# Energy-Efficient Region Shift Scheme to Support Mobile Sink Group in Wireless Sensor Networks
## 3 Efficient Mobility Support for Sink Group Utilizing Region Shift
### Group Region Discovery by Region Shift Mechanism  
As mentioned above, it is practically impossible to trace exact ideal group movements (DGR) by sensing behavior of member sinks. Previous studies provide snapshots of the group region for a certain period to approximate the movement. It collects positions of all member sinks by the flooding and calculate an average position of them. Then, the region is presented as a circle with the position and a radius covering all members. In order to overcome the shortages in terms of the communication overhead, RSMGP conducts the region discovery by our region shift mechanism. Mobile sink groups have the collective movement property. It represents motion information of one member sink implying entire movement of the group. Using this property, the proposed scheme calculates a new region location based on updates of only a few member sinks that deviate from the current region rather than all member sinks performing updates. Figure 2: Circle and secant line. Though estimation of accurate movement could not be achieved, RSMGP could provide secure snapshots by utilizing the movement information of partial members and gradual correction. If an initial group region is given, some member sinks may surpass boundary of the region after a lapse of a certain amount of time. Then, the region shift process is triggered and it moves the center point location as much distance as the sinks move. In addition, region size is in accordance with a radius of DGR. Thus, we could eliminate region-wide flooding to obtain group region information by this local triggering. In Figure 3, for example, the group moves from the location of a shaded circle to that of an empty circle. A member sink deviates to the previous group region. The sink reports its own location to pivot \(P_{1}\) in order to notify that the group is moving. The tracing process is triggered in this way. In addition, because one or more sinks can move out of the region, it needs to calculate a new location of the sink group by combining movement information of all of them, to be discussed specifically in Section 3.4. Someone has to control these kinds of operations for region information updates. Thus, we designate the nearest node to a center point as a _pivot_ node. Member sinks can recognize whether it is within the region itself because they have region information, a center point \(C\) and radius \(R\). Once a sink which is out of the region reports moving information to the _pivot_, this _pivot_ calculates a new group location using this information. The basic algorithm of the region shift is illustrated in Algorithm 1. As shown in Figure 3, following the arrow 1, the movement of the sink group makes a sink to be out of the region firstly. 2 The sink recognizes that movement out of the region and performs a location update to current pivot \(P_{1}\). 3\(P_{1}\) calculates a new location \(C_{2}\) of the group. \(P_{1}\) announces the region information in the new region. 4 At this time, since a source is aware that the group exists in the previous location, it delivers the next data to the location \(C_{1}\). When nodes in the previous region receive the data, it resets destination location to the new location and forwards it there. Figure 4 shows the relation between ideal DGR and estimated region by the region shift algorithm. Although RSMGP could not find an exact location of DGR, it provides well-deserved approximation covering all member sinks by the simple method. Figure 3: The region shift procedure.