# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 5 Reliable Routing
### QoS Based
Another novelty is the cluster heads supporting only a specific number of nodes, resulting in efficient operation of MAC layer. Furthermore, the protocol tries to position the cluster head in the middle of the cluster, to avoid long distances between cluster nodes and, by utilizing a cost propagation model, it achieves a good link between cluster heads and cluster nodes that saves more energy. However, the cluster head election procedure creates overhead. Balanced Energy Efficient Network Integrated Super Heterogeneous (BEENISH) [151] is a clustering protocol for heterogeneous WSNs that reduces energy consumption, increases stability, and prolongs network lifetime. To perform efficiently, BEENISH selects a cluster head based on residual energy by taking into account the energy reservoirs of the nodes in the network. In addition, BEENISH introduces a four-level energy scheme to classify the residual energy of nodes. These four energy levels are: normal, advance, super, and ultra-super. Nodes in the ultra-super energy level are elected as cluster heads more often than those in the super energy level. Super energy level nodes are elected as cluster head more often than advanced, etc. By employing this strategy, the network's energy balance is achieved and energy consumption is reduced due to less communication taking place between the nodes during cluster head election. On the other hand, the distance between nodes is not considered. Energy Consumption Rate based Stable Election Protocol (ECRSEP) [152] is another clustering protocol, which utilizes the so-called Energy Consumption Rate (ECR) for cluster head election. During every round, ECRSEP elects nodes as cluster heads in case they have low ECR. After the end of a round, in the next one, ECRSEP uses the previous ECRs to select a cluster head and elects again those with the lowest ones. Therefore, sensor nodes, which have been elected as cluster heads in the previous round, will not be chosen, due to their high ECR, compared to non-cluster head nodes. The formula for calculating ECR is:  
\[ECR=\frac{F_{int}-F_{r}}{r-1}, \tag{5}\]  
where \(F_{int}\) symbolizes the initial energy, \(F_{r}\) stands for the residual energy of each node and \(r\) denotes the current round. ECRSEP reduces communication energy consumption and provides energy balanced networks. However, it suffers from a problem with energy holes. Sequential Assignment Routing (SAR) [153] uses a concept of QoS in its routing decisions for the first time in routing protocols. It decides the routing depending on three factors:  
1. Energy resources
2. QoS on each path
3. Priority level of each packet  
It is a table driven multi path-approach. With the use of trees, multiple paths from the base station to nodes are formed. These trees are rooted to single-hop neighbor nodes of the base station by considering the above three factors. Each path is selected by considering energy resources and QoS on the route. In case of failure, recovery can occur on each route, with enforced consistency of routing table between upstream and downstream nodes. Failures occurring locally trigger a procedure of path restoration. SAR provides more energy efficiency from Minimum-Energy Metric algorithm. In addition, multiple routes from nodes to the base station are maintained and every packet's priority is considered. However, there is a large maintenance cost of tables and states in every node when the number of nodes is large. Reliable Routing Distributed Learning Automaton (RRDLA) [154] is a QoS routing protocol that introduces Distributed Learning Automaton (DLA) model to find the smallest number of nodes needed to preserve desired QoS requirements. RRDLA algorithm consists of four phases:  
1. Initial, where the network initializes its operation and forms DLA,
2. Learning, where a small number of nodes with high Packet Delivery Ratio (PDR) are selected,
3. Transmitting, where the selected nodes transmit their data via the reliable links established,
4.