## 7 Classification of Mobile Sink based Data Collection Schemes
### Controlled Sink Mobility-Based Schemes
Voronoi clusters and are dynamically selected based on signal strength measurements by the mobile sinks to cope with sink mobility. Once a new anchor node is selected, the mobile sink floods its interest to nodes located within its Voronoi scope via anchor node. Each interest packet contains the sink's ID, anchor's ID, and time-stamp information, together with hop-count distance to the anchor node. Nodes within the Voronoi scope upon receiving an interest packet compare these entries against their local route table and update their record if necessary, otherwise discard the interest packet. By employing multiple mobile sinks, AVRP localizes communication overhead caused by sink mobility within the Voronoi scope of the sink. However, each time the sink moves and selects a new anchor node, more interest packets need to be flooded within the Voronoi scope of mobile sinks. Depending on the sink's speed, frequent movements trigger more flooding of interest packets which potentially undermines network lifetime. [PERSON] and [PERSON] in [51] proposed a scheme to improve data delivery latency by using a controllable data mule (mobile element) so that data can be collected from sensor nodes in the shortest amount of time. The use of data mules is considered as a substitute for multi-hop forwarding and is being exploited in several schemes such as [52, 53, 54, 5, 17, 55, 5]. In the solution proposed by [PERSON] and [PERSON], it first tries to find such a path for scheduling the motion of the data mule so that data from all sensor nodes is harvested in less travel time. To do so, first, it finds the Travelling Salesman Problem (TSP) tour \(T\). Then, by using an approximation algorithm, it applies shortcutting to \(T\) in order to obtain shortest label-covering tour. In a single trip, the data mule harvests data from all sensor nodes using single-hop communication. In the proposed scheme, the controlled motion of the sink reduces the overall data delivery latency by making use of direct single-hop communication; however, the single hop communication with the sink might not be realizable in many application environments such as forests, battlefields. _etc_. Furthermore, for large scale networks and dense random deployment, traversing all the sensor nodes in a single tour without redundantly visiting some nodes becomes infeasible. In [56], [PERSON] and [PERSON] proposed a clustering mechanism that exploits multiple mobile relays to prolong the network lifetime. Basically, the proposed mechanism extends the clustering approach introduced in [57], to address the energy-hole problem in the vicinity of the fixed cluster-heads/relay nodes. The proposed scheme initially partitions the network into clusters where in each cluster the relay-nodes perform data collection and coordination. The relay nodes are considered rich in resources and perform direct communication with a static sink. Furthermore, the relay nodes move to new locations within the formed clusters such that the overall energy consumption is minimized. To relocate the relay node to a new position, the proposed scheme considers all those locations that are not farther than a maximum distance from all member nodes within that cluster. This constraint is imposed to preserve the already formed cluster topology. Once such location is found, the relay node is moved to that location for better energy savings. Although, the proposed scheme improves the network lifetime by virtue of relocating relay-nodes; however, the extra cost incurred due to the use of multiple mobile relay nodes limits its applicability. Network-Assisted Data Collection (NADC) is another controlled sink mobility scheme [58] that aims to prolong network lifetime while minimizing data delivery latency by adopting a Mobile Data Harvester (MDH). During data collection, the MDH follows an overlay graph comprising of two kinds of nodes namely Navigation Agents (NA) and Intermediate Navigators (IN). Both the NAs and INform the sink's trajectory and assist the MDH in navigation through the sensor field.