# Modeling and Fault Tolerance Analysis of ZigBee Protocol in IoT Networks
## 4 Analysis of the Simulation Results Concerning the Fault Tolerance Issues in ZigBee Protocol  
An appropriate communication protocol, as well as routing algorithm, is required to ensure continuity. The main task of routing protocols in WSN is to find the best route between sensor nodes and receiver nodes and forward from the source node to the object node. Due to the constraints involved, several factors influence the design of routing protocols: node distribution strategy, data reporting method, network dynamics, node locations, fault tolerance, or a node's type [39; 40; 41; 42]. This research aimed to analyze selected routing protocols. The protocols used can be divided into three categories: network organization, routing method, and protocols based on which it operates. The first group includes flat, hierarchical, and location-based routing. The second group includes reactive, proactive, and hybrid protocols. The last group includes negotiation-based, multipath, queue-based, Quality of Service (QoS), and coherent protocols. Reactive routing protocols do not store overall network topology knowledge in routing tables. Routes are created on-demand when a node wants to connect to another node. Proactive protocols maintain knowledge of the entire network topology in the routing tables. All routes are predefined before transmission begins. Hybrid protocols combine the advantages of reactive and proactive protocols while negating their disadvantages. In routing protocols with flat topology, all nodes are treated equally. This is mainly applicable to homogeneous networks where all nodes have the same features and functionalities. Hierarchical routing protocols are mainly used in heterogeneous networks. Often, clusters of nodes are formed in which there is a master aggregating node that forwards data. In location-based routing protocols, nodes can locate each other using location protocols. The location information helps to improve route finding by also allowing nodes to provide additional services. Multipath routing protocols provide multiple routes to a destination, thereby balancing loads, reducing latency, and improving network performance. In query-based routing protocols, sensor nodes send data only after receiving a specific request from the destination  
Figure 2: A schematic of a typical WSN structure along with a sensor node diagram [38]. node. In negotiation-based routing protocols, in order to avoid unnecessary redundancy, nodes negotiate among themselves what data will be transmitted by the nodes. The vast majority of actual fault tolerance protocols introduce redundancy. The most common technique for tolerating a wireless link is retransmission, which generates additional network traffic and causes additional energy consumption. Receivers must acknowledge the receipts of messages, which requires additional energy compared to a method without retransmissions. When the amount of retransmissions is significant, the latency is higher and provides the information to become obsolete. IEEE 802.15.4/ Zigbee is a specification used in WSN with low power consumption. The ZigBee standard is considered to be one of the most relevant technology for LLNs (Low-power and Lossy Networks). At the beginning of its evolution, it was described by the Cluster-Tree protocol as a hierarchical routing protocol that uses link-state packets to create a two-level cluster-based network, in which every cluster is managed by a single cluster head (CH). Such a network is self-organizing and supports fault-tolerance and self-healing mechanisms. A cluster tree is created by several parent-child adjacencies across ZigBee routers up to a specified level of deployment. The clusters are expected to work in exclusive time slots to prevent interference among them. With the appearance of the modified ZigBee protocol, cluster-tree routing discontinued support due to the complicated maintenance problems, and the standard instead embraced flat and mesh routing using AODV [43; 44; 45].