamic clustering and node mobility. There are three types of nodes deployed in the network defined by [22], sink, CHs and other nodes, and all the nodes are mobile after deployment. The data transmission between the nodes and their CHs adopts a single-hop mode, while the data transmission between CHs and sink adopts a multi-hop (i.e., level-by-level) mode. In our proposed SS-MAC, the DMH-LEACH protocol can be adopted to construct tree routing between CHs. Based on the above techniques, a sharable slot-based MAC (SS-MAC) is proposed in this study by combining the superframe structure of the IEEE 802.15.4-2006 standard with the sharable slot idea in SSMA [8], which makes the protocol more suitable for cluster tree structured topologies networks. In addition, the SS-MAC adopts a design of short node address (8-bit) that is similar to BEST-MAC [3] to reduce packet size and control overhead.