Industrial wireless networks are a key enabler of many aspects of advanced manufacturing. Wireless networks can be quickly deployed to transmit data to areas without existing cable infrastructures. Wireless technologies are ideal for highly flexible and efficient network connectivity for constantly changing hard-to-wire locations and worksite landscapes. Industrial wireless networks promise lower installation costs than wired alternatives, increased operational flexibility, improved factory visibility, and enhanced mobility.
This article discusses different types of wireless networking protocols used in industrial installations.
Industrial wireless technology
The critical objective of wireless communications networks must be to achieve similar capacities, bandwidths, responsiveness, and availability to wire-based communications systems. Various wireless technologies based on unlicensed spectrum are available for the industrial automation sector. Popular wireless communication technologies that are being applied to industrial applications are as follows:
ZigBee is a mesh-networking standard based on IEEE 802.15.4 radio technology targeted at industrial control and monitoring, building and home automation, embedded sensing, and energy system automation. It was developed as an open global standard to address the unique needs of easy implementation, high reliability, low-cost, low-power, and low-data rate wireless device networks. ZigBee operates the unlicensed bands, including 2.4 GHz, 900 MHz, and 868 MHz, at a maximum transfer rate of 250 Kbps, enough to satisfy sensor and automation needs using wireless. ZigBee also creates larger wireless networks, not demanding high data throughput. Two different device types can participate in a ZigBee network: Full-function devices (FFD) and reduced-function devices (RFD). FFDs can operate in three modes serving as a WPAN coordinator, coordinator, or device. RFD is only intended for simple applications, such as light switches. ZigBee supports three different topologies: star, mesh, and cluster tree, shown in Figure 1. The star topology establishes communication between devices and a single central controller called the Wireless Personal Area Networks (WPAN) coordinator. In a mesh topology, each of the network nodes, computers, and other devices, are interconnected with one another. The cluster-tree network is a special case of a mesh network in which most devices are FFDs, and an RFD may connect to a cluster-tree network as a leaf node at the end of a branch. Any FFD can act as a router and provide synchronisation services to other devices and routers. Only one of these routers is the WPAN coordinator.
Figure 1: Zigbee topology
Wireless HART was created to fulfill the existing gap in industrial wireless standardization. It was born as an extension of the widely used HART communication protocol. It is designed to be simple-to-use, self-organising and self-healing, flexible, reliable, secure, and supports the widely used HART technology. Wireless HART is a centrally managed mesh network. It is built upon the IEEE 802.15.4 physical layer and adds its own Datalink, Network, and Application Layer. Industrial security and authentication are reached through 128-bit AES (Advanced Encryption Standard)4 algorithms that cover end-to-end and hop-to-hop communications. Medium Access Control (MAC) is based on a TDMA schedule with frequency hopping. Reliability is achieved using methods of frequency diversity, path diversity, and message delivery retrials. Power Consumption can be efficiently optimised by proper management of the communications schedule.
Figure 2: Architecture of a Wireless HART network
Bluetooth has been considered as one alternative for WSN implementation. However, due to its high complexity and inadequate power characteristics for sensors, the interest in Bluetooth-based WSN applications has decreased. Bluetooth-Low-Energy specification is a part of the Bluetooth specification as an ultralow-power technology addressing devices with very low battery capacity. This extension to Bluetooth allows for data rates of up to 1 Mb/s over distances of 5–10 m in the 2.45-GHz band. Although Bluetooth Low Energy is like Bluetooth and can employ the same chips and antennas, it has some important differences. Bluetooth Low Energy has a variable-length packet structure compared to Bluetooth’s fixed length.
Figure 3: Industrial Bluetooth Technology
ISA 100.11a is an industrial wireless automation standard developed by the International Society of Automation (ISA). The corresponding IEC emerging standard is based on ISA-100 and is called IEC 62734. Unlike Wireless HART, ISA100.11a applies different mechanisms for channel hopping, such as slotted, slow, and hybrid channel hopping, to avoid collision with surrounding IEEE 802.11 networks. In addition, ISA100.11a includes backbone routers for bridging subnets, whilst Wireless HART uses access points. The compatibility of ISA100.11a with IPv6 in the network layer allows the users to connect to the internet, thus providing more options. ISA100.11a supports star and mesh network topologies and offers an interface for integration with Wireless HART.
Figure 4: ISA100.11a protocol
Ultra-wide band (UWB)
UWB or Ultra-Wide Band technology is based on IEEE 802.15.4 standard, which combines sensors and actuators into a single wireless network. It differs, however, in that UWB is allowed to operate at higher frequency bands and uses a wide spectrum bandwidth (500 megahertz or more). Since UWB devices operate in a spectrum that overlaps with preexisting allocations and uses, UWB devices also operate at very low transmit power limits to prevent interference. A UWB transmitter works by sending extremely short pulses across a wide spectrum channel; a corresponding receiver then translates the pulses into data by listening for a familiar pulse sequence sent by the transmitter. The advantages of UWB are good localisation capabilities, the possibility to share previously allocated radio-frequency bands by hiding signals under noise floors, the ability to transmit high data rates with low power, good security characteristics due to the unique mode of operation, and the ability to cope with multipath environments.
6LoWPAN stands for “IPv6 Over Low Power Wireless Personal Networks.” 6LoWPAN aims for standard IP communication over low-power wireless IEEE 802.15.4 networks utilizing IP version 6 (IPv6). In Ethernet links, a packet with the size of the IPv6 Maximum Transmission Unit (MTU) (1280 bytes) can be easily sent as one frame over the link. In the case of 802.15.4, 6LoWPAN acts as an adaptation layer between the IPv6 networking layer and the 802.15.4 link layer. It solves the issue of transmitting an IPv6 MTU by fragmenting the IPv6 packet at the sender and reassembling it at the receiver. 6LoWPAN also provides a compression mechanism that reduces the IPv6 header sizes sent over the air and thus reduces transmission overhead. The fewer bits that are sent over the air, the less energy the device consumes. Thread fully uses these mechanisms to transmit packets over the 802.15.4 network efficiently.
Figure 5: 6LoWPAN architecture
Wireless technology can introduce a completely new range of industrial applications as it has the potential to provide fine-grained, flexible, robust, low-cost, and low-maintenance monitoring and control. It can also improve the productivity of industrial systems by providing greater awareness, control, and integration of business processes.