# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 2 Communication Model
### Coherent/Non-Coherent  
In this subcategory, nodes process collected data in the node level before they route them. In Coherent protocols, a node applies minimum processing only on the data it captures. On the other hand, in non-coherent routing protocols, nodes preprocess data they capture and send them to nodes, called aggregators, which further process them. In what follows in this section, two typical examples of this subcategory are described. A synopsis of their features is shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline Directed Diffusion [28] & It offers extended network lifetime. & \begin{tabular}{c} It is not suitable for \\ applications that need \\ delivery of continuous \\ data. \\ \end{tabular} & Good & Limited & \begin{tabular}{c} The best route is \\ used. \\ \end{tabular} & Low \\ \multirow{4}{*}{\begin{tabular}{c} LOUGAR \\ [29] \\ \end{tabular} } & It provides energy efficiency during huge data generation. & \begin{tabular}{c} It has more overhead in \\ nodes with the extra layer \\ while data computation \\ in the network needs \\ synchronization. \\ A query travels more \\ hops if the look-ahead \\ parameter is too small \\ and it has similar \\ behavior as Flooding if the lock-ahead parameter is equal to the network \\ size. \\ \end{tabular} & Limited & Limited & \begin{tabular}{c} The best route is \\ used. \\ \end{tabular} & Low \\ \multirow{4}{*}{\begin{tabular}{c} ACQUIRE \\ [30] \\ \end{tabular} } & It provides efficient querying when responses are collected from many nodes. & \begin{tabular}{c} It has more overhead in \\ nodes with the extra layer \\ while data computation \\ in the network needs \\ synchronization. \\ A query travels more \\ hops if the look-ahead \\ parameter is too small \\ and it has similar \\ behavior as Flooding if the lock-ahead parameter is equal to the network \\ size. \\ \end{tabular} & Limited & Limited & \begin{tabular}{c} The shortest route is \\ used. \\ \end{tabular} & Low \\ \multirow{4}{*}{\begin{tabular}{c} Energy Aware \\ Routing [31] \\ \end{tabular} } & Compared to DD, it is more complex than more energy efficient & \begin{tabular}{c} Direct Diffusion and \\ the single path usage \\ removes path failure \\ tolerance. \\ \end{tabular} & Low & Low & \begin{tabular}{c} Probabilistic \\ forwarding through \\ random routes is \\ used. \\ \end{tabular} & Low \\ \multirow{4}{*}{\begin{tabular}{c} GBR [32] \\ \end{tabular} } & It has great load balancing and reduced energy cost produced. & \multirow{2}{*}{Low} & \multirow{2}{*}{Low} & \multirow{2}{*}{None} & The route with the \\  & & & & & \\ \cline{1-1} \cline{4-5}  & The queries reach & The combination of the \\ \cline{1-1} \cline{4-5}  & neighbors first and it \\ \cline{1-1} \cline{4-5}  & is more energy & \
\begin{tabular}{c} processes creates \\ overhead. \\ \end{tabular} & Good & None &
\begin{tabular}{c} The optimal routing \\ path is used. \\ \end{tabular} & Good \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Synopsis of communication model protocols belonging to query based subcategory. SPIN too, SPIN-PP does not take into consideration energy constrains. In SPINN-PP protocol, set-up simplicity is offered, and implosion is avoided. However, needless energy consumption takes place while data delivery is not guaranteed. _SPIN-Energy Conservation_ (SPIN-EC) [36] protocol uses the same algorithm of SPIN-PP but adds a heuristic of energy conservation. To take into consideration energy constraints, SPIN-EC uses an energy threshold. Nodes, whose residual energy is below this threshold, can receive ADV or REQ messages, but they will not send REQ messages if they are interested or will not handle DATA messages. SPIN-EC considers energy constraints and properly adapts its operation. However, nodes even below the low energy threshold keep consuming energy because they are still able to receive ADV and REQ messages.