## 6 Future Directions
\begin{table}
\begin{tabular}{|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|p{11.4 pt}|} \hline \multicolumn{1}{|p{11.4 pt}|}{**Protocols**} & \multicolumn{1}{p{11.4 pt}|}{**C**} & \multicolumn{1}{p{11.4 pt}|}{**C**} & \multicolumn{1}{p{11.4 pt}|}{**D**} & \multicolumn{1}{p{11.4 pt}|}{**P**} & \multicolumn{1}{p{11.4 pt}|}{**D**} & \multicolumn{1}{p{11.4 pt}|}{**M**} & \multicolumn{1}{p{11.4 pt}|}{**D**} & \multicolumn{1}{p{11.4 pt}|}{**A**} & \multicolumn{1}{p{11.4 pt}|}{**P**} & \multicolumn{1}{p{11.4 pt}|}{**E**} \\ \multicolumn{1}{|p{11.4 pt}|}{} & \multicolumn{1}{p{11.4 pt}|}{**M**} & \multicolumn{1}{p{11.4 pt}|}{**P**} & \multicolumn{1}{p{11.4 pt}|}{**S**} & \multicolumn{1}{p{11.4 pt}|}{**L**} & \multicolumn{1}{p{11.4 pt}|}{**L**} & \multicolumn{1}{p{11.4 pt}|}{**C**} & \multicolumn{1}{p{11.4 pt}|}{**I**} & \multicolumn{1}{p{11.4 pt}|}{**R**} & \multicolumn{1}{p{11.4 pt}|}{**U**} & \multicolumn{1}{p{11.4 pt}|}{**S**} & \multicolumn{1}{p{11.4 pt}|}{**C**} \\ \hline \multirow{11}{*}{**Protocol**} & \multirow{10}{*}{\begin{tabular}{} \end{tabular} } & _CPDA_ & M & H & N & B & M & M & N & Y & H & F & M & M \\ \cline{2-14}  & & _Conti al.’s scheme_ & H & H & N & B & H & M & N & Y & H & F & M & H \\ \cline{2-14}  & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } & _DADPP_ & H & H & N & B & M & M & N & Y & H & F & M & H \\ \cline{2-14}  & & _PHA_ & M & M & N & B & L & H & N & Y & L & U & G & M \\ \cline{2-14}  & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } & _SMART_ & H & L & N & B & M & M & N & N & H & F & S & H \\ \cline{2-14}  & & _iPDA_ & H & M & N & B & H & M & Y & N & H & F & S & H \\ \cline{2-14}  & \multirow{2}{*}{\begin{tabular}{} \end{tabular} } & Privacy & _CDA_ & L & L & N & O & L & L & N & Y & H & F & S & L \\ \cline{2-14}  & & _AH scheme_ & L & L & N & O & L & L & N & N & H & F & M & L \\ \hline \multirow{11}{*}{**Protocol**} & \multirow{10}{*}{\
\begin{tabular}{} \end{tabular} } & _Depend:_ H = High; M = Medium; L = Low; N = No; Y = Yes; B = Both; O = Outsider; U = Numerous; F = Few; G = Large; S = Small; “-” = Not Mentioned & & & & & & & & & \\ \cline{1-1} \cline{2-14}  & \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } & Hybrid & _PIA_ & L & M & N & O & M & M & Y & Y & M & U & S & L \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison result of PPDA protocols for WSNs. will be worthwhile to address both comparative aggregation functions and data integrity checking without needing the raw data of sensor nodes, as future research. Fifth, data aggregation saves the precious energy of WSNs by reducing the number of messages transmissions whereas a protocol for privacy preservation of sensor data needs a significant amount of energy. Therefore, future work needs to explore the combined impact of privacy preservation and data aggregation on the performance of WSNs and examine the tradeoff between data aggregation and privacy preservation. Last but not least, because WSNs are not reliable, it cannot be expected that all nodes will reply to all requests. Therefore there needs to be a reliable and scalable mechanism for communicating the _IDs_ of the contributed nodes to the sink node. Since TinyOS-based sensor nodes have a limited payload size (29-bytes), support for the transmission of the _IDs_ of contributed sensor nodes along with aggregated data to the sink node for large-scale WSNs is a possible future research direction for privacy-preserving data aggregation.