# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 5 Energy Saving on Network Level
### Energy Efficient Routing  
Routing protocols are important in wireless sensor networks to ensure end-to-end connectivity especially over long distances [118; 119]. A small sending range is important for reducing energy consumption and limiting sending power. Energy efficient routing protocols reduce the energy consumption further by collecting data and selecting the energy saving path between the destination and source nodes. One possibility for aggregating the data in a network is via a mobile node [120], which collects the data from the normal nodes and transmits them to the base station (BS). The so-called mobility driven schemespresent an efficient technique for minimizing the energy consumption by avoiding multi-hop communication and reducing the overload for the nodes closer to the base station, which typically occurs in other routing protocols [121]. In this energy saving technique, the mobile node needs to have a high energy budget, but the consumed energy in normal nodes is uniformly distributed. This technique is suitable in applications where regular data collection in a certain interval is taking place, such as walk-by solutions in smart metering or in robotics applications. However, in the case of large-scale networks, the mobile node takes a long time to collect data from different nodes, resulting in a considerable delay in data transmission. Thus, this technique is suitable for small-scale networks.  
Depending on the design constraints for the network structure, other routing protocols can be adopted, such as data centric routing (flat), clustering-based routing (hierarchical) and location-based (geographic) routing [122]. Location-based routing protocols, also known as geographic routing protocols, use the information about the physical location of nodes provided by the Global Positioning System (GPS) [123] or some other localization techniques [72]. To reduce the energy consumption, a relay node is selected by the source node to forward the data packet toward the destination node. To this end, the source node determines the distance from neighbor nodes using different localization techniques such as RSSI [114].  
Data centric routing protocols have a flat hierarchy [124], where all nodes have the same role and transmit data packets to a sink. At the beginning, the sink node sends queries to nodes within the deployment region and waits for incoming data. The intermediate nodes aggregates data from multiple sources and forwards the collected data towards the sink. Thereby, redundant data can be avoided, and energy can thus be saved.  
Clustering-based hierarchical routing is an energy-efficient communication protocol based on three kinds of nodes: normal nodes (NN), cluster head (CH) and BS. NN sense the environment, get data and forward them to their associate CH, which in turn aggregates data from all cluster members and transmits them to the BS or, in a multi-hop network, to other CHs that take responsibility for transmission to the BS. Cluster-based hierarchical routing is one of the most interesting energy-aware techniques that minimizes simultaneously the number of transmitted messages and the distances to be covered between nodes. In addition, clustering may include data aggregation and fusion techniques, which reduce the overload as well as the packet losses [125].