# An Efficient SS-MAC Protocol for IEEE 802.15.4-Based WSNs of Cluster Tree Topology
## 2 Related Work  
Based on hybrid CSMA/TDMA and low duty-cycle techniques, the iQueue-MAC [2] proposes a traffic feedback mechanism to implement traffic adaptation: the protocol uses a CSMA mechanism to transmit data at light load, and changes to a TDMA mechanism to allocate transmission slots when traffic increases. BEST-MAC [3] and MPCB-HM [6] combine CSMA and TDMA techniques by detailing the various phases of the network process. They use CSMA to build the cluster topology and reserve slots required to data transmission, while TDMA is adopted for data transmission. However, the protocols above mainly target the problems of cluster topology construction and data transmission from member nodes to CH in a cluster, while effectively forwarding data from member nodes to sink through CHs remains a problem. It is worth mentioning that BEST-MAC [3] improves the address formats defined in the IEEE 802.15.4-2006 standard, adopting an 8-bit shorter address to reduce control overhead. It also proposes a knapsack algorithm for CHs to determine the number of slots and order allocated to each member node.  
SSMA (Slotted Sense Multiple Access) reported in [8] proposes a distributed scheduling algorithm for WSNs of tree topology. It allocates a sharable slot to each level of the tree topology, and all nodes in the same level perform data transmission by using CSMA/CA in their sharable slot. In SSMA, an exponential wait time function is defined to ensure that the nodes at lower level have a larger sharable slot, and the allocated sharable slots are gradually reduced from the sink (at level one, that is, the lowest level) to leaf nodes. Each tree node can calculate the size and start time of the sharable slot as long as it knows its level. However, the performance of SSMA is heavily dependent on the function. Unreasonable parameters selection can easily lead to network performance degradation. The LELLMAC described in [9] makes use of three algorithms to control idle listening, collisions, and address the hidden terminal issue. The protocol uses the logical link choice algorithm to ensure that two source nodes do not send packets to the same receiver at the same time. Before transmitting data packets, the nodes need to send \"Request to send\" (RTS) packets to adjacent layer nodes, and to reply \"Clear to send\" (CTS) packets after transmission. The protocol requires that the maximum amount of time allowed for data transmission in each wake-up period is 60 ms.  
Like the improvement of MAC layer techniques, the design of routing algorithms or topology structures is also a major research area of MWSNs. Researchers generally propose their routing protocols based on network topology (i.e., data forwarding path) design [17; 18] and sink mobile path design [19; 20; 21; 22]. The DMH-LEACH protocol proposed in [22] is a cluster-based routing protocol, and all nodes are movable. Ordinary nodes send their packets to nearby CHs in a single-hop manner, and then the CHs forward the packets to the sink in a multi-hop manner. The protocol details the tree routing algorithm between the CHs and the sink.  
In order to build a cluster tree topology network of our proposed SS-MAC, we employ the routing algorithm of DMH-LEACH [22] to build the tree topology between CHs, and design a strategy of cluster topology construction between ordinary nodes and CHs (tree nodes). However, the MAC protocols proposed in the above references are mainly based on WSNs of cluster or tree topology, rather than cluster tree topology. Thus, we upgrade and improve the above technologies; especially, we introduce the sharable slot algorithm of [8], then improve and optimize the algorithm to realize data packets forwarding level-by-level between CHs. In addition, we also adopt the short address (8-bit) design from reference [3] to reduce packet size and control overhead, and improve its knapsack algorithm for CHs to allocate slots to member nodes.