## 2 Related Work  
There exists a considerable research effort for the development of routing protocols in WSNs. The development of these protocols is based on the particular application needs and the architecture of the network. However, there are several factors that should be taken into consideration when developing routing protocols for WSNs. Energy efficiency is the most important among these factors, since it directly affects the lifetime of the network. There have been a few efforts in the literature pursuing energy efficiency in WSNs. In [11] Low Energy Adaptive Clustering Hierarchy (LEACH), a hierarchical protocol in which most nodes transmit to cluster heads, is presented. The operation of LEACH consists of two phases:  
* The Setup Phase: In the setup phase, the clusters are organized and the cluster heads are selected. In every round, a stochastic algorithm is used by each node to determine whether it will become a cluster head. If a node becomes a cluster head once, it cannot become a cluster head again for \(P\) rounds, where \(P\) is the desired percentage of cluster heads. * The Steady State Phase: In the steady state phase, the data is sent to the base station. The duration of the steady state phase is longer than the duration of the setup phase in order to minimize overhead. LEACH is a protocol that tends to reduce energy consumption in a WSN. However, LEACH uses single-hop routing in which each sensor node transmits information directly to the cluster-head or the sink. Therefore, it is not recommended for networks that are deployed in large regions. Power-Efficient GAthering in Sensor Information Systems (PEGASIS) is an energy efficient protocol [12], which provides improvements over LEACH. In PEGASIS, each node communicates only with a nearby neighbor in order to exchange data. It takes turns in order to transmit the information to the base station, thus reducing the amount of energy spent per round. The nodes are organized in such a way as to form a chain, which can either be formed by the sensor nodes themselves using a greedy algorithm starting from a certain node, or the BS can compute this chain and broadcast it to all the sensor nodes. In LEACH, a node becomes a cluster-head using a stochastic mechanism. This is prone to producing unbalanced energy level reserves in nodes and, thus, to increasing the total energy dissipated in the network. In PEGASIS, the cluster head selection does not take into consideration neither the residual energy of the nodes nor the location of the base station. PEGASIS has better performance compared to LEACH [13], but the nodes are grouped into chains that cause redundant data transmissions. Threshold Sensitive Energy Efficient (TEEN) is a hierarchical protocol designed for sudden changes in the sensed environment [14]. The response of the network in time-critical applications is extremely important, obliging the network to operate in a reactive mode. The sensor network architecture in TEEN is based on hierarchical grouping. The nodes close to upper level clusters are used to transfer data from other nodes that are further away, a process that goes on the next level cluster until the sink is reached. The main advantage of TEEN is that it works well in conditions where sudden changes in the sensed attributes occur. On the other hand, in large area networks and when the number of layers in the hierarchy is small, TEEN tends to consume considerable amounts of energy, because of long distance transmissions. Moreover, when the number of layers increases, the transmissions become shorter and there exists a considerable overhead in the setup phase, as well as the operation of the network. The Shortest Hop Routing Tree protocol (SHORT) [15] efficiently collects useful data from a remote wireless sensor network to the base station and provides energy efficiency. This protocol selects the node with the largest value of residual energy as the leader. The Extending Lifetime of Cluster Head (ELCH) routing protocol [16] has self-configuration and hierarchal routing properties.