Industry 4.0 Vision for the Supply of Energy and Materials. Группа авторов

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radio environments to detect availability of channels. Subsequently, RRM is performed on the clear channels through optimizing proper channel features for data transmission [174, 175]. In addition to reliable link, RRM involves strategies for controlling power transmission, which is particularly important for IIoT devices working on batteries.

      1.6.1.3 Protocol Interface Design

IIoT involves diverse industrial assets such as manufacturing control systems, industrial process controllers, devices, and components. These controllers and equipment are distributed throughout an industrial site and use both proprietary and open protocols for communication. Basically, proprietary protocols belong to a particular product line within the systems [176, 177, 178], while open protocols are governed by standard organizations and utilized across products. Considering the advent of communication technologies and various protocols for both proprietary and open standards, there are technical challenges in the interconnection of different industrial protocols and assets. This calls for the creation of systems and techniques that integrate different protocol interfaces in manufacturing and industrial processes and that allow the interoperation of devices, equipment, and services from various vendors and operators. In this context, Open Platform Communication-Unified Architecture (OPC-UA¹⁶) is a possible solution that offers interconnection and easy integration of newer standards (wired and wireless) and middleware technologies in Industry 4.0 [179]. Moreover, a communication interface for IIoT is proposed in [180] that provides communication among programmable controllers and devices for operating and monitoring processes and equipment.

Design of protocol interface could be discussed in horizontal and vertical directions [55]. Figure 1.1 indicates the concept of integration in Industry 4.0. Typically, horizontal protocol interfaces provide interconnectivity among nodes to meet certain network functionalities such as clock synchronization among nodes. Time-sensitive network (TSN) protocols are examples of such interfaces that provide real-time connectivity for mission-critical and time-sensitive use cases [162, 181]. On the other hand, vertical protocol interfaces assist in secure data and service integration and offer consistent information flow via protocol stacks [55]. IETF has released encapsulation and compression techniques that unify such operations for devices [182]. 6LoWPAN is an example of networking technology from IETF that determines fragmentation mechanism for IPv6 headers [183].

      1.6.2 Metrics for Wireless System Design in IIoT

      The autonomous communication in the future IIoT environment significantly relies on wireless networks. There are different wireless technologies competing with each other for various industrial services and use cases. Since network connectivity should be robust and efficient, crucial challenges are raised in network design and integration of automation and control systems. Furthermore, real-time analysis of IIoT systems is a major issue for smart environments and services. In designing a system, we need to actively evaluate system performance and its efficiency. Multiple factors such as outage probability, power consumption, and spectral efficiency could verify the performance and efficiency of IIoT wireless design. These metrics can be categorized into two broad classes, namely, resources and services indicators.

      1.6.2.1 Resource Indicators

      Wireless solutions in IIoT should specify the required resources to create connectivity within the systems. To identify the level of network resource utilization in wireless systems, a set of metrics are introduced as resource indicators: spectrum, time, power, and network computing [55].

      1 Spectrum: In the frequency domain, the wireless channels are identified by the channel bandwidth. Given that the data rate of each wireless channel is related to its bandwidth [184], different services in IIoT environment are highly dependent on spectrum resources to accommodate the desired data rate. For instance, in mission-critical communications such as safety systems, the radio spectrum resource should be assigned properly for effective deployment (i.e., high data rate and low latency) [185]. Given that spectrum resources are shared in wireless domain for a large variety of applications, wireless networks suffer from spectrum scarcity. This highlights the need for communication protocol interoperation within different frequency bands and channels [186].

      2 Time: Wireless spectrum resource usage could be defined in terms of the transmission time and length. Time manages the temporal operation of wireless resources based on the desired performance such as QoS [187]. In this context, the access time of channel spectrum could be considered a resource indicator to be optimized for wireless networks.

      3 Energy consumption: Power consumption of wireless nodes at transmission side is closely dependent on transmission range, quality of transmission channel, and transmission power at RF front ends. The aforementioned resources are essential in the design of a wireless system. Various energy-efficient resource allocation techniques (with given QoS constraints) are proposed to assist in design of wireless systems for IIoT.

      4 Computing resources: Network resources are in both hardware and software types (physical and virtual). Basically, computing resources include all network resources tied up to the aforementioned resources (i.e., spectrum, time, and energy consumption). For instance, in TDMA-based transmission, different communication links are assigned to users at different time slots and through computing resources.

      To achieve the diversity gain for different applications in IIoT, the resource space is used in multiple dimensions. For example, a wireless solution could exploit a TDMA scheme in the multi-antenna wireless systems, while both power and spectrum are considered in the system design.

      1.6.2.2 Service Indicators

      In addition to wireless network resources that specify the cost to build connections, service performance highly affects the design of IIoT networks. Several service measures are identified to indicate the level of service satisfaction (while ensuring QoS), and they are extensively utilized in formulating wireless design. Service performance factors that form a set of basic metrics are data throughput, delay, coverage, and scalability.

      1 Data throughput: Service requirements are specified by several data rate metrics such as real-time throughput, aggregated throughput, raw data rate, good throughput (goodput), peak and minimum throughput, average throughput, and instantaneous data rate. Throughput is the key indicator for high-performance services such as autonomous driving and robot-assisted surgery.

      2 Delay: It specifies the latency over the communication link and is composed of the transmission delay, queuing delay, and jitter. As a service indicator, delay defines the speed of data delivery across networks and verifies the timeline for data packets transmission. This is an essentially important metric for mission-critical industrial applications such as real-time control and safety that are featured by stringent latency requirements.

      3 Coverage: It is defined as the wireless network range that connectivity is provided for services. Generally, the worst-case channel for all terminals in the coverage area is considered as the performance metric of wireless network.

      4 Scalability: It stands for the number of nodes that could be connected to the wireless network. Given that there are various IIoT services with different levels of scalability, this performance metric is highly associated with the traffic load of network.

      Generally, the combination of multiple indicators forms a composite service metric in wireless systems, and the selected indicators are optimized for the given service requirements.

      1.7 MAC Protocols in IIoT

Industrial environments have some special properties СКАЧАТЬ