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In Inter-networked of Things, stake holding technologies expect trillions of Sensors, billions of Smart Systems, and millions of applications in near future. There are numerous supportive technologies with IoT to perform smarter than before. IoT Associative Technology can be classified into four sub-topics, namely,

 (i) Sensor and Actuators,

 (ii) IoT Networking,

 (iii) IoT Connectivity Technologies, and

 (iv) IoT Communication Protocol.

 (i) Sensor and Actuators: Sensor and Actuators are the most essential and core components of the IoT. As per oxford dictionary, the meaning of sensor is “It is a device which detects or measures a physical property and records, indicates or otherwise responds to it.” [31]. So, sensors basically sense the physical observable fact around us from an environment. As per the other sources [32], sensors can be defined as “A sensor detects (senses) changes in the ambient conditions or in the state of another device or a system, and forwards or passes this information in a certain manner”. According to this definition, a sensor can sense or detect the physical phenomena or measured properties such as temperature, humidity, smoke detection, and obstacle detection. So, we have different specific sensors that can be used to sense particular properties and cannot be used to sense or to detect, or be insensitive to the other properties surrounding us, i.e., specific physical properties can only be detected by specific sensors not bothering about other properties surrounding us. For example, a temperature sensor can sense heat (temperature) around us and then these sensed values are converted into its equivalent electrical signals.The smallest change that can be detected and can be measured by a sensor as an output is known as resolution. Based on the output, the sensor can be classified into two categories: Digital Sensor and Analog Sensor. Analog Sensor can generate or produce a continuous output signal equivalent to continuous measured property in nature; e.g., temperature, humidity, pressure, and speed are analog quantities. While Digital Sensor produces binary output (0 or 1, ON or OFF) signal. So, it generates a non-continuous (discrete) value in the form of bits that combine to gather generated byte as an output. Based on the output data types, sensors can be classified into two major groups: Scalar Sensor and Vector Sensor. Scalar Sensor generates output proportional to quantity measured from surroundings without considering its orientation or direction, e.g., physical quantity such as temperature and pressure. Vector Sensor generates the output that is proportional to quantity measured as well as its orientation or direction, e.g., physical quantities such as sound and velocity.Based on sensed information from sensors, actuators basically perform some actions (actuates) on the physical environment. So, here, actuators take actions based on what has been sensed and in that way controls a system that can be acted upon an environment. In this context, the actuators require some control signal and source of energy to function further. So, when actuators receive such control signals, they convert the energy into mechanical motion. Based on their functional domains, we have three broad categories of actuators such as pressure-based actuators (hydraulic and pneumatic), electric-based actuators (electrical, thermal, and magnetic), and mechanical-based actuators. Other than these types of actuators, other popular actuators are used in industries. Agriculture uses Soft actuators. Soft actuators are polymer-based actuators designed to handle delicate objects and used in robotics.“Transducer” is another associative term which can be used for both Sensors and Actuators [33]. So, actuators sense the surroundings in the form of information and are converted into electrical signals; such control signals are received by actuators and action is taken accordingly. For example, in “soil moisture and water level monitoring application”, agriculture soil water/moisture level in a farm is sensed by specific sensors, is converted into electrical signal, and is provided to the actuator as “solenoid valve”. Solenoid valves consist of a mechanism that allows or stops the water flow. So, depending on the electrical signal received from the sensor (water/moisture level), this solenoid valve as an actuator can actuate, i.e., flow water or stop water.

 (ii) IoT Networking: IoT Network consists of several components such as Device (The Thing), Local Network, Internet, Backend services, and Applications. Here, in case of “Device”, it consists of a collection of sensors and actuators that can act as one component in the entire IoT Network. These become different nodes in the IoT Network that can be communicated with each other. As shown in Figure 1.3, a node in IoT Network can be communicated with other target node via another component of IoT network, i.e., Local Network. If target node does not belong to the local network, IoT network will search it through another component of IoT network, i.e., Internet. In Backend services, the data may be received from local networks or from the internet and perform complex analysis using different machine learning algorithms.Such result generated after complex analysis is given to applications that serve as an output of IoT Network. Thus, IoT is a very complex system that involves things (sensors and actuators), local area network, wide area network (internet), machine learning, and analysis algorithms which act mutually into one system entity.Such result generated after complex analysis is given to applications that serve as an output of IoT Network. Thus, IoT is a very complex system that involves things (sensors and actuators), local area network, wide area network (internet), machine learning, and algorithms which act mutually into one system entity.So, to perform function through IoT we need more associative technology such as Bigdata, M2M communication, cloud computing, Cyber Physical System (CPS), 3G/4G/5G, and Internet of Vehicles (IoV). To perform suitable communication among such heterogeneous technologies and devices, we need to deal with certain challenges of IoT. They are securities, interoperability, scalability, energy efficiency, and interfacing. IoT connectivity technologies are involved in IoT communication to execute it properly. They are as per the sub topics given below.

 (iii) IoT Connectivity Technology: Connectivity among devices is fundamental when we think about the IoT. There are several IoT connectivity technologies in the form of communication protocols that utilize IoT networks to perform communication between IoT devices (Things). IoT service offering protocols such as RFID (Radio Frequency Identification), CoAP (Constrained Application protocol), XMPP (Extensible MessagingFigure 1.3 IoT networking.Presence Protocol), MQTT (Message Queuing Telemetry Transport), AMQP (Advanced Message Queuing Protocol), and 6LoPAN (IPv6 over Low-Power Wireless Personal Area Networks) are basically utilized to establish connectivity between IoT devices in IoT network.RFID stands for Radio Frequency Identification that is used widely in shopping malls as a system whole. RFID system consists of RFID tag, RFID reader, and RFID software. RFID tag is covered by a hard jacket that consists of integrated circuit and antenna and stores digitally encoded data. RFID tags are categorized into Active Tag (own power supply) and Passive Tag (dependent for power supply). RFID reader reads from the tag and transfers data to RFID software for further processes to operate.CoAP, as per its name Constrained Application Protocol [13], is utilized for web transfer just as http but in constrained networks resources environment such as limited computational resources, limited bandwidth and limited power supply in IoT. CoAP in IoT network functions as a session layer and an application layer. CoAP is designed for M2M communication and uses a request-response model for two connected endpoints (objects) in the IoT network.XMPP stands for Extensible Messaging and Presence Protocol [06], an open standard XML (extensible markup language)–based middleware protocol that is used for real-time structured data exchange. XMPP uses decentralized client-server architecture which means the central server is not located for message transfer. So, in this context, XMPP provides flexibility in sustaining interoperability between different things (objects), between diverse systems, and between heterogeneous protocols in the IoT network. XMPP does not support text-based communication.MQTT is a Message Queuing Telemetry Transport protocol [7], publish-subscribe–based ISO standard protocol. So, in this protocol, publisher publishes the data that can be utilized by the subscriber and this phenomenon creates this protocol as a lightweight protocol that can be used in combination with TCP/IP protocol. In MQTT, there is a central entity known as “broker” which is responsible for transferring messages from sender to receiver. Here, client publishes a message to the broker including topic (routing information for broker). Based on the matching topic, the broker delivers the message to the client. So, in this architecture, client does not know the real message passing entity, this feature provides a highly scalable solution independent of data producer and data consumer. MQTT is used by Microsoft Azure, Amazon Web Services, Facebook Messenger, and Adafruit for providing various services.AMQP stands for Advanced Message Queuing Protocol [14] and is ISO/IEC-based open standard protocol for passing business messages between different business applications or organizations. At the time of passing business messages, AMQP is persistent and provides three different types of message delivery guarantees. They are At-most-once (message delivered once or never), At-least-once (message certainly delivered may be multiple times), and Exactly-once (certainly delivered and only once).6LoPAN is an IPv6 over Low-Power Wireless Personal Area Networks [15]. Due to large components involved in the IoT network, unique address identification can be done through IPv6 (64 bits) address protocol instead of IPv4 (16 bits) address protocol. This protocol provides transmission of data wirelessly with limited data processing potential in PAN. So, as per its name, it permits low-powered devices to connect to the internet which is also a basic characteristic of IoT networks.

 (iv) IoT Communication Protocol: Other well-known communication protocols that require to perform communication in IoT network are IEEE 802.15.4, Zigbee, Z-Wave, Wireless-HART (wireless sensor networking technology based on the Highway Addressable Remote Transducer Protocol), Near-Field Communication (NFC), and Bluetooth as given below.IEEE 802.15.4 is an extensively used standard protocol for establishing communication in the IoT network [9, 10]. It provides a framework for lower layers such as physical layer and Mac layer to a small range of Personal Area Network (PAN) and Wireless Personal Area Network (WPAN) that generally range from 10 to 75 meters in the environment of low-power, low-speed, and low-cost requirements. It uses star and peer-to-peer network topologies for establishing communication between neighboring devices in the IoT network.Zigbee is an enhanced version of IEEE 802.15.4 that functions on top of layer 1 and layer 2 of IEEE 802.15.4 in layer 3 and onwards [11, 12]. So, Zigbee uses the MAC layer to the application layer in the IoT Network. Zigbee is basically used for Wireless Sensor Network (WSN) and supports stat and mesh topology. In Ad-hoc network, Zigbee utilizes Ad-hoc On-demand Distance Vector (AODV) Protocol for broadcasting a route request to all its immediate neighbors. Such neighbors spread this message to their neighbors and, in that way, messages can be spread all the way through the IoT network. One of the important applications utilized by Zigbee is “Building Automation”. Other applications are healthcare monitoring, home energy monitoring, LED lighting monitoring, telecom services, and many more.Z-Wave is a well-known protocol for home automation to do different functions using various IoT devices. It functions on mesh topology that can have up to 232 nodes (devices) in a network and uses radio frequency for communication, i.e., signaling and controlling of home automation IoT devices. In a home, there is a Z-Wave controller that controls the signal communication with existing other Z-Wave nodes (devices). Such Z-Wave devices may communicate directly with each other or they can communicate via Z-Wave controller in smart home automation systems.Wireless-HART is a wireless sensor networking technology based on the Highway Addressable Remote Transducer Protocol [16] that is developed for networked smart field devices. IoT implementation and cost of performing communication between IoT devices will be cheaper and easier using HART. There are certain differences between physical HART and wireless HART in the context of physical layer, data link layer, and network layer. HART physical layer utilizes IEEE 802.15.4 protocol. HART data link layer has a provision of a super frame that ensures suitable communication between different IoT devices (nodes) of the IoT network. Wireless HART network layer uses mesh topology for communication in IoT networking. Wireless HART protocol network layer can be composed of OSI Network layer, transport layer, and session layer. HART application layer is responsible for generating responses by extracting commands from messages and executing them. So, the basic difference between Wireless HART and Zigbee is that Zigbee hops when the entire network hops but Wireless HART hops after every message.Near-Field Communication (NFC) is designed for use of devices in its close proximity and uses magnetic induction principle just as RFID [17]. Based on power/energy resource availability, NFC has two types, viz., Active NFC and Passive NFC. Active NFC does not depend on external power/energy resources and Passive NFC depends on external power/energy resources. Like RFID, NFC also has three components: reader, tag, and software. NFC reader creates magnetic fields using electric current that connects the physical space between these two devices, NFC reader and NFC tag, and can transmit encoded information from NFC tags such as identification number. NFC can be operated in three different modes much as peer-to-peer, read/write and card emulation. For example, in peer-to-peer mode, two smartphones can communicate with each other. In read/write mode, one active and one passive device is involved to perform communication and in card emulation NFC can be used for contactless credit card operation.Bluetooth is a wireless short range communication technology that is heavily used in establishing communication in IoT network devices in PANs [18]. Bluetooth can be utilized to perform communication between two smart phones for transferring data to short range. Bluetooth uses ad-hoc technology known as Ad-hoc Piconets. Connection establishment in Bluetooth can be possible in sequence using three different phases such as Inquiry, Paging, and Connection. In the “Inquiry” phase, Bluetooth devices discover other Bluetooth devices near it. After finding a Bluetooth device nearer to the current device, in this second phase “paging”, connection can be established between these two devices. In the third phase of “connection”, either devices can actively participate in the network or enter into low-power sleep mode.After discussing IoT associative technology such Sensor and Actuators, IoT Networking, IoT connecting technology, and IoT communication protocol, the important characteristic of IoT devices and technologies that make all this possible is “Interoperability in IoT” as discussed in the next topic.