# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 5 Discussion
### General Challenges and Open Research Issues
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & **Basic Operation** & **Advantages** & **Disadvantages** \\ \hline
**Data Compression** & Nodes compress data prior to their transmission to the BS. & Reduction of size of transmitted packets and transmission time. & QoS reduction (accuracy, latency, fault tolerance security). \\ \hline
**In-Network Processing** & Nodes process data, prior to their transmission to the BS. & Data aggregation is performed. Reduction of data transmission. & Data processing may cause non-negligible energy consumption. \\ \hline
**Data Prediction** & Prediction models are created to restrict continuous sensing. & Data are transmitted only when they differ from predicted ones. & High level computations consume energy. Powerful nodes are needed. \\ \hline
**Adaptive Sampling** & Adjustment of sampling rate in line with application needs. & Energy is saved, when applied in centralized implementations. & High complexity and overhead are caused. Central control is needed. \\ \hline
**Hierarchical** & Dynamically deciding which sensors must be activated. & Energy hungry sensors actuated only when high detail is needed. & Accuracy may be sacrificed to achieve energy saving. \\ \hline
**Model-based Active Sampling** & Models predict data to save energy in data acquisition. & The number of data samples are reduced via mathematical models & Complex computations are needed. \\ \hline
**Location Driven** & Nodes are activated according to their location. & Unnecessary activation of nodes is avoided. & Location must be known. GPS units are costly and cause interference. \\ \hline
**Connectivity Driven** & Nodes are activated to ensure connectivity and coverage. & Only necessary for connectivity and coverage nodes are active & Location must be known. GPS units are costly and cause interference. \\ \hline
**On-Demand** & Nodes awakened only when necessary to communicate. & Converient for deployments with very low duty cycle. & An additional radio for wakeup signaling is needed. \\ \cline{2-3}
**Scheduled** & A mutual wake up schedule exists & When a node is awake, nearby nodes are also awake. & Problems in clock synchronization \\ \cline{2-3}
**rendezvous** & & & \\ \cline{2-3}
**Asynchronous** & Nodes are independent but have common active periods. & Simple implementation. & Robustness trades off for energy consumption. Latency. \\ \hline
**Scheduled MAC** & Nodes can access the shared & The multiple access of network & Costly. Hidden terminal (CSMA). \\  & medium channel. & nodes is regulated. & Clock synchronization (TDMA) \\ \hline
**Contention** & Protocols that aim at the & Robustness. Scalability. Idle & Increment of packet delivery \\
**based MAC** & avoidance of collision. & listening reduction. & latency. \\ \hline
**Hybrid MAC** & Scheduled and contention-based & The flaws of scheduled and & Complexity increases accordingly \\  & MAC features combined. & contention-based MAC amended. & to the number of nodes. \\ \hline
**Query-based** & Enquiries are used to support the & Dynamic network topologies and & Not suitable for continuous data \\ \cline{2-3}
**protocols** & & transfer of data. & multiple path routing are enabled. & delivery. \\ \hline
**Coherent/Non-Coherent-based** & Local processing: full in & & High overhead, high end-to-end \\  & Non-Coherent least in Coherent. & Data transmissions are reduced. & delay, low scalability. \\ \hline
**Negotiation-based** & Meta-data negotiation is used. & Redundant data are reduced. & Data delivery is not guaranteed. \\ \hline
**Flat Protocols** & All nodes have equal roles. & Ideal for small scale applications. & Remarkably low scalability \\ \cline{2-3}
**Hierarchical protocols** & Nodes have roles according to network hierarchy. & Data aggregation. Great & High overhead. High complexity. \\  & Every node knows the location of & acknowledg. & Optimal routes not guaranteed. \\ \hline
**Location-based protocols** & Every node knows the location of & The most energy efficient routes are used. Latency is reduced. & High overhead. Limited scalability. GPS units are costly \\  & & all other nodes. & and interfere. \\ \cline{2-3}
**Mobile agents-** & A movable entity collects the data & Energy expenditure for data & Low scalability. High latency. \\  & from nodes to the BS. & transmission is reduced. & High Complexity. \\ \hline
**Mobile sink-** & Sinks move and collect data from & Energy saving and reliability in & Delays on data delivery. Routing \\ \cline{2-3}
**based protocols** & The nodes. & increased. Connectivity enhanced. & paths and topology changes occur. \\ \hline
**QoS-based** & Routing is performed based on & High quality and fidelity in data & High processing overhead is \\  & various quality metrics. & transmission are achieved. & caused. \\ \hline
**Multipath-based protocols** & Data from nodes are routed to & Load balancing done. Failed & Processing load is considerably \\  & sinks via various paths. & nodes and congested paths are overcome. & increased. \\ \hline \hline \end{tabular}
\end{table}