# An Efficient SS-MAC Protocol for IEEE 802.15.4-Based WSNs of Cluster Tree Topology
## 1 Introduction  
The IEEE 802.15.4-2006 standard defines medium access control (MAC) and physical layer (PHY) specifications for low-rate wireless personal area networks (LR-WPANs). It defines two types of devices: full functional devices (FFDs) and reduced functional devices (RFDs). FFD can play any role in a WPAN, such as personal area network (PAN) coordinator, coordinator and device. It can communicate with any type of device, whereas RFD can only communicate with FFD because of hardware limitations. A cluster tree network is a typical network structure for WPANs as well as wireless sensor networks (WSNs). All cluster heads (CHs) establish a tree topology according to a certain routing algorithm and notify the nearby devices to join them by broadcasting messages. To reduce the cost of the network layout, the sink (i.e., the root node) of the sharable slot-based MAC (SS-MAC) proposed in this paper is served by the PAN coordinator, the remaining tree nodes (i.e., CHs) are coordinators, and the cluster member nodes can be RFDs. The MAC protocols of WSNs are mainly divided into the following two categories: (1) contention-based and (2) contention-free (or slot scheduling-based) [1, 2, 3]. In contention-based MAC protocols, nodes and their neighbors typically follow a carrier sense multiple access/collision avoidance (CSMA/CA) technique to compete for medium resources. Nodesuse medium resources on demand and can achieve flexible and efficient data transmission under low traffic load conditions. However, when the load increases, multiple nodes try to access the medium at the same time and network performance is degraded due to the number of collisions and packet retransmissions increase. Researchers have proposed many contention-based MAC protocols, such as SPEECH-MAC [4], DCO-MAC [5] and others. In contention-free-based MAC protocols, time-division multiple access (TDMA) is one of the most commonly used techniques. A manager node or sink divides the system time into slots and allocates slots to nodes in the network according to the demands of the nodes or in a fixed way. A node can access only the allocated slots without any contention with its neighbors. Therefore, less collision and avoiding idle listening are the main advantages of this type of MAC protocol. It also provides predictable packet delay. However, if the allocated slots cannot meet the communication requirements exactly, it is easy to cause either medium resource wasting or an increase in packet delay. Because of the aforementioned shortcomings, it is difficult to meet the requirements for reliable and efficient data transmission in WSNs using only one category of the MAC protocols above. Therefore, the MAC protocols of hybrid CSMA/TDMA have attracted many researchers' attention, such as iQueue-MAC [2], MPCB-HM [6], Asym-MAC [7], SSMA [8] and IEEE 802.15.4 [9; 10; 11; 12; 13; 14; 15; 16]. To reduce the overhead of the nodes, MPCB-HM utilizes CSMA/CA to exchange the data and TDMA to inform the data transmission sequence. iQueue-MAC selects a CSMA/CA technique to transmit data at low load, and changes to TDMA when the load becomes high, which helps to improve the utilization of medium resources. The Asym-MAC was proposed to combat performance degradation due to asymmetric links in low duty-cycled WSNs. The SSMA utilizes a distributed slot scheduling mechanism to solve the problem of obtaining global topology information in large-scale networks. In beacon-enabled mode, the IEEE 802.15.4 standard realizes the combination of CSMA/CA and TDMA mechanisms by setting a contention access period (CAP) and a contention-free period (CFP) in the superframe structure of beacons. The CSMA/CA technique based on IEEE 802.15.4-2006 requires the source node to perform exponential backoff several times and clear channel assessment (CCA) operations twice before data frame transmission. After sending a data frame, the source node needs to wait for an acknowledgement (ACK) frame from the destination.