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.
1.4 Interoperability in IoT
In IoT, many heterogeneous devices, protocols, operating systems have to work together to fulfill objectives. This heterogeneity is one of the major concerns when we perform communication in the world of IoT as it requires not only anytime, anywhere but also anything enabled to communicate. “Interoperability” is a characteristic of a product or system whose interfaces are completely understood to work with other products or systems without any limitations. Interoperability is must when we would like to communicate in the era of IoT that contains heterogeneous devices [19]. So, by maintaining interoperability in the IoT network, we can have exchange of data and service in a seamless manner. In this seamless exchange of data and service, many elements are involved and perform the communication such as physical objects can communicate with other physical objects.
As per the overall goal of IoT, anytime anywhere anything (device) can be communicated with other devices, i.e., can do Device-to-Device (D2D) communication. More than these types of communication, others such as Device-to-Machine (D2M) communication, M2M communication should also be performed seamlessly in the IoT network. Hence, in this situation, the IoT network has to deal with many types of heterogeneity such as heterogeneity of different wired and wireless communication protocols. Moreover, different programming languages are used for different platforms as well as different hardwares that also vary different standards and support different languages and communication protocols. So, if we would like to perform seamless communication between such corel, heterogeneous connected components, protocols, languages, operating systems, databases, and hardwares, then interoperability among them is a must.
There are basically two types of Interoperability such as User Interoperability and Device Interoperability. User Interoperability is an interoperability problem between user and device(s) and Device Interoperability is an interoperability problem between two different devices. User interoperability problems occur when remotely located users would like to communicate with other device(s) whose product id may be written in different language, there may be differences in user syntaxes, differences in user semantics, as well as differences in user specification for those devices. So, all these