# A Green Routing Protocol with Wireless Power Transfer for Internet of Things
## 3 Related Works  
Several survey works summarized the RPL operations and other routing solutions for IoT/LLN networks as in refs. [20; 21; 22; 23]. Recently, different papers have analyzed new RPL features mainly addressing security aspects, route maintenance and recovery mechanism, i.e., link failure detection and recovery mechanism, and various performance metrics such as packet reception rate, storing mode feature, average delay of paths, and energy usage. In this brief review, we only consider the papers presenting composite metrics (in addition to the IETF proposed objective functions, i.e., OF0 and ETX) for RPL based on the residual energy of the nodes to improve the global lifetime of the network. In ref. [24], the authors address energy efficiency and reliability issues when the network size and the location of the root node change in addition to considering storing mode and convergence time for local repair approaches. In ref. [5], an OF named EEQ is shown to preserve the node which has already sent sensed data towards the root node and has consumed too much of its energy. EEQ computes the rank of a node by using weighted combination of the following parameters: (i) expected number of transmissions, (ii) consumed energy, and (iii) active queue length. This composite metric under high intensity traffic loads presents less overhead of control messages resulting in energy conservation. In ref. [6], a variant of RPL is proposed for routing multimedia traffic which considers the remaining energy of nodes. Since multimedia traffic is extremely energy consuming, it is important to improve the network lifetime for multimedia content distribution. The RPL protocol is analyzed under a heterogeneous traffic [10] by proposing an OF based on packet queue of the node to measure its congestion and on the link workload condition (QWL-RPL). This OF allows to distribute the load in a balanced way and to minimize the number of controls overhead when supporting heterogeneous traffic. In [9], the residual energy and the transmission delay are used as routing metric in the next hop selection process for RPL. The objective function named QoSRPL for this metric is based on ant colony optimization (ACO), i.e., on algorithms exploiting the ability of simple ants to solve complex problems through cooperation. The results of experiments  
Figure 1: Routing Protocol for Low-Power and Lossy Networks (RPL) Destination Oriented Directed Acyclic Graph (DODAG) management flow chart upon receiving a DODAG Information Object (DIO) message. show performance of QoSRPL in terms of energy consumption and delay better than ETX focused on the reliability of links. Energy Efficient and Path Reliability Aware Objective Function (ERAOF) for IoT applications is proposed in ref. [7] using an integration of energy consumption and link quality routing ETX metric. Results show that ERAOF improves the network performance in terms of packet delivery ratio and energy spent per delivered data packet. In ref. [8], the authors use both link and node metrics to define an index, called life cycle index (LCI), which includes also congestion detection to design the route selection. [PERSON] et al. [25] proposed a solution (RAME) based on the lowest energy node to find an optimal path. Simulation results on 40 nodes showed the energy-harvesting efficiency in a multi-hop network. Another work [12] presents GreenNet, a wireless sensors network with photovoltaic energy-harvesting nodes. The GreenNet motes are manufactured by STMelectronics. The work also reports a novel mechanism to enhance the existing protocol stack, i.e., RPL. Recently, SWIPT was introduced to harvest energy from the environment and simultaneously decode the desired information signal [15, 16, 26, 27]. Sensor nodes and wireless devices in IoT are powered by limited time batteries and their replacement is costly, due to the extremely high number of sensors in a WSN, or hard, e.g., in environments where the sensors are difficult to reach.