# Fog Computing Enabling Industrial Internet of Things: State-of-the-Art and Research Challenges
## 1 Introduction  
Revolution in any realm is required with the passage of time. Every field changes to go forward with better solutions dealing with the challenges of the era. Industrial Internet of Things (IIoT) is revolutionizing the classical communication methodologies. With the emergence of smart devices (mobile, machines, sensors) coupled with a diverse range of applications requirements, IIoT is the way forward. It is expected that 26 billion IoT devices of heterogeneous capabilities will be installed to perform functions with different Quality-of-Service (QoS) requirements by 2020 [1]. IIoT gives rise to 4_th_ industrial revolution based on Cyber-Physical Systems (CPS) with the need arising back in 2015 originated basically in Germany [2]. Industry 4.0 defines diverse use cases ranging frominterconnected digital technologies, CPS, Mobile Cloud Computing (MCC) and Internet of Things (IoT) for promoting the whole industry in terms of efficiency, effectiveness, supporting heterogeneous data, higher production, automation, and integrating knowledge [3]. These key enabling technologies have been deployed to some extent in industrial domains such as healthcare, transportation, smart cities, micro-grids, and smart factory. This trend gives rise to intelligent, distributed and self-organizing solutions to support these application domains. Deploying industry 4.0 involves three-layer implementation; physical layer, network layer, and intelligent-application layer [4]. The physical layer comprises identification and location awareness entities i.e. actuators, sensors, and terminal devices; the network layer comprises of the development of a network that can support industrial automation, network can be cellular, indoor, cloud or private. Factory automation and coordination are processed on the application layer. Infrared (IR), Radio-Frequency Identification (RFID), Bluetooth, 6 LoWPAN, IEEE 802.11 af, IEEE 802.11 a/b/n/ac for short range connectivity; Ultra-Wideband (UWB), cellular (2G, 3G, 4G, LTE-MTC, 5G), Sigfox, Long range (LoRa) for long range connectivity, are a few of the majorly used communication standards for IIoT [5; 6]. The future of automation is based on decentralized intelligence in which all machines can communicate with one another to arrive at independent or consensus inference, called Machine-to-Machine (M2M) communication. These decentralized intelligent solutions play a vital role in industry 4.0 digital transformation. The decentralized solutions provide flexibility and quick decision assistance over centralized solutions. For M2M communication, 802.11 ah technology has evolved in the recent past. Exchanging machine data demands real-time communication ensuring latency, security, reliability, bandwidth and privacy measures in all IIoT domains. To satisfy these critical requirements, there is a need to explore new enabling solutions that support these applications. In the future, 5G cellular technology will support such heterogeneous networks with massive number of IIoT devices. It is anticipated that future 5G networks not only provide flexibility but can optimize the usage of available resources of bandwidth, power, energy, connectivity to different applications at the same time [7]. In the last decade, computation and processing requirements of end users have increased exponentially. It has become increasingly challenging for designers to scale the processing and data storage capabilities for users within the given device size and battery constraints. To meet these growing requirements, researchers have come up with the solution to offload services to a centralized location known as the cloud. Cloud computing is an alternative for data computation, storage and management. It supports intensive computation and manages heterogeneous devices of next generation networks [8; 9; 10]. Additionally, cloud computing architecture involves the direct connection between devices and the cloud server.