## 5 Conclusion  
As it has become apparent, significant research has occurred since 2002 on WSN DCS schemes. Since WSN may be deployed in large-scale networks with thousands of sensors, efficient data storage and retrieval mechanisms are of pivotal importance. The key DCS schemes have been classified in this paper and summarized for convenience in Table 6. In the current literature, identified DCS schemes do not provide solutions for all the challenges mentioned in Section 2. Some key challenges such as similarity, spatial-temporal similarity search and the non-uniformity of sensor networks are addressed only in a few papers and conclusive approaches are yet to be found. Other challenges are still open. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline  & \multirow{2}{*}{**Title**} & \multirow{2}{*}{**Routing**} & \multirow{2}{*}{**Dimension** **(attribute)**} & \multirow{2}{*}{**Range** **vs. Point**} & \multirow{2}{*}{**Data** **Aggregation**} & \multirow{2}{*}{**Similarity**} & \multirow{2}{*}{**Multi** **Replication**} & \multirow{2}{*}{**Load** **Balance**} \\  & & & & & & & **Age** & & **Search** & **Replication** & **Balance** \\ \hline \multirow{4}{*}{1} & Geographic Hash Table & Point-to-Point & \multirow{4}{*}{Single} & \multirow{4}{*}{Point} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{No} \\  & (GHT) [1] & Routing & & & & & & & \\ \cline{1-1} \cline{5-10}  & Data Storage and Range & \multirow{4}{*}{Query Mechanism for Multi-dimensional Attributes. [35]} & \multirow{4}{*}{Multi} & \multirow{4}{*}{Range} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{No} \\  & & & & & & & & \\ \cline{1-1} \cline{5-10}  & Spatial-Temporal Data Storage Scheme. [19] & \multirow{4}{*}{Point-to-Point Routing} & \multirow{4}{*}{Multi} & \multirow{4}{*}{Range} & \multirow{4}{*}{No} & \multirow{4}{*}{Yes} & \multirow{4}{*}{No} & \multirow{4}{*}{Yes} \\  & Load Balanced and & \multirow{4}{*}{Efficient Hierarchical Data-Centric Storage.} & \multirow{4}{*}{Point-to-Point Routing} & \multirow{4}{*}{Single} & \multirow{4}{*}{Point} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{No} & \multirow{4}{*}{Yes} \\  & & & & & & & & \\ \cline{1-1} \cline{5-10}  & \multirow{2}{*}{Dynamic Load Balancing Approach [23]} & \multirow{2}{*}{Point-to-Point Routing} & \multirow{2}{*}{Single} & \multirow{2}{*}{Point} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{Yes} \\  & Load Balanced & \multirow{2}{*}{Point-to-Point Routing} & \multirow{2}{*}{Single} & \multirow{2}{*}{Point} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{Yes} \\  & Data-Centric Storage & \multirow{2}{*}{(LB-DCS) [24]} & \multirow{2}{*}{Point-to-Point Routing} & \multirow{2}{*}{Single} & \multirow{2}{*}{Point} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{Yes} \\  & (LB-DCS) [24] & & & & & & & \\ \cline{1-1} \cline{5-10}  & \multirow{2}{*}{Tug-of-War [27]} & \multirow{2}{*}{Point-to-Point Routing} & \multirow{2}{*}{Single} & \multirow{2}{*}{Point} & \multirow{2}{*}{No} & \multirow{2}{*}{No} & \multirow{2}{*}{Yes} & \multirow{2}{*}{No} \\ \cline{1-1} \cline{5-10}  & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of DCS Schemes Highlighting Their Key Features.