## 3 Sink-oriented Dynamic Location Service
### Motivation  
When a sink sets on static location, geographic routing can be used successfully, i.e. sending the packets to the closest node to the destination. As for the static sink, it only needs to broadcast once to the whole network at the initial stage of the network operation, which would not result in significant energy dissipation. If a sink moves quickly to be out of last hop node's transmission range, geographic routing can't be executed correctly without a mobile sink's location updates because the sink can't receive last hop forwarding without any additional reactions. For solving this problem, the mobile sink can periodically flood its location to all sensors but it is not efficient and scalable for large sensor networks and each time the sink changes its location in the network. Due to high packet overhead of flooding, when these nodes use up their energy, no more data can be transmitted back to the sink. Thus, sinks should update limited number of sensors own current location instead of flooding to all sensors. Therefore, the mobile sink only needs to broadcast to a subset of nodes in the network each time when it arrives at a new site, thus saving much energy.  
In a general way, a source node that has detected an event of interest tries to send its data to the sink. Therefore, the mobile sink broadcasts its location information over the subset of nodes in the network [see Figure 2(a)]. However, when the sink moves out of a destination node, it has to rebroadcast its location information over the network to a new destination node. We argue that this repeated network-wide broadcasting brings extra high energy loss which could impair the lifetime gains from sink mobility, especially in large-scale WSNs.  
Figure 2: Selection of location service servers.  
To address this problem, we provide source node's location in the WSNs [see Figure 2(b)]. A significant difference between our proposed scheme and the traditional mobile sink supporting scheme is that the source node provides its location service server. Therefore, it is necessary to propose an energy efficient location service protocol to balance sensor node energy consumption.  
In this paper, we propose a data dissemination model called EDA (Eight-Direction Anchor) for geographic. Sinks should be able to acquire current source node's location with bounded time and overhead. EDAs are location server sensors that track source node's position. Figure 2 illustrates our motivation for proposing the location service servers. As shown in Figure 2(a), selection of location service servers will be established using six lines of grid nodes, three lines of grid nodes traversing the network vertically and three lines of grid nodes traversing the network horizontally. But from Figure 2(a), EDA can reduce the traversing path from 6 to (2+2\(\sqrt[]{\mathbf{2}}\)) compared with ALS. EDA prevents intensive energy consumption at the border sensor nodes; consequently, the energy consumed is well balanced for all the sensor nodes.  
After the source node gets the sink's location, the source node should be able to transfer the sensed data to shortest path. Figure 3 illustrates our motivation for proposing the shortest path relay.  
When the sink has mobility, the sink agent has to relay the data from the source node to the sink. Then, this occasions a detour case (see Figure 3(a)). Therefore, it is necessary to propose an energy efficient shortest path relay. In our proposed algorithm, once the sink is aware of the source node's location, the sink can directly request the data forwarding from the source node [see Figure 3(b)]. It can efficiently forward data from a source node to a sink with minimal delay path. Therefore the sink can quickly get source node's data.