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In the design of MAC layer protocols for wireless sensor networks, many factors including energy-saving, simple in operation, reducing transmission collisions, transmission efficiency, load balancing, and support for routing, etc. should be considered. This paper proposes a new MAC layer protocol called DMAS with which sensor nodes operate according to a predefined periodic schedule named a superframe. In the superframe, the time for a sensor node to become active/sleep and to transmit/receive data is pre-determined. So, a sensor node has only to determine the starting time of the superframe, and then, it will work according to the schedule. After all the nodes have determined their individual superframe starting time, the topology of the network is also determined in the same time. That is, routing paths for data gathering are also determined. The topology of the network is constructed as trees. In the topology, we limit the number of children of each node to three in order to balance the data gathering load of all nodes. The design and implementation of the proposed MAC protocol are all accomplished in the ns-2 simulation environment. In the environment, wireless packet collision problem and transmission time costs are fully took into account, so the experimental results could be treated as practical outcomes. Our experiments show that the proposed protocol is efficient in energy-saving, topology creation, and data dissemination.