# Cross-Layer Energy Optimization for IoT Environments: Technical Advances and Opportunities
## 5 Open Research Issues in Energy Oriented IoT  
Many issues related to energy consumption have been solved and isolated by researchers, but still there are challenges to solve existing issues. An insight is provided related to existing issues faced to attain in energy efficient system. The research issues that require attention are as follows:  
\(\bullet\)_Cross layer interoperability_  
Several algorithms and techniques working separately on a particular layer of IoT architecture are introduced for energy efficiency in the IoT. Combining them in single operational solution is an existing challenge. The concept of interoperability should be stabilized and privileged by the system to avoid oscillation among layers [34]. Moreover, there should be enough flexibility in the system, while changing a certain techniques such as energy efficiency of a particular layer without affecting the whole IoT system. Figure 21 represents the concept of interoperability of IoT layers: sensing, local processing and storage, network, cloud processing and storage, and application, in terms of various techniques related to energy efficiency, mobility, and security and QoS parameters. This cross layer interoperability makes a IoT system more effective and efficient in various IoT applications [58; 59; 60; 61; 62; 63; 64; 65]. \(\bullet\) Energy is omnipresent _Potential of energy harvesting_  
In the form of light, temperature, motion, wind, electric, sound, mechanical etc., but only needs to be harvested. Cyber-physical systems, which are a new area in IoT, combine networking and computing with physical processes [36]. These processes can be human controlled or autonomous. IoT-based smart sensors gather, control and measure the information and communicate to the base station [37]. Because of the battery limitations of the devices, energy harvesting is necessary, using natural energy resources like wind, vibration power, solar and thermal power [39]. Due to unreachability of these devices in many cases, energy efficiency to support these devices through energy harvesting techniques is an important research issue. In Figure 22, various natural energy resources those can be potentially utilized for energy harvesting are illustrated. The usage of energy resources such as solar power, vibration power, thermal power, wireless power and wind power may solve the issue of battery limitation in terms of energy through various energy harvesting techniques, in the IoT environment [62]. Figure 21: Cross layer interoperability. \(\bullet\)_Interactivity limitations_  
A network can be created for processing and managing data on various remote servers joined together rather than by a local computer or server in the cloud computing environment [52]. The cloud provides sharable resources among multiple users, while making accessible large amounts of information gathered from IoT devices for analysis and processing [53]. In the case of gathering large amount of data from devices it creates multiple effects such as more energy consumption [54]. As a result new approaches and research can be introduced for reducing the device interactivity to minimize energy consumption in IoT applications [63; 64; 65; 66]. In Figure 23, location-based interaction based on the cloud and IoT is illustrated. Location-based interactions between IoT devices and users based on cloud computing are one of the effective approaches to overcome the issue of interactivity for making energy efficient IoT system [67]. \(\bullet\)_QoS constraints approaches_  
Devices have become autonomous and intelligent, because of the usage of new computational concepts like artificial intelligence for the effective deployment of IoT networks [40]. The limitations of energy consumption should be adapted through the association of artificial neural networks. Moreover the QoS constrained system may be controlled in an intelligent manner for every IoT application that may have a distinct energy profile [42].