# Energy Efficiency for Green Internet of Things (IoT) Networks: A Survey +
## 3 Literature Review
### Sources of Energy Wastage  
Several studies have revealed that the communication unit is relatively greedy for energy [68]. In WSNs, most of the energy is wasted in the processing, receiving, or transmitting of data to fulfill the requirements of the application [70]. It is clear that reducing data transmissions will economize the energy of these smart objects [68]. Regarding communication, several studies have found that a great amount of energy is dissipated in ways that make no useful contribution to the application, such as [71]:  
* Collision: when two or more packets reach the sensor node at the same time and thus a packet collision occurs [72]. Thus, the packets are either discarded or sent back to their originating node, then retransmission of these packets is needed which rises packet latency and energy depletion which adversely affects the network lifetime [73]. * Overhearing: is a significant waste of energy, especially when node density is high and traffic load is heavy. When a node sends a packet, all sensor nodes in the network located within its transmission range distance receive the packet even if these nodes are not the proposed destination [74,75], see Figure 9. Node A wants to deliver its information to Node B. However, many surrounding nodes are within radio range of  
Figure 8: Necessary considerations for IoT-based WSNs. Node A. All these nodes will receive the data from Node A. Energy is consumed when a sensor node transmits or obtains the data that are intended for other nodes [75]. Please note that Node A will also receive data from its surrounding nodes when they transmit their data. * Control packet overhead: is a combination of excess memory, bandwidth, computation time or other resources to execute a specific job. Thus, it is crucial to process the minimum number of control packets that enable the transmission [71]. * Idle listening: happens when a sensor node must stay open to an idle channel to receive possible traffic [71], thus a sensor surrounds with many neighbor nodes will be active most of the time. This is due to overhearing transmissions, neighbor nodes discovery [76] or a node may use numerous paths to deliver data to a neighbor nodes [77]. Obviously, a node with less idle listening time has better energy retention than other nodes [71]. * Interference: each node with two or more nodes within transmission range suffers from interference generated by the surrounding nodes. Interference increases with increase in the number of neighboring nodes [78]. It increases both congestion and conflicting transmissions, and then retransmission may happen. Therefore, avoiding higher node interference could reduce packet loss and thus minimize the overall energy wasted of the network [79]. * Redundant Data: nodes are generally deployed randomly which can mean that there are some regions monitored by two or more sensors at the same time [80]. However, this type of deployment will increase the reporting of redundant data in the network. As a result, energy is wasted aggregating, processing and transmitting redundant data [13]. Energy consumption could be minimized by avoiding the unnecessary operation of a node. * Distance: the transmission distance (\(T_{d}\)) between nodes is a very important aspect of energy efficiency. The communication between a node and its associated CH node and the intended destination can be either single or multiple hops. Since energy consumption for transmission is proportional to the square of the distance (see Equation (1)) [81], so the power required for transmission increases rapidly with distance, which means single-hop transmission maximizes energy depletion if the size of the network is large. \[E_{Tx}=k(E_{elec}+\epsilon_{amp}*d^{2})\] (1) where \(E_{Tx}\) is the energy used to dispatch a chunk of data (\(k\)) from the node to the next-hop node. \(d\) is the distance between the source node and next-hop node.