# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 6 Conclusions  
This research work provided a survey on both classic and modern protocols, which have been proposed for the achievement of energy efficient routing in WSNs. Generally, the protocols presented were classified, according to their main structural or operational features, into four main categories which namely are: Communication Model, Network Structure, Topology, and Reliable Routing. Various representative examples of the aforementioned categories of protocols were studied and their pros and cons were highlighted. Their common characteristic is that they all pursue energy conservation by performing energy efficient routing. On the other hand, this study made evident that there are many differences in their structure and operation that were stated. Specifically, protocols belonging to Communication Model category were further classified as Query based, Coherent or Non Coherent, and Negotiation based. Query based protocols use queries in order to enable the transmission of data from nodes that possess data to nodes that request these data. Their operation supports dynamic network topologies and multiple route replies. On the other hand, query based protocols are not appropriate for applications that require continuous data delivery such as environmental monitoring, since they route data after receiving queries and not in a constant rate. Coherent based protocols support minimum processing only of the data a node captures, while, in non-Coherent based routing protocols, nodes preprocess data they capture and send them to other nodes, called aggregators, which further process them. Coherent based protocols are not suitable for applications that require low end-to-end delay and high scalability, while non-Coherent based can be utilized for applications that require a large number of nodes. Negotiation based protocols \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline \multirow{6}{*}{HMRP [158]} & It is simple with small & \multirow{6}{*}{\
\begin{tabular}{} \end{tabular} } & It uses only one & \multirow{6}{*}{Good} & \multirow{6}{*}{Low} & \multirow{6}{*}{
\begin{tabular}{} \end{tabular} } \\  & great network lifetime, & broadcast of the layer & & & \\  & data aggregation and & energy balanced & & & \\  & routing. & & & & \\  & It has simple cluster & & & & \\  & level hop by hop & It has errors and & & & \\  & routing and less & increased control & & & \\  & interference over & overhead during & & & \\  & conventional & packet reassembly. & & & \\  & multipath routing. & & & & \\  & It provides low delay & It mainly focuses on & & & The route that starts \\  & and extended & video data & High & None & with different single \\  & network lifetime. & transmission. & & & \\  & The multipath & & & & \\  & converging and & One source node per & & & \\  & multipath expanding & round is selected as & & & \\  & tradeoffs are & the reference source. & & & \\  & combined. & & & & \\  & It has low energy & Only unicast traffic is & & & \\  & consumption. & & supported. & & \\  & The medium number & & & & \\  & of advertisement & & & & \\  & messages can be & & & & \\  & reduced. & advertisement & & & \\  & It has low energy & & & & \\  & consumption with & & & & \\  & sleep mode, collision & & & & \\  & free data transmission & & & & \\  & and interference & nodes. & & & \\  & & & & \\  & & & & \\  & & & & \\  & It extends network & The DVB is not the & & & \\  & Optimum routing & Supported & None & heads to upper level & \\  & cluster heads and & & & \\  & the BS within the & & & & \\  & DVB. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 14: _Cont._use negotiation schemes in order to support communication among network nodes, in which data exchanged are named in order to reduce redundant information at destination nodes. In this way, both point-to-point and broadcast communication is indeed energy efficient. However, the successful delivery of data is not guaranteed.