# Cross-Layer MAC/Routing Protocol for Reliability Improvement of the Internet of Things
## 2 Background
### Related Work
To this end, functions corresponding to routing metrics and path optimization to achieve the requirements of each application were separated as objective function (OF), and the contents defined in the standard define general contents commonly used for various OFs [54]. There are many routing protocols that enhance RPL. In [55], the authors proposed a modification protocol to enable cluster trees in IEEE 802.15.4 to work efficiently with RPL. Based on this modified MAC layer, the authors used an opportunistic forwarding scheme [56] to extend the RPL capable of forwarding packets. Instead of always using the preferred parent, the node opportunistically forwards the packet through another parent as long as the sink path is better. In [57], the authors propose a proactive multipath load-balancing approach that seeks to achieve maximum life by equalizing traffic rates between nodes at the same distance in the local sink. In this algorithm, load balancing is performed through routing graphs generated by an Multi-class Multipath Routing Protocol (M2 RPL) for low-power and lost networks. In [58], the authors proposed a multipath RPL (MRPL) algorithm to maximize network life and minimize total transmission costs. The algorithm proposed a novel heuristic load balancing mechanism (HeLD) to find the optimal traffic rate for each link and simultaneously equalize the rate of energy consumption between nodes of the same depth. In [59], the authors proposed a multipath routing protocol to provide efficient event packet forwarding in event-based wireless sensor networks using different mechanisms in the path-building and data-transfer phases. However, these multipath routing protocols only consider traffic flows to the sink, such as MP2P traffic. Accordingly, a bypass problem occurs during P2P communication, and packets converge around the sink. Mesh-topology-based routing protocols offer promising techniques to overcome these reliability problems. Fault tolerance for a single-path failure can be achieved by maintaining both basic and alternative paths. Wireless mesh networks have been considered promising access networks for next generation technology, providing better services and guaranteeing flexibility and robustness to different wireless network environments. The mesh network can take several paths from the source to the destination using the intermediate nodes within the network. [PERSON] et al. [60] proposed a hybrid architecture that combined a WSN and a wireless mesh network (WMN). In [40], the sensor node only used resources (i.e., power) to implement sensing functions, whereas the mesh router collected encrypted data and then relayed the aggregated data to the sink, reducing the volume of traffic exchanged over the network and the resulting overall power consumption. [PERSON] et al. [61] proposed a distributed beacon scheduling-based MAC protocol for the IEEE 802.15.4 beacon mode. The proposed algorithm enabled low-powermesh networking through distributed beacon scheduling and was compatible with the IEEE 802.15.4 standard. Furthermore, the proposed scheme was able to improve the reliability of the IEEE 802.15.4 cluster-tree topology (which provided only a single communication path) by constructing a mesh topology. The ISA 100 standard was designed to provide reliable and safe behavior in the field of system monitoring and control [62]. It defines the structure of networks, systems management, gateways, security managers, etc., for low-speed wireless connections to fixed, portable, and mobile devices that support limited power consumption conditions. The ISA 100 standard is fully compatible with the IEEE 802.15.4 standard, but it achieves similar results through simpler messages that use the data link layer subnet time shared as a security element without IEEE security. [PERSON] et al. [63] proposed an adaptive access parameter tuning (ADAPT) algorithm based on analytical studies of the IEEE 802.15.4 standard.