## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Unconstrained Sink Mobility-Based Schemes
This overhearing process goes on step-by-step in the subsequent data forwarding till the source nodes comes to know the latest location of the mobile sink. Elastic routing proposes that upon overhearing, each node should compare its distance to the new sink location with the distance from the originator of transmission. Thus, if the originator distance to the sink is shorter than its own distance to sink, it caches the sink latest location information and otherwise it discards it. The main limitation of this scheme is the long convergence time which consequently results into long message delivery latency. Furthermore, it embeds the latest location information of sink in each data packet and exploits the overhearing mechanism for propagating the sink latest location information, which does not yield significant energy savings due to the idle listening. In the case of uncontrolled sink mobility, [PERSON] _et al._ proposed in [41] a Density-based proactivE data dissEmination Protocol (DEEP) that aims to obtain a representative view of the network sensed data by visiting only a subset of nodes. First, it addresses how to obtain an optimal number of nodes to be visited by the mobile sink while maintaining a good trade-off between the storage requirements on such nodes and the ratio between the visited nodes and the representativeness of the gathered data. The proposed scheme is based on proactive data dissemination, however, unlike other virtual structure-based data dissemination schemes such as those described in [12, 23, 27, 42], the sensed data is not deposited in a subset of nodes to be retrieved later by a mobile sink. It is argued that depositing sensed data at  
Figure 9: Mobile sink tracing. nodes that form the virtual structure results into an imbalanced use of energy consumption and storage space as nodes forming the virtual structure and its one hop neighbors would quickly deplete their energy. Hence, to make a balanced use of energy consumption and storage space on nodes, it assumes that every sensor node is equipped with a buffer space that stores a partial view of the sensed data. The partial view is composed of not only the data sensed by the sensor node itself, but also a compressed version of its neighbors' observed data as well to be uploaded to a mobile sink. The proposed scheme avoids the network overhead associated with keeping track of sink mobility and keeps a partial local view of sensor nodes observed data to be retrieved by a mobile sink upon its visit to that segment of the network. However, it is only suitable for applications which are delay tolerant and where the rate of event generation is low. The virtual grid-based Two-Tier Data Dissemination (TTDD) protocol for large-scale WSNs in [42] aims to minimize the flooding of sink topological updates. This protocol periodically builds source/event-based virtual global grids (spanning the entire network area) where a data source node proactively constructs a virtual grid structure spanning the entire sensing field and arranges the forwarding information at all disseminating nodes (nearest nodes of grid points). A sink upon arrival at any cell of the grid structure, injects a query destined to the data source. The query utilizes the grid structure and traverses two tiers to reach to the data source: a lower-tier which is within the local cell of the sink's current location, and a higher-tier composed of all the disseminating points on the grid structure. Furthermore, each disseminating node is aware of its upstream and downstream team members. In traversal, the query first determines a nearest disseminating node by the flooding mechanism which is bounded by the size of the grid cell. From that point onwards, the query is relayed towards the source node along the upstream disseminating points until it arrives at the source node. Accordingly, the source sends the response message along the reverse path of the query message. To cope with mobility, the sink appoints a Primary Agent (PA) node and an Intermediate Agent (IA) node.