# Energy Efficiency for Green Internet of Things (IoT) Networks: A Survey +
## 3 Literature Review
### Taxonomy of Energy Consumption Solutions
#### 3.7.1 Routing Protocols
The H-HEED protocol finds the center of each cluster and then allocates the node nearest the cluster center. The H-HEED protocol re-computes the cluster centers with a new assignment of nodes and allocates a node to clusters until clusters do not change for a given number of iterations. However, in this protocol, several iterations are performed to form the clusters and select a CH, this is an overhead that consumes a significant amount of energy [97]. Nevertheless, the proposed scheme increased the network lifetime of the sensor nodes by 63% [98]. Another study [99] proposed an energy-based rotated  
Figure 14: HEED protocol architecture. HEED (ER-HEED) protocol for WSNs. Here, the clustering formation and CH node selection are implemented based on the HEED protocol. Therefore, the selection of CH node among sensor nodes in each cluster is based on the node with the highest level of energy. ER-HEED improves the HEED protocol by reducing the HEED cluster selection to minimize energy wasted and lengthen network lifetime. In [100], a new multihop routing strategy was proposed, the cluster heads enhanced hybrid, energy-efficient distributed HEED method (E-HEED) for WSNs. The E-HEED chooses the CH node according to the HEED protocol, and then grades the CH nodes according to the least transmission distance from the BS. It was claimed that the E-HEED protocol lengthened network life by 0.8 % compared to HEED. * **Power-Efficient Gathering in Sensor Information Systems (PEGASIS)** PEGASIS is another CH node selection technique. This approach is to form a chain among the sensor nodes for the transmissions, see Figure 15 for the architecture of the PEGASIS routing protocol [90]. Each node receives the data from one neighbor node and transmits it to another. Two nodes at the end of the chain forming the routing structure will forward the information through the other nodes to the single leader node (CH node) and then the CH sends these data to the intended destination. The CH node is randomly elected to transmit the gathered data to the intended destination. PEGASIS is aimed to minimize the transmission distances between sensor nodes in the sensing field, and thus the energy depletion of each sensor is minimized. However, only one node is picked as a CH node per round. It this may become a bottleneck that causes delay and retransmission of some of packets. It also increases the rate of packet transmission on the node selected as a leader and thus depletes its energy quickly. Table 2 presents characteristics and comparisons of LEACH, HEED and PEGASIS based on the more important metrics:  
Figure 15: PEGASIS protocol architecture. More recently, authors [105] investigated, a new routing technique called destination-oriented routing scheme for energy-balanced WSN (DORA). The DORA aims a new multichain routing method to transmit the data to balance energy for the sensor nodes. In this protocol, the optimal transmission distance between any two nodes in the sensing field is derived by the mathematical analysis model. The proposed protocol reduces energy consumption for the nodes and thus extends the global network lifespan. However, in this protocol, any node in the sensing filed might be connected with two or more multichain based on the transmission range of the node, and thus a node may be sends same data through two or more paths to the final destination. Recently, [PERSON] et al. [55] gave a comprehensive review of the IoT and WSN technologies for medium access control (MAC) protocols. The review focuses on the MAC layer protocols and common causes of energy consumptions. Early studies by [106] have investigated a new routing technique LLND protocol which is defined the MAC behavior for IoT networks that run inaccessible environments. Sensor nodes interconnected by wireless links with dynamic and lossy wireless link conditions, resulting from interference, channel fading, or heat/dust/ moisture physical environment, are classified by Low-power and Lossy Network (LLNs).