# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 5 Reliable Routing
### QoS Based
As soon as a cluster head is dead, another sensor node in close proximity to the exhausted one, which was previously in sleep mode, will then become the new cluster head in the next round. As long as the sensor nodes exchange data among them, no energy is wasted because of either switching between states or the process of new cluster head election. Based on its aforesaid operation,SPEC keeps energy consumption amid state changes low. In addition, it offers reduced computation energy loss. However, network is broken up when a cluster head dies. _Sleep-awake Energy Efficient Distributed_ (SEED) [147] is a protocol for heterogeneous networks. SEED reduces redundant data to the base station and separates the network into three energy regions, depending on their distance from the base station to distribute energy evenly. These regions are:  
1. A region close to the base station with normal energy nodes,
2. A region next to the first with advanced energy nodes,
3. A region far away from the base station with super energy nodes. After every region is separated, it is then divided into clusters while nodes of the same application within a cluster form a sub-cluster. SEED limits the node's communication by enabling only one sensor node to transmit data to the base station, while the others remain in sleep mode for energy conservation. In addition, cluster heads do not send data to the cluster heads of other regions, but rather send them directly to the base station avoiding both hotspot problems and energy holes near the base station. In addition, the cluster head election's main criterion is the remaining energy of a node. SEED attains high energy efficiency and great data redundancy. However, the nodes in sleep mode may disrupt the network. In addition, the scalability provided is not high. _Energy Efficient Routing Protocol_ (EERP) [148] is a protocol that combines an optimal aggregated cost function with the A* algorithm to maximize the lifetime of a WSN. In order to find the optimal path with minimum hop count, EERP takes into account the following three network attributes:  
1. Node's residual energy, in order to have a metric of WSN's lifetime,
2. Packet Reception Rate (PRR), which is the ratio of the base station received data packets to total sent packets,
3. Free buffer, which helps the network calculate the traffic load nodes can handle. These parameters are vital at the start of each round, where the base station receives and analyzes all these parameters in order to find the optimal path. When a node has new data to send, the base station chooses which sensor nodes will receive and forward the packet. EERP provides both optimal path calculation in each round and increased network lifetime. On the other hand, energy consumption when data collection rises is increased. Moreover, it has a lack of scalability. _Energy Efficient Cross-layer Protocol_ (EECP) [149] is a routing protocol that interacts with the Physical, MAC, and network layer. To ensure data efficient transmission, packets are routed through nodes with high energy closer to the base station, and it utilizes sleep time for energy efficient performance. EECP saves energy by utilizing five parameters:  
1. Sleep time maximization of nodes out of the routing path
2. Tries to use different routing path each time
3. Avoids to wake up the nodes before the time that they are needed to
4. Reduces collisions
5. Prevvents energy consumption from idle listening and overhearing  
Using EECP offers reduced collisions, prolonged network life, and good load balancing. However, the nodes being in sleep mode may deteriorate the network performance. In addition, during next hop selection, EECP may pick the same node from a previous hop. _Cluster Chain Weight Metrics_ (CCWM) [150] is a QoS based routing protocol, which achieves energy efficiency by avoiding long distances between cluster heads and cluster nodes as well as dense areas. To elect a cluster head, CCWM takes into account parameters such as path loss factor, residual energy, and node degree.