## 4 Classification Based on Taxonomy and Design Drivers
### Multi-Replication
\begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Schemes** & **Policy** &
\begin{tabular}{c} **RoutingAmong** \\ **Replica Nodes** \\ \end{tabular} & **Remark** \\ \hline
1 & SR-GHT [1] & \begin{tabular}{c} Hierarchical Grid \\ Replication Mechanism (4d) \\ \end{tabular} & \
\begin{tabular}{c} Recursive \\ Hierarchical \\ \end{tabular} &
\begin{tabular}{c} As data never replicated to all \\ nodes, basic data lost problem \\ exists \\ \end{tabular} \\ \hline
2 & SDS [19] & \
\begin{tabular}{c} Head node stores copy \\ of all client data \\ \end{tabular} & N/A &
\begin{tabular}{c} Single point of head zone \\ failure. Head zone energy \\ depletes quicker than others \\ \end{tabular} \\ \hline
3 & ToW [27] & \
\begin{tabular}{c} Hierarchical Grid \\ Replication Mechanism (4d) \\ \end{tabular} & Combing &
\begin{tabular}{c} Extends SR-GHT by adding \\ two modes of operation. It \\ inherits drawbacks from \\ SR-GHT \\ \end{tabular} \\ \hline
4 & SSA [22] & \
\begin{tabular}{c} Create mirror of index \\ node using Mirror \\ Hilbert Curve \& Mirror \\ Mapping Function \\ \end{tabular} & Not Specified &
\begin{tabular}{c} It doesn’t explain how data \\ would be mirrored rather just a \\ proposal is mentioned by \\ couple of lines \\ \end{tabular} \\ \hline
5 & RDCS [36] & \
\begin{tabular}{c} Each zone has at most \\ one replica node of \\ mirror node \\ \end{tabular} & GPSR &
\begin{tabular}{c} Selection of mirror node is not \\ specified clearly. \\ \end{tabular} \\ \hline
6 & QAR [16] & \
\begin{tabular}{c} Hierarchical Grid \\ Replication Mechanism \\ with Quadratic \\ Evolution (d2) \\ \end{tabular} & Combing &
\begin{tabular}{c} Inherits drawbacks from SR- \\ GHT \\ \end{tabular} \\ \hline
6 & Double Rulings [28] & \begin{tabular}{c} Stores data replica at a \\ curve instead of one or \\ multiple isolated sensors \\ \end{tabular} & \
\begin{tabular}{c} Greedy Routing \\ on a Curve \\ \end{tabular} &
\begin{tabular}{c} Can only employ 2 global \\ replicas while tow and qar are \\ adaptable to traffic load with \\ multiple replicas \\ \end{tabular} \\ \hline
7 & \begin{tabular}{c} Dynamic \\ Random \\ Replication [41] \\ \end{tabular} & \
\begin{tabular}{c} Replicate data in \\ randomly selected set of \\ data replication nodes \\ \end{tabular} & Minimum &
\begin{tabular}{c} Two major limitations: static \\ WSN and consideration of \\ homogenous spatial \\ applications \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of Multi-Replication DCS Schemes. overall network traffic. However, Unlike GHT, ToW and QAR replicate producer data to all replicas in their second mode of operation. Nevertheless, the mode of operation is not deterministic since the mode of operation depends on the resolution (see Section 3.6) and hence ToW and QAR might also suffer from the data loss problem during their first mode of operation, _i.e._, _write_one_query_all_. SSA [22] creates a mirror of the index node using a mirror Hilbert Curve and mirror mapping function. However, the replication process is not explained in the paper. RDCS [36] proposes at most one mirror node for each zone, which will keep a copy of the zone's index node. An index node stores all data directed toward its responsible zone and therefore creation of a hotspot surrounding the index node is possible. Nevertheless, the hotspot issue has not been mentioned in the selection process for the mirror node. Dynamic random replication for DCS [41] and double rulings [28], described in Section 3.8, are two important research outcomes related to multi-replication research for DCS. Relative to previous work, random replication is a simpler and more flexible technique that enables an effective reduction of network traffic.