## 2 IoT and Opportunistic Networks--State of the Art
### Energy Efficiency in Opportunistic Networks and IoT  
In opportunistic networks, energy-efficient communication is essential due to the sporadic nature of network connectivity and the limited battery life of mobile devices. Researchers have proposed various techniques to optimize energy consumption while maintaining communication reliability. For instance, [PERSON] et al. introduced an energy-aware routing protocol for opportunistic networks, which dynamically selects routes considering both energy levels and transmission opportunities [19]. Similarly, [PERSON] et al. proposed an energy-efficient data dissemination scheme based on social network analysis, where nodes with higher energy reserves are prioritized for message forwarding [20]. These studies highlight the importance of energyawareness in routing and data dissemination strategies to prolong network lifetime in opportunistic environments.  
In IoT deployments, energy efficiency is crucial for prolonging device lifespan and reducing maintenance overhead. Recent research has focused on optimizing energy consumption at both the device and network levels. For example, [PERSON] et al. proposed an energy-efficient IoT architecture leveraging edge computing and data aggregation techniques to minimize data transmission and processing overhead [21]. Additionally, [PERSON] et al. introduced an energy-efficient MAC protocol for IoT networks, which adapts transmission parameters based on channel conditions to reduce energy consumption during data transmission [22]. These studies demonstrate the significance of energy-efficient protocols and architectures in enabling sustainable IoT deployments.  
Recent advancements in energy efficiency emphasize cross-layer designs that integrate optimization techniques across different protocol layers. For instance, [PERSON] et al. proposed a cross-layer optimization framework for IoT networks, which jointly optimizes routing, MAC, and physical layer parameters to minimize energy consumption while ensuring QoS requirements [23]. Similarly, [PERSON] et al. introduced a cross-layer energy management scheme for opportunistic networks, which dynamically adjusts routing and transmission parameters based on energy availability and network conditions [24]. These studies highlight the potential of cross-layer approaches in maximizing energy efficiency in both opportunistic networks and IoT deployments. In response to the limitations of conventional routing protocols, the authors in [25] introduce VORTEX, a novel routing protocol that utilizes opportunistic routing strategies with hierarchization. VORTEX features network-driven opportunistic forwarding, where packets progress towards destination terminals using hierarchy as guidance, eliminating the need for conventional route discovery procedures. In [26], a hybrid route selection mechanism, enabling both proactive evaluation by the central controller for source nodes and reactive decision-making by source nodes for route selection in a distributed manner, is proposed. The preference for D2D communication facilitates traffic offloading from the control plane to the data plane, enhancing network efficiency.