## 2 Related Work  
A lot of research has already been reported for efficient communication in WSNs for the deployment of a green IoT [17, 18, 19, 20, 21], but little work is found concerning energy efficient communication for a scalable IoT. Routing protocols can be categorized into three types [22]: (i) energy efficiency-based; (ii) reliability and network operation-based and (iii) network operation-based. A comparative study of clustering-based routing protocols [23, 24] revealed that these protocols are an optimized solution for IoT applications. Many hierarchal protocols were proposed in past such as HEED [25], PEGASIS [26], CODA [27], HCR [28], SEP [29], EECHA [30], EECS [31], DWEHC [32], EEUC [33], PANEL [34], EB-PEGASIS [35], CCS [36], BCDCP [37], LEA2C [38], T-DEEC [39], EESAA [40], MODELACH [41], Cross layer protocol [42], _etc._ which try to optimize the energy efficiency by the use of optimal cluster head selection, forming chains of nodes, by balancing the load on the clusters, _etc._ In these protocols, nodes have different functionalities or different roles so they are classified as normal nodes and cluster heads. The main objective of these protocols is to enhance the network lifetime. These routing algorithms are not suitable for IoT applications as they require extra time to form clusters and are not scalable and they introduce more complexity. The most widely used network architecture for routing protocols is the tree-based one. In this type of routing all nodes transmit data to one node that is a base station (BS). Many existing solutions like E-CHtree [43] and multi-hop LQI [44] construct trees to route the data in a many-to-one pattern, but these solutions are not applicable for IoT applications like environment monitoring and coal mine good applications. Other patterns required to be considered like many-to-many and one-to-many communication. The discipline of meta-heuristic Evolutionary Algorithms (EAs) has also been utilized by several researchers to tackle cluster-based routing problems in WSN [45, 46, 47, 48], but at the cost of a stability period and delay which cannot be avoided in IoT applications. UCEB-CMF [49] has proposed as an unequal clustering and multipath algorithm with multi-hop communication can be the best solution for coal mine good applications, but this protocol is not suitable for environmental IoT applications as the redundancy of data will be increased, and processing of the same data by many paths puts an unnecessary burden on the nodes. To aggregate the data from different locations, sink mobility in a controlled manner is proposed by [PERSON] _et al._[50]. It is the best approach in terms of energy efficiency, but controlling the movement of the sink in the large scale network becomes difficult. A hybrid node scheduling based on efficient chain routing is proposed in [51]. Its routing strategy can be used for both event and time driven applications. It does not take into account continuous monitoring applications. The sink is considered at the center, which is considered the best case to gather the data, but some applications like border security surveillance cannot place the sink in the center, so this protocol requires analyzing the worst case, _i.e._, when the sink is far away from the nodes. Multi-criteria objective function performs a decentralization of the clustering protocol [52, 53] different from LEACH. CHs are elected in every round, but the election of the CHs again and again also consumes node energy. Shortest path selection by forming the minimum spanning tree [54] has proved an improvement over genetic-based algorithms, but this protocol does not pay attention to the fault tolerance, which is essential for the reliability of the protocol. Self-organized tree-based routing [55] performs better than HEED in terms of energy efficiency, but root nodes which communicate with all the nodes may be far away from some nodes which can affect the communication energy of the nodes. The energy balanced algorithm was proposed for water environment systems [56].