## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Constrained Sink Mobility-Based Schemes
For data collection, the mobile sink periodically collects data from the sensor field by forming a converge-cast tree first. To construct such a tree, the sink selects the CH node at the closest rendezvous point as the root and transmits a query to that CH node. The CH node accordingly propagates the query to sensor network via the backbone network and forms a tree structure using all CH nodes which provides shortest path from all CH nodes to mobile sink. Consequently, the CH nodes knowing location of neighbor CH nodes, and mobile sink together with  
Figure 5: An example of virtual backbone structure in VCCSR [25]. the associated rendezvous point location route the sensed data using minimal hops towards the mobile sink. Upon sink mobility, VCCSR avoids reconstruction of the whole routing tree as the sink updates only the part of the tree (a few CH nodes) that need to reconstruct the routing tree as shown in Figure 6. This partial tree reconstruction yields great energy savings. In VCCSR, the controlled reconstruction of the routing tree results in decreased energy consumption and thus helps in prolonging network lifetime. Furthermore, it also facilitates continuous data delivery to the mobile sink. However, VCCSR is not suitable for event driven applications which cannot tolerate significant delay in message delivery. In addition, it considers sink mobility along a circular path outside the sensor field only where the sink maintains communication with the border nodes. We argue that imposing the sink's communication with border nodes only, results in imbalanced energy consumption of sensor nodes as border nodes would consume more energy compared to innermost nodes. This would decrease the network lifetime caused by early depletion of border nodes' energy reserve. [PERSON] _et al._ proposed a greedy forward algorithm called Wireless HIgh SPEed Routing (_Whisper_) for sending data towards a high speed sink that aims to ensure guaranteed data delivery. It is based on the idea that the sink, due to its high speed, will not stay at the location at which it injected the query and thus it sends the response towards a more up to date location of the high speed sink. It assumes that all the nodes know their locations, neighbor's locations, and the sink's trajectory (included in query packet) and displacement. Furthermore, it assumes that the sink will not vary its speed and will follow a trajectory along a straight line. The field sensor nodes will generate the response and will forward it towards the estimated meeting point with the sink. The meeting point is estimated on the basis of considering the various delays in message transmission together with node's own location, the neighbors' locations, and the information supplied in query packet. This scheme limits its applicability by imposing two constraints, _i.e._, the constant speed of the mobile sink and its future trajectory, which may not be realistic assumptions as in most of the cases a mobile sink keeps on changing its speed and directions. Figure 6: Partial tree readjustment in VCCSR upon sink mobility [25]. [PERSON] _et al._ proposed an energy-aware and delay-aware data forwarding scheme [27] specifically tailored to incorporate WSNs in an intelligent transportation system. It follows a query driven data delivery strategy. Using this scheme, to forward a response towards a mobile sink, the traditional geographic routing is optimized such that the next hop node is selected on the basis of its relative distance to the sink and its residual energy level. In the considered scenario, as shown in Figure 7, a car injects a query (such as information about parking slots) into the sensor field via roadside nodes (vice-sinks) and consequently collects the response from one of the vice-sinks along its trajectory. The query contains information such as ID of the mobile sink, its coordinates, speed, direction, time-stamp, coordinates of target area along with radius of target area.