## 2 Related Work  
The main purpose of sensor nodes in the WSN is to sense the area of the environment, collect data, process and aggregate it and deliver it to the base station. Direct transmission is the simplest way for nodes to communicate with a base station or sink. However, they need a lot of power to forward records. Thus, their resources will be quickly depleted and the node will quickly die out due to unnecessary energy use [25]. One way to reduce power consumption is to send data from one node to another and finally to the BS. The most important issue in wireless sensor networks is an energy efficiency. Thus, routing protocols with an energy efficiency solution represent an engaging research area in this area. The main goal of these algorithms is to reduce the power consumption of nodes in the network. [PERSON] et al. in [15] proposed the I-SEP (IoT-SEP) protocol, introducing a threshold energy value _T(n)_ for each node type for the SEP protocol, where depending on this energy threshold, the existing cluster head and its corresponding cluster members must either be changed or stay transmitting in the next round. Thus, an effective method for electing the head of the cluster is being studied. The nodes in this network are of three types: (1) regular normal nodes, (2) intermediate nodes, with energy value between normal and advanced nodes, and (3) advanced nodes. At each round, remaining residual energy _E_res of the cluster head is weighed up. If _E_res is lower than the threshold value _T(n)_, then the new cluster head selection procedure is initiated and clusters are renewed. Thus, additional energy consumption when forming a new cluster is reduced. The cluster head selection procedure is the same as in LEACH and SEP algorithms. By considering the probabilities of each node, the threshold value _T(n)_ for cluster head election is specified for each node type. Thus, _p(N)_, _p(I)_ and _p(A)_ are considered as probabilities for regular normal, intermediate and extended advanced nodes, respectively as:  
\[p_{(N)}=\frac{p}{1+a\alpha+b\beta} \tag{1}\]  
\[T(n_{N})=\left\{ \begin{array}{ll}\frac{p_{(N)}}{1-p_{(N)}\left(r\bmod\frac{1 }{p_{(N)}}\right)},&if\ n_{N}\in G_{1}\\ &\\ 0,&otherwise\end{array} \right. \tag{2}\]  
\[p_{(I)}=\frac{p(1+\beta)}{1+a\alpha+b\beta} \tag{3}\]  
\[T(n_{I})=\left\{ \begin{array}{ll}\frac{p_{(I)}}{1-p_{(I)}\left(r\bmod\frac{1 }{p_{(I)}}\right)},&if\ n_{I}\in G_{2}\\ &\\ 0,&otherwise\end{array} \right. \tag{4}\]\[p_{(A)}=\frac{p(1+\alpha)}{1+a\alpha+b\beta} \tag{5}\]  
\[T(n_{A})=\left\{ \begin{array}{cc}\frac{p_{(A)}}{1-p_{(A)}\left(r\;mod\frac{1}{p _{(A)}}\right)},&if\;n_{A}\in G_{3}\\ 0,&otherwise\end{array} \right. \tag{6}\]  
where \(a\) and \(b\) are the set of advanced and intermediate nodes. \(G_{1}\), \(G_{2}\) and \(G_{3}\) denotes the set of nodes in each type of normal, intermediate and advanced nodes respectively, that had not assigned as a cluster head in former epochs, and \(r\) is the current round. Nodes have \(E_{0}\), \(E_{0}(1+\beta)\) and \(E_{0}(1+\alpha)\) as the initial energies of normal, intermediate and advanced nodes respectively. Once the cluster head is assigned, the remaining member nodes in the cluster join the cluster head according to the information sent in the advertisement (ADV) message. In this way, each round cluster heads, clusters and its members regularly change. Once node is elected as a cluster head, according to traditional SEP, this node cannot take a part in a cluster head selection procedure next 1/_p_ epochs. The next research paper focusing on energy-based threshold sensitive cluster head election protocol is a method called improved threshold-sensitive stable election protocol (ITSEP) as described by [PERSON]. This protocol enhances the threshold formula by taking into consideration the node distance from the base station, the number of its neighbor nodes, nodes remaining residual energy and the nodes-to-nodes average distance [26].