# Energy-Efficient Region Shift Scheme to Support Mobile Sink Group in Wireless Sensor Networks
## 2 Related Work  
In order to support multiple mobile sinks, a series of schemes have been suggested. These studies can be divided into two primary types: individual and group mobility support as shown in Figure 1. In addition, group mobility support schemes could be divided into two detailed phases. Then, the studies can be categorized by approaches used in each phase. Individual mobility support schemes [7; 8; 9] have focused on efficient sink location management approaches. Purposes of mobile sink are various e.g., traffic balancing, data gathering, or low latency communications, and thus the research area has been extensively conducted. SinkTrail [7] is a broadcast-based mobility support scheme. A sink broadcasts its own status information when it moves longer than a predefined distance threshold. Sensor nodes received the information update relative position (the number of hops) to the sink and repeat broadcasting to other nodes. To reduce the broadcasting overhead, if a relative position of updated information is the same as the previous one, the update is discarded at the node because farther nodes are less affected by a single movement. However, it still seems not to be unmanageable in plenty of sinks due to the status maintenance of respective ones. [PERSON] et al. [8] have proposed a prediction-based tracking scheme. The study analyzes the data set of sink positions and predicts a mobility pattern. With enough data sets, it could precisely predict movement plan and significantly reduce control overhead. On the other hand, it would not apply until enough of the data set is collected. In addition, it requires quite a lot of data sets to deal with mobility patterns of multiple sinks even if they have group motion patterns. RingRouting [9] utilizes a rendezvous area of the ring shape constructed in the center of a network. Location information of mobile sinks is stored on the rendezvous structure. When a source generates data and wants to report to the sinks, sink location query and reply message are transmitted to/from the storage. The scheme also could not be effectively applied to the mobile sink group support. Conclusively, they commonly waste signaling since it does not treat redundant movement information massively generated by member sinks having the similar motion pattern with each other. It causes excessive energy consumption and traffic congestion. Meanwhile, group mobility support schemes exploit the movement pattern for efficient communication. The studies are based on the model that a mobile sink group moves to a target point with a certain size of a group area, while member sinks freely move in the group area. Thus, sources require information of the mobile sink group (e.g., group location and size) in order to send data to the group. In terms of that, the mobile sink group support schemes consist of two phases: group region discovery and data dissemination. Primitive schemes [5; 6] have supported the sink groups by the flooding-based approach in both phases. This approach conducts group region discovery by periodic flooding. Even a representative sink query in the expected region is four times larger than the original group region size since the representative sink is not conscious of the whereabouts located itself in the actual group region. Furthermore, every member sink replies with their own location information to the representative sink. Data dissemination should also be performed by flooding in the discovered group region whenever data is generated. The solutions are intuitive and simple, but there is room for improvement. Figure 1: Taxonomy of sink mobility support schemes. [PERSON] et al. [10] proposed band-based geocasting to reduce the data dissemination overhead. This approach stores data on expected movement paths in advance in the form of band. Therefore, it considerably reduces data dissemination overhead since it does not need to broadcast data into the whole group region.