## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Constrained Sink Mobility-Based Schemes
All the nodes including the vice-sinks, are considered as location-aware and thus forward the query towards the area of interest using the geographic routing. A central node in the area of interest collects the requested information, generates a response message and based on the mobility information provided in the query packet, estimates the target location of the mobile sink. Accordingly, it forwards the response towards the anticipated location of the sink in a multi-hop manner. Finally, the response message is delivered to the nearest vice-sink. If the mobile sink has not yet arrived at the vice-sink, it waits for the mobile sink and delivers the response message upon its arrival. However, if the mobile sink has already passed the vice-sink, it routes the response message towards the next vice-sink via its one hop sensor node. This process goes on till the response message is delivered to mobile sink or is timed out and thus dropped. The proposed scheme in [27] achieves balanced use of nodes' energy consumption and latency-aware message delivery to a mobile sink. However, it does not fit well in situations where the sink trajectory is not known _a priori_ or cannot be predicted. In the scheme proposed in [28], data delivery to mobile sinks is considered for delay-tolerant applications. It aims to minimize the number of transmissions required to deliver data to mobile sinks thereby exploiting the knowledge about the likely trajectories of the mobile sinks. It exploits the fact that in certain application environments, the mobile sink is aware of its future trajectories constrained by the motion patterns like roads, trails or hallways. Hence, in the initial phase, the mobile sinks announce a set of anticipated trajectories (collected offline) to nodes in the form of a broadcast  
Figure 7: System architecture: A mobile sink (MS) queries network via vice-sinks (VSs). Sensor nodes (SNs) inside network communicate in multi-hop fashion to reach to VSs [27]. destined for the entire network. In this approach, sensed data is not directly routed to mobile sinks but rather to some relays which lie along any of one of the trajectories of the mobile sink. These relays stash the reported data and deliver to sink when the sink passes by them in the future. It also tries to accurately predict a set of stashing points covering all trajectories of the mobile sinks that can be potentially visited by sinks in the near future. Furthermore, it makes it mandatory to have at least one stashing point on each trajectory, thus ensuring that the data will not be lost and will be retrieved by at least a single mobile sink. To find the optimal stashing points, linear programming is adopted for each data source to cope with the uncertainty about the trajectory of mobile sink. This scheme achieves guaranteed data delivery to mobile sinks at the expense of more energy consumption in the form of redundant transmissions in different directions. TRAIL [29] is another lightweight routing protocol specifically designed for low traffic load mWSNs that aims to deliver data to mobile sinks with minimal protocol overhead. The proposed protocol adopts both random walk- and trail-based forwarding strategies for data delivery to mobile sinks. In situations where a node has no fresh knowledge (trail) of any sink, a packet is forwarded using a random walk strategy until it reaches a node which has fresh knowledge (trail) about a sink. From that point onwards, the packet follows the trail till it is delivered to a sink. In TRAIL, each node builds and maintains two tables: sink-table and route-table. In the sink-table, a node keeps a record of whether the node is/was a neighbor of sink together with time-stamp information whereas the route-table keeps a record of the next-hop nodes towards the sink. An integral part of the TRAIL protocol is the trail formation. A sink's trail is formed by periodic broadcasting of beacon messages where the rate of beacon messages generation is tuned taking into account the velocity and communication range of nodes.