Section 8 organizes existing schemes on the basis of their main goal(s) and provides a comparative study in terms of the various features. Finally, Section 9 concludes our discussion with identification of issues that need to be addressed in pursuit of data delivery to a mobile sink.2.?Network Architecture of Mobile Sink Based Wireless Sensor NetworkThe mWSN network architecture differs from that of a static WSN in the sense that in the former case, the sink keeps on moving around/inside the sensor field for efficient data collection. A reference mWSN network architecture is shown in Figure 2. The main components of a mWSN are given as follows:Regular Nodes��These are the ordinary sensor nodes that are deployed in the sensor field for sensing some phenomenon of interest.

Upon sensing the events, these nodes disseminate their data in a cooperative manner towards a mobile sink. Depending on their placement in the sensor field, nodes might work as relays thereby forwarding others data towards a mobile sink.Mobile Sink(s)��Depending on the application scenario, there might be single/multiple mobile sink(s) that move inside/around the sensor field for data collection. Such devices are considered unconstrained devices in terms of their resources. Mobile sink can be a sensor node attached to a human, car, animal or a robot.(Optional) Sink Assistants��In some applications, special nodes are deployed at strategic positions that provide assistance to the sink in data collection. These devices are also considered as energy rich.

In static deployment, such nodes become intermediate/local data collectors from the sensor nodes and later on deliver collected data to a mobile sink upon its arrival. In the mobility case, they are meant to ensure coverage of almost the entire sensor field for real-time communication services in certain applications.Figure 2.Network architecture of a mobile wireless sensor network.3.?Sink Mobility AdvantagesIn almost all WSN applications, the sink is considered as an unconstrained entity in terms of resources (energy reserve, processing power, communication capability, etc.). Likewise, in several applications of sensor networks, sink mobility can be realized by attaching a sink device to a mobile entity such as human, animal, robot, or vehicle which can move around/inside the sensor field for data collection. Thus considerable energy savings can be obtained by deploying a mobile sink in sensor field. Kinalis et Drug_discovery al. identified several potential advantages of sink mobility [12] in the sensor field that are outlined as follows:Sensor Lifetime Enhancement��By exploiting sink mobility, not only is the energy-hole problem alleviated, but it also improves the lifetime of nodes thereby reducing the multi-hop communication.