# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
It is ideal for video communication over Wireless Multimedia Sensor Networks aiming at both energy savings and high QoS. The operation of PEMuR consists of two phases, which namely are the initialization and the steady state phase. During the initialization phase, the nodes become members of the upper and lower level clusters which are created by the BS. The cluster heads inform the BS the energy status of all of their cluster members. During the steady state, the nodes having the highest residual energy in each cluster are elected to be the new cluster heads. Additionally, the soft, hard, and energy thresholds are defined. Nodes are supposed to transmit whenever their residual energy is below the energy threshold or the values of sensed parameters meet conditions related with hard and soft thresholds. Messagesare routed from the cluster heads to the BS via direct communication in case they are upper level cluster heads or via intermediate upper level cluster heads if they are lower level. In both cases, energy efficient routing is achieved, by taking into consideration both the energy conserves of the nodes and the energy cost of data transmission. Moreover, PEMuR protocol proposes an analytical model that can accurately predict for every packet the effect that its loss has on the resulting distortion of decoded video. Thus, PEMuR can successfully cope with limited available channel bandwidth by selectively dropping less significant packets prior to their transmission. PEMuR achieves great energy efficiency and good scalability along with high preservation of QoS in multimedia content transmission. However, there is lack of mobility considerations. _Novel Energy Aware Hierarchical Cluster based_ (NEAHC) protocol [75] is a hierarchical protocol designed to limit the unbalanced energy consumption of the sensor nodes of multi-hop protocols. In a multi-hop network, data travel from the cluster head to the BS, while energy consumption depends on the distance and the number of hops, between the sending and the receiving node. Nodes close to the BS need to relay more data than the distant nodes and consume more energy. Before cluster head election, NEAHC chooses one cluster head, depending on its residual energy and a lower number of switches between sleep and active modes, in order to balance energy consumption. The great advantage of NEAHC is that energy holes are less likely to occur. On the other hand, cluster head selection increases the network energy consumption In addition, nodes in sleep-mode may cause network disconnections. _Hierarchical Energy-Balancing Multipath_ (HEBM) [76] is a hierarchical routing protocol with an adaptive clustering scheme, for homogeneous WSNs and a static base station with unlimited energy reservoir and communication power. It calculates the best cluster size balance, resulting in a minimum energy network topology. HEBM operates in time intervals or rounds, which consist of six phases. Before it calculates cluster sizes and considering network's density and size, it separates the nodes into clusters. By utilizing cluster range and the minimum energy path to cluster head, the levels of the network are formed. HEBM algorithm ensures a fairly distributed network because cluster head election considers network residual energy and distance from neighbor nodes. In this way, load balancing is achieved. Moreover, less message delays occur. On the other hand, extra overhead from collecting network data and calculating cluster size is caused. In addition, mobility is not supported. _Energy Efficiency Semi-Static_ routing algorithm (EESSC) [77] is a hierarchical protocol based on an improved HAC (Hierarchical Agglomerative Clustering) approach. EESSC operation consists of four steps:  
1. The BS sends a message to all nodes of the network to activate them,
2. Utilizing the HAC method sensor nodes form clusters and the LNC (List of Nodes in Cluster). 3. Sensor nodes begin to operate while the cluster head within a cluster would be rotated according to LNC. 4.