## 2 Related Research Work
[29] in which each round of communication is segmented into two phases, i.e., (i) the setup phase, in which limited number of sensor nodes are selected as CH depending upon their probability values and CMs of that CH, (ii) each CH assigns a particular timeslot to every CM in order to avoid collision. Each CM forwards the sensed data only in that timeslot. Various researchers have also proposed many variants of LEACH in [28; 29; 30; 31; 32; 33]. [PERSON] et al. [31] proposed the heterogeneous aware Stable Election Protocol (SEP), which is based on weighted election probabilities of sensor nodes for selection of CH depending upon their residual energies. In SEP, the energy heterogeneity problem is characterized using node classification, in which nodes are either labeled as normal or advanced. This protocol ensures that, during CH election procedure, CH is selected randomly based on the fraction of energy of each sensor node, thus assuring a uniform energy usage of all sensor nodes. Foregoing this view, [PERSON] et al. [34] presented an extension of SEP which is known as SEP-Enhanced or SEP-E. In SEP-E, the energy heterogeneity problem is characterized using a three-node classification, in which nodes are labeled as normal, intermediate and advanced sensor nodes. According to [34], the main goal of SEP-E is to achieve a robust self-configured heterogeneous WSN with a longer network lifetime of sensor nodes and better energy utilization. [PERSON] et al. [33] proposed a Heterogeneity-aware Energy efficient Clustering (HEC) protocol, which is based on three different phases and CH is selected in every phase in a different manner, i.e., (i) in the first phase, sensor nodes labeled as advanced are allowed to participate in CH selection, (ii) all sensor nodes, irrespective of their residual energies, are allowed to participate in CH selection procedure with equal probabilities, (iii) for the last phase, direct transmission to BS is preferred instead of clustering, due to low residual energy of sensor nodes. [PERSON] et al. [35] suggested a novel heterogeneous WSN supporting a Distributed Energy-Efficient Clustering (DEEC) scheme in which energy heterogeneity is characterized using normal and advanced sensor nodes. Additionally, [PERSON] et al. [36] proposed an extension of the DEEC protocol which is known as the Enhanced Distributed Energy-Efficient Clustering (E-DEEC) scheme, in which the energy heterogeneity problem is addressed using three-node classification, i.e., normal, intermediate and advanced. In E-DEEC, the CH selection probabilities are not adapted as per the energy levels of sensor nodes. [PERSON] et al. [37] proposed a heterogeneous network model based on Enhanced Developed DEEC (ED-DEEC or DEEC-E) which is established on two-node power classification and three energy levels of sensor nodes for dynamically modifying the CH election probability. [PERSON] et al. [38] introduced a novel energy-efficient protocol known as Threshold sensitive Energy-Efficient Sensor Network (TEEN) protocol for reactive networks. In TEEN, each CM in a cluster takes turns to become the CH for a time interval, called the cluster period. After every cluster period, the CH broadcasts a soft and hard threshold to its CMs along with other attributes. CMs only send the sensed data to its CH by changing the status from working to sleeping, only if the data values are in the range of interest, keeping in view the hard threshold. In addition to this, CMs send the data to CH by changing their status from sleeping to working only if their data values change by at least the soft threshold. Although there are no collisions between data transmissions due to TDMA scheduling, TEEN introduces an extra delay during the reporting of time-critical data. It has been revealed through a detailed literature review that most of the clustering schemes consider attributes such as residual energy, distance to the BS, etc. as parameters of criteria.