Contents I. INTRODUCTION
Wireless sensor networks (WSN) have the characteristics of dynamic topology and strictly constrained node resources, especially the energy resource. Because of the dynamic topology, it is required that the data acquisition and data transmission in WSN must have sufficient redundancy to ensure the reliability. Redundant acquisition often depends on the densely distribution of nodes, and redundant transmission often rely on multi-path to transport the same data packet at the same time. However, strictly constrained energy resource makes it necessary that the total number of data packets forwarded in the data transmission process should be as small as possible in order to save energy and prolong the network lifetime. Data sinking is a main form in wireless sensor networks' data transporting in typical MHR wireless sensor networks, data aggregation is implemented along the MHGF composed of different gradient levels, and the level space is decided by nodes' effective communication radius (Re). The nodes within the same gradient level have the same value of minimum hop count (MHC) to the sink node, which referred to as the node's hop count value (HC). The dividing line between the neighboring levels forms the gradient level boundary. According to the direction of data gathering, the nodes in MHGF can be classified into on-bound node, outbound node and in-bound node. For some a MHGF level, the boundary near to the sink is called inner boundary, and the boundary far from the sink is called outer boundary. An on-bound node is a node located just on the outer boundary; an in-bound node is a node located near to the inner boundary; and an out-bound node is a node located near to the outer boundary. The relation between MHGF boundary and node in some a level of MHGF, for example, the 3rd level, is displayed in Figure 1. MHGF level, boundary and node
