## 6 Discussion and Open Issues  
In this paper, a variety of energy-efficient mechanisms applied in routing protocols are summarized and classified into appropriate categories such as multi-sink, mobile sink, multi-path, bio-inspired and power control (as a cross-layer approach) techniques. These mechanisms which are categorized according to the protocol operation could be simultaneously employed in routing schemes for achieving more energy-efficiency [23]. However, some routing algorithms may use more than one mechanism from different classes. Future trends in routing strategies for wireless sensor networks concentrated on different directions should all follow the common goal which is prolonging the network lifetime by avoiding the network partitioning phenomenon. Supporting multiple sinks in order to balance energy consumption across the \begin{table}
\begin{tabular}{|l|l l l l l l l|} \hline \multirow{3}{*}{**Protocol**} & \multicolumn{6}{c|}{**Metrics of Comparison**} \\ \cline{2-9}  & **Network** & **Lifetime Improvement** & **Application Type** & **Multi-path** & **Data** & **Mobility** & **Location** \\  & **Structure** & **Method** & & & **Aggregation** & & **Awareness** \\ \hline
**[20]** & Hierarchical & Only Bio-Inspired & N/S & No & Yes & No & Yes \\
**EEABR [47]** & N/A & Only Bio-Inspired & Event-Driven & No & No & No & No \\
**[49]** & N/A & Only Bio-Inspired & Event-Driven & Yes & Yes & No & No \\
**MO-IAR [51]** & N/A & Only Bio-Inspired & Time \& Event-Driven & No & No & No & Yes \\
**OBCR [54]** & Location-Based & Only Bio-Inspired & N/S & No & No & No & Yes \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of bio-inspired mechanisms. network and prolong the lifetime of the sink neighbors is a hot topic these days. In most cases, the researchers employ mobile sinks for gathering data from source nodes all over the sensor field. Partitioning phenomena could also be avoided by employing the multi-path technique [41, 42, 43]. Using this method, the traffic load could be distributed on more than one path. Therefore, the nodes on the paths tolerate less traffic and experience lower energy consumption. As said before, experiments [27] show that data transmission with maximum power level wastes a significant amount of nodes' energy. In order to address this problem, power control schemes [16, 17, 18, 19] could provide an energy-efficient paradigm in which a sensor node forwards the data packets to the next hop using sufficient (minimum) power level. Bio-inspired protocols [47, 49, 51] are also aimed to save the node's energy through constructing the shortest path between the source and the sink nodes. Furthermore, power control and bio-inspired mechanisms provide high link quality for WSNs. Although numerous researchers have worked on different strategies in the past years, there is a high potential to improve current methods in the future. Some of the most important open research issues are as follows:  
* A large number of previous approaches in the venue of routing protocols include only one energy-efficient scheme in their related algorithms. However, a combination of different methods could be applied in new researches. For instance, implementing multiple paths towards multiple static sink with the help of bio-inspired techniques for path optimization is an open issue. On the other hand, using power control scheme accompany with multi-path mechanism helps the protocol to conserve much more energy [16]. It seems using a hybrid form of multiple and mobile sinks is a smart choice for more lifetime elongation [8]. The dual-sink [56] protocol, for example, is a novel scheme in which one of the sinks is mobile and collects the data packets from one or a few hops neighboring nodes while the other one is stationary at the center of field and receives data from far away source nodes without any localization overhead. In this way, the algorithm could benefit the advantages of both static and mobile sink approaches. Therefore, the hybrid multiple mobile sinks is an open issue for future trends.