## 7 Classification of Mobile Sink based Data Collection Schemes
### Path Unconstrained Sink Mobility-Based Schemes
PA is the nearest node to the sink's current position and collects data from intermediate disseminating nodes to deliver it to the sink. Each query message contains location information about the PA. Initially both the PA and IA are the same. Upon changing its location, when the sink is about to lose the IA (also the PA initially) due to coverage issues, the sink appoints another IA and updates the PA about the new IA's location for directing the future data towards the new IA. This phenomenon has been illustrated in Figure 10. Similarly, if the sink moves over a considerable distance (such as a cell size) from its PA, it repeats the aforementioned process by flooding the query bounded by cell size to find new disseminating node and accordingly chooses a PA for that new intermediate disseminating node. The proposed scheme achieves considerable energy savings thereby avoiding the flooding of the whole sensing field. However, the cell size needs to be set carefully as it has a direct impact on the size of the flooding area and grid construction/maintenance, _i.e._, a larger cell size means a wider flooding area, whereas a smaller cell size gives rise to more overhead involved in the grid construction/maintenance. Furthermore, constructing a grid for every single source/event on a periodic basis results in more energy consumption [43]. In addition, in case of sink mobility, this scheme just ensures the uninterrupted data delivery, but the route adopted might not be the optimal route (number of hops involved), thereby potentially compromising the data delivery latency. A Scalable Energy-efficient Asynchronous Dissemination (SEAD) protocol has been proposed in [44] that aims to prolong the network lifetime by employing multiple sinks. SEAD constructs a dissemination tree (called _d-tree_) both for routing and caching data. Any mobile sink interested in communicating with the source of the tree joins the _d-tree_ by sending a _join-query_ to one of its neighbor sensor nodes. This neighbor node becomes the sink's access-node and is responsible for data delivery to the sink. The access-node upon receiving the _join-query_, recursively constructs a _d-tree_ by exploiting the geographic location information. The access-node also keeps track of sink mobility. The _d-tree_ is updated whenever the sink changes its access-node. However, to avoid frequent reconstruction of the tree caused by sink mobility, the sink does not choose another access-node until the hop-counts between sink and access-node exceed a certain threshold. When a new access-node is selected, the old access-node is informed accordingly. This strategy is adopted to maintain the trade-off between energy-expenditure in the tree-reconstruction and path-delay. Furthermore, to disseminate data to multiple sinks, a set of nodes between the source and sinks are chosen as replica that keep the source data temporarily. An example SEAD tree model is shown in Figure 11.  
SEAD achieves improved energy consumption, thereby avoiding frequent re-construction of the _d-tree_. However, in pursuit of energy conservation, it incurs additional delays (in the form of a few extra multi-hops towards the sink's latest position) and is therefore not applicable for delay-sensitive applications. Furthermore, to maintain the _d-tree_, if a source has no more sensory data to report, it still sends idle messages to sinks, which results in unnecessary energy consumption. The aforementioned data collection schemes which do not impose any constraints on the path of the mobile sink(s) are summarized in Table 2. It illustrates each scheme in terms of any constraints it imposes on sink mobility, the number of sinks involved for data collection, data reporting modes  
Figure 10: Handling sink mobility in TTDD. (proactive, reactive, query-driven, periodic, or wait-and-upload), network architecture, network overhead control mechanism, an estimate of network control overhead, together with the main goal(s).