## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Constrained Sink Mobility-Based Schemes
Each beacon message is comprised of a sink's ID and time-stamp information. Nodes upon receiving beacon messages update their sink-tables and do not further propagate beacon messages. Thus, a trail is being formed from all those nodes which have been recently visited by a mobile sink. Consequently, this trail is used for data packet forwarding by sensor nodes. TRAIL results in less communication overhead caused by sink mobility and is therefore very appealing for delay-tolerant networks. However for large scale networks, TRAIL would cause considerable data delivery latency for the farthest nodes as major part of the data delivery path adopted would be comprised of random walks. To ensure uninterrupted data delivery to a mobile sink, a mobility prediction based scheme in proposed in [30]. The proposed scheme delivers sensory data to a mobile sink via relay nodes that are predicted based on the mobility graph of the mobile sink. The mobility graph is pre-computed based on a sequence of those nodes that receive relatively strong signals from the mobile sink. Each time, the mobile sink moves from one relay node to next, it floods the network with the information of the next predicted relay node and arrival time. This helps sensor nodes dynamically adjust their routes to the predicted next relay node in accordance with anticipated arrival time of the mobile sink. The reported experimental results reveal high packet delivery ratios at low speed of the mobile sink which gradually degrade as the speed increases. Furthermore, the proposed scheme considers sink mobility in an indoor office environment where the mobility is relatively constrained and thus easier to predict compared to an outdoor scenario. To improve network lifetime and data delivery performance, a solution is proposed in [31] that is based on a three-tier network architecture. Tier-1 is formed by all the sensor nodes, while mobile sinks being working as data collectors form tier-2 and a static base-station forms tier-3. For data collection, predictable sink mobility is adopted and multiple mobile sinks approach the sensor nodes in different network segments thereby forming clusters around themselves. Consequently, mobile sinks query single-hop sensor nodes, collect data from them and report to the base-station. However, to avoid very long packet delivery latency and long waiting times, nodes forward sensory data towards an anticipated sink position. In case of large packet sizes, the proposed scheme splits packets into several smaller packets and delivers them to various mobile sinks simultaneously, assuming it will be reassembled at the base-station. Moreover, for guaranteed message delivery, the proposed scheme reduces sinks' velocity, thereby ensuring long contact time between sensor nodes and mobile sinks. The proposed scheme achieves improved network lifetime and high packet delivery ratios by employing multiple mobile sinks together with their slow and fixed speed. However, the reduced sink velocity results in increased data delivery latency. The scheme proposed by [PERSON] and [PERSON] in [32], aims at balancing energy consumption and data delivery latency to a mobile sink. The proposed scheme minimizes the topological overhead caused by sink mobility and makes dynamic route adjustments. It adapts the network topology by exploiting the full/partial knowledge of the navigational map and the traveling schedule of the mobile sink. Accordingly, it assumes that the mobile sink moves in steps along a straight line to reach intermediate positions, where the size of the step is taken in accordance with the sink's speed. Initially, according to the primary position of the mobile sink, routes are set in an energy-efficient way to facilitate continuous data delivery till the sink moves to the next intermediate position. When it moves from one point to another, the network topology is reassessed.