# Cross-Layer MAC/Routing Protocol for Reliability Improvement of the Internet of Things
## 3 Topological Structure Mesh Routing  
Energy consumption is one of the major problems in wireless communication systems, including the IoT. To address this, the IEEE 802.15.4 standard improves energy efficiency through time synchronization using a beacon frame broadcast by a coordinator. The IEEE 802.15.4 standard defines two types of devices -- that is, a full function device (FFD) and a reduced-function device (RFD). FFDs can operate in three modes -- namely, the coordinator, router, and device modes. Additionally, FFDs can be network coordinators without restrictions on the network topology configuration and can communicate with all the equipment on the network. Conversely, RFDs operate only in device mode and can only participate in a limited form of star topology. Additionally, RFDs cannot act as network coordinators and can only communicate with the parent router. The IEEE 802.15.4 standard defines only one-hop communication among adjacent devices. Therefore, to support multi-hop communication, it provides a cluster-tree topology in which most devices are FFDs. RFDs may be associated with a cluster-tree network as leaf nodes because they can only communicate with the parent router, or FFD. The cluster-tree topology is capable of low-power consumption while performing multi-hop communication. However, in a cluster-tree topology, the entire network may be shut down because of the departure of a few router devices. The IEEE 802.15.5 standard address-allocation method can assign a unique address to all nodes throughout the network, but the disadvantage is that it takes some time to form an initial network in a large network and does not properly respond to the dynamic addition and deletion of nodes. Moreover, the memory overhead for storing the addresses of all the nodes increases. The ZigBee protocol, established by the ZigBee Alliance -- a leading standardization organization on WSNs--provides a distributed address-allocation technique that sets up a balanced tree-based address area and then configures a tree topology to allocate addresses. Currently, the distributed address allocation scheme using the Zigbee standard CSkip starts with a maximum tree depth (Lm), a maximum number of children (Cm), and a maximum number of routers (Rm) specified in advance. This has the advantage of simplifying routing by using only address information without requiring a separate routing table or route search process while ensuring the uniqueness of the address when a new node enters the network. However, the ZigBee address allocation scheme cannot support asymmetric networks. This is because all devices in the tree network use the same network parameters of Cm, Rm, and Lm values, and therefore use the same CSkip. Moreover, the value of the maximum network depth (Lm) is often very difficult to select before actually forming a network. When applied to a large-scale network, the predetermined parameters of Cm and Rm cannot always meet the address allocation requirements, which results in some nodes not being able to obtain network addresses and becoming orphan [69; 70; 71; 72]. Additionally, owing to the use of a fixed topology, an orphan problem can occur that cannot be accessed because of the exhaustion of addresses of neighboring routers, even though sufficient address areas remain in the network. An address-allocation scheme for large-scale networks should be able to shorten the initial network formation time, avoid generating excessive traffic in the network, and effectively respond to the dynamic addition and removal of nodes. If the destination node locates the subtree of an intermediate node, the routing protocol of the cluster-tree network should find the next-hop node for a given destination address without a routing table. However, the sender node does not know whether the destination is located nearby or is not within the subtree itself. Consequently, cluster-tree-based routing cannot find multi-path routes between the source and destination devices and cannot optimize the routing path.