## 7 Classification of Mobile Sink based Data Collection Schemes
### Controlled Sink Mobility-Based Schemes
In order to reduce the data collection time and thus the data delivery latency, initially the Traveling Salesman Problem (TSP) approach is adopted such that in a single tour, the MDP covers all the NAs with least cost in terms of hop counts. Accordingly, those nodes which provide the shortest path among adjacent NAs become INs. Furthermore, all the single-hop neighbors of NAs are considered as Designated Gateways (DGs). The DGs are responsible to collect and buffer sensory data of their k-hop neighbors. For data collection, MDH sends a query message to the nearby NA asking about DGs, next NA and IA along its trajectory. NA accordingly provides the requested information and MDH collects buffered data from DGs on the way. The proposed solution prolongs the network lifetime with reasonable data delivery latency, but it is only applicable to small scale networks. The MobiRoute routing protocol proposed by [PERSON] _et al._ in [PERSON] _et al._ (2013) focuses on proactive routing towards a mobile sink where sensor nodes send their data in a multi-hop fashion on a periodic basis. It assumes the sink mobility pattern as a discrete mobility pattern in which the sink moves along several anchor points and makes temporary pauses at those points. The sink's pause time interval is much greater than the movement time between consecutive anchor points. According to authors, movement along the anchor points and the relatively large pause time, significantly reduce the overhead on part of the routing protocol thereby avoiding the need for frequent routes adjustments triggered by sink mobility. The sink while in transit establishes the link with the nearby nodes via beacon-messages. To avoid propagation of sudden and frequent topological changes of the mobile sink, MobiRoute propagates this up-to-date sink position information further in the network only when the sink arrives at an anchor point. This is because while in transit, the sink may come across multiple nodes and the links with them may no longer be valid when the sink arrives at next anchor point. To achieve improved network lifetime, MobiRoute also employs an adaptive mobility control mechanism thereby making energy-profiles of various network's segments. Based on the energy-profile, the sink avoids stopping at an anchor point if its energy-profile is extremely low. However, network-wide propagation of sink's topological updates triggered by a sink's arrival at a new anchor point results in extensive energy consumption. Alongside, another limitation of this scheme is the strict implication that the MAC protocol is overhearing energy-free, which may not a realistic assumption as performance-wise better MAC schemes such as B-MAC suffer greatly from overhearing problems [PERSON] _et al._ (2014). [PERSON] _et al._ exploit sink mobility for prolonged network lifetime in [PERSON] _et al._ (2014) where the mobile sink moves around a sensor field and makes temporary pauses at nodes placed at grid points. In the proposed scheme, sensor nodes are assumed to be placed deterministically at equal distances along the grid points, where the grid is considered as bi-directional having same-sized square cells. Furthermore, it assumes nodes have unlimited buffer size and communicate periodically with the sink in a multi-hop manner where one hop is considered as the cell size. The sink moves along the grid points to prolong the network lifetime. To do so, the sink makes a temporary pause at a particular node in accordance with the residual energy of that node. In order to determine the candidate nodes to be visited and the pause-period at those nodes, a linear optimization model is adopted with the aim of maximizing the network lifetime till first node in the network dies due to energy depletion. Compared to the static sink scenario, the proposed scheme improves network lifetime thereby providing balanced energy consumption among sensor nodes. However, the deterministic nodes deployment at equal distances and unlimited buffer-size assumption do not hold true in most sensor network deployments.