IoT reference architecture is discussed and evaluated by research bodies. The concept of IoT is surrounded by facilitating inter-communication between diverse heterogeneous objects from different manufacturers and vendors. The basis of IoT communication is classified as thing-to-thing communication, thing-to-human communication, and human-to-thing communication. The reference architectures are broadly categorized as Three-layer models (a), Middleware base models (b), Service-Oriented models (c), and Five-layer models (d).
IoT reference architecture (A.-F. Ala, et al. 2015)
The application layer or the business layer addresses the human interaction with the IoT ecosystem. Mobile applications or interactive web interfaces belong to this layer to facilitate, human to thing and thing-to-human communication. Five-layer reference model decouples the application layer as application layer and business layer where the business layer address the user interaction and the application layer facilitates the functionalities.
The network layer is a significant layer of the home automation ecosystem. This layer contains device communication, including device-to-device communication, machine-to-human communication and human-to-device communication. The depth of the layer is described differently based on the architectural model. For example, the middleware base model subdivides the layer into three sub-layers: Middleware Layer, Coordination Layer and Backbone Network Layer.
The perception layer represents the IoT devices as a whole. This layer defines the firmware layer and objects' connection mechanisms used in IoT devices, divided into further layers based on the architectural model. Five layers architectural model contains the object abstraction layer, which is placed between the object later and the service management layer. The object abstraction layer is a separate layer that addresses data transportation for heterogeneous sensor networks such as LoraWAN, Wi-Fi, ZigBee, Bluetooth, GSM, 3G, WiMAX and infrared. The layer is not visible to the model when one sensor technology is used for the implementation.
The service management layer, the middle layer represents the same functionality between different architecture models. This layer provides a programming interface for IoT developers to interact with the devices and their functionality without programming the device itself. The middle facilitates accessing device data, control flows, controls and data transmission functionalities.
Moreover, the explanation for IoT architecture (Yang, et al. 2011) defines as having any object interconnected with another in terms of information exchange, communication to rectify intelligence, monitoring and collecting environment information, and identification of physical features such as orientation, location, and relativeness based on agreed confirmation among objects.
The object layer, aka the perception layer, defines the first layer of the ecosystem. The main objective of the layer is to establish things that contain physical sensors, smart consumer devices, and any other electronics end devices enabled by firmware. These devices and things consider as low-power consuming objects with minimum or limited processing capabilities. Minute scale devices communicate with the abstract object layer maintaining a longer operating lifespan due to low power consumption. The everyday activities of sectors are collecting and transmitting light level, temperature, weight, or conducting actuation or rotational based on provided instructions. Vendor agnostic devices should work together to expand the IoT network while ensuring security measurements and controls among heterogeneous objects.
The network layer of IoT lies across the existing mobile network (GSM) and internet communication network. The objective of the layer is to exchange information between perception later and the application layer. Communication may extend to long distances as well as the cloud. The IoT network's speciality can contain multiple technologies such as Wi-Fi, ZigBee, RFID, and TCP/IP connectivity. Depending on the IoT ecosystem, the system can be extended to cloud services as well and also act as an intermediate for everyday functions.
The application layer facilitates the thing to human communication in the ecosystem along with user interaction. Users engage with the layer to communicate and read information about the perception layer. As an example, users might read the temperature and offer a command to turn on/off the air conditioner. The application layer can trigger communication based on the predefined rules without end-user engagement. The application layer exposes interaction API such as REST or SDKs to maintain open integrity and engage application developers. Moreover, the visibility of the IoT system is exposed via this layer.
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