# Modeling and Fault Tolerance Analysis of ZigBee Protocol in IoT Networks
## 3 The Reference Scenario  
The IoT is a system of interconnected devices equipped with unique identifiers, capable of transmitting data over a network without human intervention. These devices are constantly connected to a virtual world which allows them to be controlled remotely. A critical requirement of the Internet of Things is the need for constant connectivity between objects. The architecture of an Internet of Things system must guarantee an operation that creates a gateway between the physical and virtual worlds. Designing such an architecture is based on many aspects, such as creating communication networks, business models, and ensuring security. The scalability, interoperability, and extensibility of selected devices must also be considered at the design stage. In the Internet of Things, objects do not have an assigned place, they are in constant motion, often interacting with other objects in real-time. Therefore, the architecture ought to be adjusted to dynamic interactions while maintaining a decentralized and heterogeneous nature [35; 36]. In the Internet of Things, service-oriented architecture (SoA) may be necessary for both service providers and users. SoA is able to provide interoperability between disparate devices in many ways. Figure 1 shows the service-oriented architecture of the Internet of Things composed of four layers [37]:  
* The sensing layer is integrated with existing hardware entities to detect states and conditions (e.g., RFID Tags, Intelligent Sensors, RFID Readers, WSNs, Bluetooth Devices);
* The network layer is the infrastructure to relief over wireless or wired connections between objects (e.g., WSNs, WLAN, Social Network, Mobile Network);  
Figure 1: The service-oriented architecture of the Internet of Things [37]. * The service layer is responsible for creating and managing services required by users or applications (e.g., Business logic, Service Division, Service Integration, Service Composition, Service Implementation). Service platforms must reflect rules and policies governing the interactions and service exchange. Protocols are normative and guide actors/customers on how they can make use of available resources and value offerings. * The interface layer consists of the interaction methods with users or applications (e.g., Application Frontend, Contract, Interfaces, Application API). SoA treats a complex system as a set of defined subsystems or simple objects. Sub-systems or objects can be reused and maintained individually--software and hardware components can be reused and updated. Because of these advantages, the service-oriented architecture has found wide application in wireless sensor networks [37; 38]. Devices vary widely in terms of communication, the computation performed, memory and capacity for stored data, and transmission capability. An Internet of Things application consists of various devices. Therefore, all devices should be properly organized by the network and should be accessible. The Internet of Things can be an aggregation of heterogeneous networks such as wireless sensor networks, mobile networks, and Wireless Local Area Network (WLAN). By using these networks, many complex activities such as data exchange, computation, and decision-making can be performed. Reliable communication between the gateway and the objects is essential for the management process in the Internet of Things. The gateway can locally run an optimization algorithm. It does this by leveraging its knowledge of the network. The computational complexity is transferred from the object to the gateway. A global optimal route and parameter values for the gateway can be derived in this way. Currently, the following protocols and standards are most often used [37; 39]:  
* RFID
* NFC, IEEE 802.11, IEEE 802.15.4, IEEE 802.15.1
* mesh networks
* machine-to-machine (M2M)
* IP protocols  
WSNs can be defined as infrastructure-less and self-configured WSN designed to monitor physical or environmental conditions, collect data, and transmit it over the network for analysis.