III. NODES DEFINITIONS AND RELATIONS
In this paper, a large WSN is separated due to wind turbines apart distance is over the transmission range of nodes. And this paper intends to use RNs to connect every sub-net work in a wind farm. Each sub-network selects an agency node which connect sub-network i and RNS to form a connected topology. Assuming the agency nodes of each sub-network are selected among the fixed nodes in sub-network. However, the agency nodes may be less capacity and transmission range than RNs, moreover, if multiple agency nodes of the sub-networks would be connected into the entire network. And the additional relays have to be added into a sub-network which is for data transmission between any two sub-networks. That is how to add RNs to cover one or more fixed agency node in each sub- network, and then connect all the sub-networks. Moreover, the number of required RNs should be minimized. This issue has been proved that is a NP-hard problem [16]. Additionally, there is only one agency node for each sub-network, and the sub- networks traffic is transmitted via the identified agency nodes. A RN is placed and close to agency node, in order to share potentially high traffic with agency node. And the RNs placement has to satisfy the network fault tolerance requirement.