# A Matrix-Based Proactive Data Relay Algorithm for Large Distributed Sensor Networks
## 2 Related Work  
Many data relay protocols have been designed for distribute data fusion in wireless sensor network [20; 21; 22]. They can be categorized as two groups according to where the fusion occurs. One strand of protocols are based on fix or predefined sink nodes. They are designed for multiple sensors to forward data to specific sink nodes for data fusion, such as Directed Diffusion [9], in which a sink floods its interests to build reverse paths from all potential sources to the sink. RumorRouting [10], Constrained anisotropic diffusion routing (CADR) [11] and GRAdient Broadcast [12] are some variations of Directed Diffusion. In Rumor routing, sink floods queries while sensors flood events which make Rumor routing performs better than Directed Diffusion when number of events is small. CADR introduces an information utility measure to select which sensors query and dynamically guide data routing. GRAB builds and maintains a cost field to provide sensors the direction to forward sensing data. It is a robust data delivery algorithm addressed nodes failures and link failures. However, since these protocols are based on single-gateway architecture that makes them not fit for large scale mobile sensor network [21]. First of all, sensors near sink nodes have heavy burden to relay data and their energy will be exhausted in short time. Second, sensors are typically not capable of long-haul communication and the latency in communication can not be ignored. Third, energy on finding ways to sink nodes is huge which makes them not suitable for mobile networks where network topology dynamically changes. To allow the system to be able to cover a large area of interest without degrading the service, networking clustering has been pursued in some routing approaches. LEACH [21] and LEACH series [23] are hierarchical routing algorithms for sensor networks. Cluster heads change randomly over time in order to balance the energy dissipation of nodes. Each node transmits directly to the cluster-head. They are completely distributed and requires no global knowledge of network. However, they use single-hop routing where each node transmits directly to the cluster-head and the sink. Therefore, it is not applicable to networks deployed in large regions. Further more, the idea of dynamic clustering brings extra overhead, e.g. head changes, advertisements etc., which may diminish the gain in energy consumption. For the second strand of protocols, there is no predefined sink nodes, sensors are teated equally as probable sink nodes, and data fusion can be done by any sensor unless it aggregates enough relative data. The most straight forward protocol is flooding [8], in which sensors rebroadcast any new data it receives. Apparently, FLOODING consumes too much energy on the transmission of redundant data. To reduce the redundant data of flooding, some reactive and proactive protocols are proposed. Sensor Protocols for Information via Negotiation(SPIN) [13] is a reactive protocol which avoids redundant data transmission by meta-data negotiation with neighbors. sensors only forward data to the neighbors that need it. However, it is infeasible for distributed fusion where the size of meta-data is close to useful data because the frequent query data transmission is not cost-effective and introduces non-negligible time delay. Proactive protocols do not need queries to avoid redundant data transmission but proactively decide if rebroadcasting data received based on the local topology and static information of redundant data, such as Scalable Broadcast Algorithm (SBA) [14], The Lightweight and Efficient Network Wide Broadcast (LENWB) [15], Dominant Pruning [16] and Dynamic Probabilistic Flooding Algorithm [17]. SBA and Dominant Pruning maintain the local network topology by hello messages. In SBA, sensors only rebroadcast the data that at least one of its neighbors do not know. In Dominant Pruning, rebroadcasting nodes proactively choose one or more of its neighbors as rebroadcasting nodes by Greedy Set Cover algorithm.