## 5 Insights and Future Directions
### Guidelines for Energy, QoS, Security and Related Areas of Routing in WSNs  
Due to crucial role of WSNs in our lives, their popularity is increasing day-by-day. The WSNs may either be static or dynamic in nature. The static WSNs are immovable and perform sensing/monitoring activities by permanently occupying the assigned spaces. The example is structural health monitoring [6] or underground train tunnels investigation [14]. The dynamic WSNs consist of moving nodes such as surface-robots or air-drones that may perform surveillance and reconnaissance of critical installations. The example includes fire investigation [23], borders, bridges and dams monitoring. Both static and dynamic WSNs are affected by a variety issues as discussed below:* Energy conservation is the key goal in the design of routing protocols for both static and dynamic WSNs. In case of static WSNs, the immediate neighbors to sink suffer more readily from hot-spot issues due to network traffic. On the other hand, dynamic WSNs may suffer from additional energy consumption due to mobility and frequent connection/data loss. Although mobile sink and multipath-routing approach are promising solutions for achieving energy conservation [43] in WSNs. Another promising direction may be dynamic multi-sink multipath approach for achieving energy efficiency in WSNs which requires in-depth review and analysis in future. * A variety of energy conservation approaches are devised for WSNs. However, a sensor network with no indigenous source of energy harvesting is destined to die down due to energy depletion of sensor nodes. A solution is to employ such energy harvesting mechanisms that may provide a constant source of energy assistance to WSNs and may help in averting the danger of dying out of sensor network due to energy exhaustion. However, the prevailing energy harvesting machinery is bigger than the small-sized sensor nodes [107] and is expensive to generate usable power for sensor nodes [107]. Therefore for handling the constant nuisance of power exhaustion in sensor networks, there is a tremendous need of doing further research for devising such miniature nano-scale energy harvesting equipments that may easily generate the required energy for recharging sensor nodes and help sensor network for functioning continuously. * Hierarchical routing helps to achieve energy efficiency in WSNs [109]. However it may suffer from various issues such as optimum cluster size, cluster head selection/communication and cluster topology/scalability, etc. These issues become more challenging in case of mobility-oriented clustering where network topology frequently changes which may seriously impact network connectivity. Eventually, clustering would be severely affected. In case of high speed node mobility, then majority of network resources would be consumed in cluster formation and upgradation rather than desired data transmission. Therefore there is a need to survey the issues and challenges of mobility-oriented clustering in WSNs. * Both energy and QoS are closely associated e.g., increasing the packet transmission energy may increase the transmission range of a packet. Eventually, less hops are required to send data to destination which may decrease end-to-end delay [38]/jitter and improve system reliability. However, such mechanism may more readily drain the energy of sensor nodes and eventually cause early death of sensor network [38]. One solution to handle such energy drainage may be to use energy harvesting nodes for this purpose. However when more nodes may send packets with high energy simultaneously, then they may disrupt each other communication. A solution may be to use adaptive power control, but it may soon induce power competition among sensor nodes. Since energy-aware QoS routing assures required bandwidth, latency and energy-efficient routes [36], therefore it should be more thoroughly surveyed along with underlying issues and challenges for bringing efficient and more realistic solutions in this regard.