## 8 Comparative Study
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & & \multicolumn{6}{c}{**Performance Metrics**} \\ \cline{2-10}
\begin{tabular}{c} Moderate \\ \& Fixed \\ \end{tabular} & Medium & Yes & Query Driven \\ \hline
[56] & Single & Yes &
\begin{tabular}{c} Controlled \& \\ Predictable \\ \end{tabular} & Discrete & N/A & Large & No & Proactive \\ \hline
[36] & Single & No & Random & Discrete & N/A & Small & No & Reactive \\ \hline
[39] & Single & No & Random & Continuous &
\begin{tabular}{c} Slow \& \\ Variable \\ \end{tabular} & Very Small & No & Reactive \\ \hline
[35] & Multiple & No & Random & Discrete & N/A & N/A & N/A & Reactive \\ \hline
[60] & Single & No & \
\begin{tabular}{c} Controlled \& \\ Predictable \\ \end{tabular} & Discrete & N/A & Medium & No &
\begin{tabular}{c} Periodic and \\ Proactive \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparative study of energy efficiency based data collection schemes. _Network Size_--Network size is considered as the total number of nodes in the network. It is highly application dependent and has a direct impact on performance of data delivery to sink. For convenience, here we consider network size as small (up to 100 nodes), medium (100 to 400 nodes), or large (greater than 400 nodes). In the applications where network size is small, controlled sink mobility yields greater energy savings [51]. _Location Awareness_--It specifies whether in the considered scheme sensor nodes are aware of their geographical coordinates or not. In routing mechanisms, especially location-based routing, nodes' location information can be helpful in a number of ways: First, if destination's (sinks') position is known, significant energy savings can be achieved thereby avoiding energy expenditures in route discovery [61]. Secondly, nodes do not need to store routing tables, thereby saving considerable memory space [62]. Thirdly, nodes' location information can be quite helpful in preserving the size of clusters and topologies [63], cluster formation and in cluster-head selection [64]. Similarly, query and response packets can be quickly propagated to destinations based on appropriate next hop selection [65]. Finally, in case of mWSNs, nodes' location information can be used to restrict propagation of sinks' topological updates to a limited number of nodes, e.g., recently visited nodes, avoiding frequent topological updates if mobile sinks' location and arrival time can be predicted as in [66], and avoiding duplicate visits in a single tour of a mobile sink [45]. However, to determine location information, it either requires nodes to be equipped with specialized hardware (GPS) or needs to run dedicated localization algorithms. The use of GPS not only incurs additional hardware costs but it is also energy consuming, whereas the localization algorithms which make use of beacon nodes consume considerable amount of energy during the message exchange process. _Data Reporting Mode_--Data can be disseminated to a mobile sink either using any one of the five modes (or in combination with others): wait-and-upload, proactive, reactive, periodic-basis (time-driven), and query-driven. In wait-and-upload approach, sensor nodes buffer sensory data till a mobile sink arrives in their vicinity for data collection [12, 45, 48, 51]. A proactive approach is adopted when paths to destinations are already known to reporting nodes and thus the observed sensory data is proactively disseminated either to sinks [7, 13, 32, 37, 56] or stored temporarily at some specific nodes in the network to be retrieved at a later stage by a mobile sink [23, 28, 41]. In the reactive approach, paths to sinks are not known _a priori_ and are determined and used on demand whenever an event of interest occurs [36, 37, 39]. In the periodic approach, sensor nodes perform the sensing and data dissemination tasks on a periodic basis [33, 36]. Query-driven data dissemination is exhibited when sensor nodes receive a query from sink asking for required data [25, 27, 44].