## 2 Related Works
### Energy-Aware Optimization-Based Routing in WSNs  
A prominent energy-efficient algorithm for clustered networks is the Low Energy Adaptive Clustering Hierarchy (LEACH) [19]. LEACH uses data aggregation techniques and a cluster head selected in a random way. It is a routing model based on hierarchical topology and energy efficiency. The main drawback in LEACH is that it is unsuitable for time critical applications. In addition, LEACH suffers from scalability problems due to the assumption that every sensor can transmit directly to the sink. The authors in [20], proposed the Routing based on Energy-Temperature Transformation (RETT-gen), which is a modification of the LEACH protocol, taking into consideration the residual energies of the sensor nodes in routing decisions. They disseminate equally the energy load over all SNs in the network, thereby extending the network lifetime. However, the persistent problem in this protocol and other clustering protocols is that the cluster formation process as well as the probabilistic cluster-head selection incurs high overheads and complexity.  
The power-efficient gathering in sensor information systems (PEGASIS) [21], introduces a new factor delay-energy, which concurrently decreases delay and energy cost. PEGASIS is similar to LEACH but uses lower energy per round. It creates a chain such that if a sensor has data to send to the sink, it goes through the leader sensor, which is chosen randomly every round. However, one of the limiting factors of PEGASIS is that every sensor has to aggregate data such that packet transmission is minimized. Moreover, the protocol requires the global information of the network and uses greedy method before the chain formation can be done, which requires data packet traversal through many sensor nodes.  
In [22], a threshold-sensitive energy efficient sensor network protocol (TEEN) is proposed. TEEN is an adaptive protocol for frequently changing network conditions in clustered WSNs. The problem of this protocol is the high incurred overhead during cluster formation. Additionally, the protocol cannot recognize dead sensor nodes.  
The authors in [23] proposed the self organized routing (SOR) protocol, where nodes conserve energy by employing other nodes as mediators to increase the survival time of the network. They introduced payment schemes for sensor nodes that act as mediators. Payment received by a node depends on whether it transmits its own packet or acts as a mediator for others. The protocol is energy efficient due to the self interest orientation of the nodes, using local information to make informed decisions.  
The authors in [24], proposed Direct Diffusion (DD) for data centric based routing protocols. The protocol uses the data address rather than the IP address. Here, the user sends a query to the sensor network for specific events and those nodes that sensed the required event start to send their data. However, the DD scheme does not consider the residual energy of nodes in the flooding phase and continuously broadcasts the request of interest, which causes high energy dissipation due to transmission and reception.  
The authors in [25], proposed the so-called Minimum Energy Communication Network (MECN) for _ad hoc_ network scenarios. The protocol also works well with wireless sensor networks. The improved version of MECN, which is known as Small-MECN (SMECN) shows a better performance, offering an energy efficient sub-network. The sub-network includes all nodes from the original network and a subset of the edges.  
The work in [26], proposed a distributed energy balanced routing (DEBR) protocol, which decentralizes the data traffic in the network in a way that prolongs the lifetime of the network. The algorithm makes a tradeoff between packet delay and energy. Despite the applicability of the algorithm in many WSN scenarios, it assumes that each SN is within the direct communication range of the sink, which is not true for multi-hop routing.