# The Security of Big Data in Fog-Enabled IoT Applications Including Blockchain: A Survey
## 3 Fog Computing Architecture  
Fog computing provides decentralized on-demand services and applications for managing and analyzing big data directly at the network's edge. It provides storage, processing, controlling and networking analogous to cloud computing. The fog layer serves as an intermediary between IoT devices and the cloud. The term fog-enabled IoT system is not new and has already been used in the literature [29; 30; 31]. For example, [PERSON] [29] discussed some important architectural requirements for fog-enabled IoT systems along with a wide variety of potential IoT use cases, applicable to many vertical markets (i.e., transportation, utilities, smart cities, manufacturing, retail, agriculture, and health etc.). [PERSON] et al. [32] proposed a fog-enabled healthcare IoT system that is based on a three-layer architecture: sensor layer, fog layer and cloud layer. However, to the best of our knowledge, there is no standard architecture for fog computing [9]. We can broadly classify the fog-enabled IoT systems into three layers, as illustrated in Figure 2: IoT device layer, fog layer and cloud layer. For example, a fog-enabled IoT application of smart city collects data through various sensors, and offload some computations to the fog layer. This may also include the analysis and processing of delay-sensitive data at the fog layer. Moreover, if further computing and processing is required and it is not time crucial, the data can be sent to the cloud. In this work, we classified fog architecture into three distinct layers, which is inline with the architecture proposed in [16], as shown in Figure 3:  
* Core-network and service layer
* Data center layer
* Device layer with inter and cross layer communication technologies  
Figure 2: Fog-enabled IoT systems. The core layer is responsible for providing network, management and other services to the end users. It encompasses fog nodes, e.g., routers, bridges, gateways, switch, base stations, etc. They are assisted with computing resources and local servers entailing controllers, embedded computing, smart phones and cameras. Network connections are used to deploy these fog nodes wherever needed, for example on roadsides, factory floors, power stations and in smart phones and vehicles. Infrastructure providers deploy data centers, which are found on edge of the network. Data center layer is responsible for multi-tenant virtualization infrastructure, which is used for scalability, flexibility and for enhanced computation, storage and other resource sharing requirements to meet user demands. It is used to achieve data and IoT application isolation and security for a concurrent and independent processing. Although these data centers are interconnected with the cloud layer through the network infrastructure, they can work and cooperate autonomously. The computing and storage capacity increases as we move forward from device to cloud in chronological order. There are two types of devices involved at the device layer: Mobile-IoT and fixed-IoT devices [8]. Mobile devices encompass peripatetic devices such as smart watches, smartphones, trackers, etc., whereas fixed-IoT devices have specific functions and remain stationary at particular locations, such as sensors and RFID tags. They are constrained in terms of storage and computing resources with limited bandwidth [33]. Due to their limited characteristics, they are not suitable for responding to emerging events and occurrences and are mainly used for data collection and submission to a higher layer. Due to the underlying differences between cloud computing and fog computing, security solutions proposed for cloud computing may not suit fog services, which are available to end users at the edge of networks. This might impact the adaptation of fog-enabled IoT systems. Therefore, addressing security concerns at fog layer could enable fog paradigm to provide not only additional computational resources, but also adequate level of security to minimize cyber-attacks in fog-enabled IoT systems.