ADI Wireless Sensor Networks for the Industrial Internet of Things

The Industrial Internet of Things (IoT) and the related wireless connectivity requirements for industrial sensors are constantly changing and evolving. However, the networking requirements for industrial equipment and applications are very different from those in the consumer field. Reliability and security are the top priorities for industrial IoT. This white paper focuses on some of the key network requirements for industrial wireless sensor networks.

The advent of low-power processors, smart wireless networks, low-power sensors, and “big data analytics” has sparked a surge of interest in the Industrial Internet of Things (IoT). Simply put, the combination of these technologies has enabled a large number of sensors to be placed anywhere: not just where communications and power infrastructure is located, but anywhere that can collect important information about how, where, or what an object is doing. Equipping machines, pumps, pipelines, train cars, and other objects with sensors is nothing new in the industrial world. A large number of specialized sensors and networks are deployed in industrial environments ranging from refineries to production lines. In the past, these operational technology (OT) systems have been run as separate networks, maintaining high standards for network reliability and security that consumer technologies simply cannot meet. Screening the applicable technologies against these high standards ultimately selects the technologies that are best suited for mission-critical Industrial IoT applications. In particular, the way these sensors are networked determines whether they can be deployed safely, reliably, and cost-effectively in the harsh environments of industrial applications. This white paper explores some of the key requirements that make industrial wireless sensor networks (WSNs) unique.

Reliability and safety are the most important

For consumer applications, cost is often the most important consideration, while industrial applications generally prioritize reliability and security. According to a survey of industrial WSN users worldwide conducted by ON World, reliability and security are the two most important issues they mentioned.1 A company’s profitability, the quality and efficiency of workers’ production of goods, and the personal safety of workers often depend on these networks. This is why reliability and security are essential for industrial wireless sensor networks.

1 Industrial Wireless Sensor Networks: Trends and Developments

Figure 1. Sensors are everywhere. Low-power wireless sensor nodes powered by energy harvesting systems, such as this wireless temperature sensor from ABB powered by harvested thermal energy, can be placed at appropriate locations to gain more data about industrial environments.

A general principle for improving network reliability is to provide redundancy and set up fault recovery mechanisms for possible problems so that the system can resume operation without losing data. In wireless sensor networks, redundancy is used in two ways. The first is the concept of spatial redundancy, that is, each wireless node can communicate with at least two other nodes, and the routing mechanism allows data to be forwarded to either of the two nodes and still reach the intended final destination. In a properly designed mesh network, each node can communicate with two or more adjacent nodes, and if the first path is unavailable, it will automatically switch to another path to send data, so mesh networks have higher reliability than point-to-point networks. The second redundancy can be achieved by using multiple available channels in the RF spectrum. The concept of channel hopping means that pairs of nodes can use different channels each time they transmit data, so in the ever-changing and harsh RF environment faced by industrial applications, temporary problems in any given channel will not affect data transmission. In the IEEE 802.15.4 2.4GHz standard, there are 15 spread spectrum channels available for frequency hopping, making channel hopping systems more resilient than non-hopping (single channel) systems. This dual spatial and channel redundant time-slotted channel hopping (TSCH) technique is used in several wireless mesh networking standards, including IEC62591 (WirelessHART) and the upcoming IETF 6TiSCH standard.2 These mesh networking standards utilize radio frequencies in the globally available unlicensed 2.4 GHz spectrum and are derived from the work done by the Analog Devices SmartMesh team, which pioneered the TSCH protocol for low-power, resource-constrained devices starting with the SmartMesh product in 2002.

Although TSCH is essential to ensure data reliability in harsh RF environments, the way the mesh network is established and maintained is also critical to achieving continuous, trouble-free operation for many years. Industrial wireless networks often have to operate for many years and will face many different RF challenges and data transmission requirements throughout their lives. Therefore, to have the same reliability as wired networks, they must also be equipped with intelligent network management software that can dynamically optimize the network topology, continuously monitor link quality, and be able to cope with interference and RF environment changes to maximize throughput.

Security is another key feature of industrial wireless sensor networks. The main goals of implementing security in WSNs are:

Confidentiality: Data transmitted over a network cannot be read by anyone other than the intended recipient.

Completeness: Confirm that any information received is exactly the same as the information sent, without any additions, deletions or modifications.

Authenticity: Asserts that information from a given source actually came from that source. Authenticity also protects information from being recorded and replayed if time is used as part of the authentication scheme.

Key security technologies that must be incorporated into WSNs to achieve the above goals include: strong encryption algorithms (such as AES128) with reliable keys and key management, cryptographic-grade random number generators to prevent replay attacks, message integrity checks (MICs) for each message, and access control lists (ACLs) that explicitly allow or deny access to specific devices. These advanced wireless security technologies can be easily integrated into many devices used in today's WSNs, but not all WSN products and protocols use all security technologies. 3 Note that the connection between a secure WSN and an insecure gateway is another weak point, and end-to-end security must be considered in system design.

Industrial IoT is not installed by wireless experts

Most mature industries add IIoT products and services on top of traditional products, and these customers have both old and new devices in their deployment environments. The intelligence embodied in industrial WSNs must make IIoT products easy to use, achieve a seamless transition, and allow existing field staff to easily use new IIoT products. The network should be able to self-build quickly so that installers can deliver a stable network; avoid service interruptions through self-healing when the connection is weak or lost; perform self-service reporting and diagnosis when service interruptions occur; and generally require little or no maintenance after deployment, thus avoiding the high costs caused by on-site maintenance. The success or failure of many applications depends in part on whether they can be deployed in hard-to-reach or dangerous areas, so IoT devices must run on batteries and generally need to run continuously for more than 5 years.

Additionally, since widespread adoption of the Industrial Internet of Things by end users often spans the entire company, the system should be available for global deployment and multi-site standardization is required. Fortunately, international industry wireless standards that understand and meet this requirement are already in place, including IEEE 802.15.4e TSCH.

Sensors are everywhere

For industrial IoT applications, accurate placement of sensors or control points is critical. Wireless communication is possible with wireless technology, but deployment costs are prohibitive and impractical if wireless nodes need to be powered by plugging into an electrical outlet or recharging every few hours or months. For example, equipping rotating equipment with sensors to monitor the equipment's operating condition is not possible using a wired connection, but the information gained by monitoring the equipment in operation allows customers to perform predictive maintenance on critical equipment, thereby avoiding unnecessary and costly downtime.

To ensure flexible and cost-effective deployment, each node in an industrial WSN should be able to run on batteries for at least 5 years, which provides great flexibility to users and expands the coverage of industrial IoT applications. As an example of an industrial TSCH WSN, Analog Devices' SmartMesh products typically operate at a current well below 50µA, so they can run for many years on two AA batteries. If there is abundant storage energy in the surrounding environment, the wireless nodes can also achieve continuous operation through energy storage (see Figure 1).

Figure 2. Network visibility—Network management software provides critical information about the health of your wireless network.

For example, this SNAP-ON software utility from Emerson Process Management

2 6TiSCH Wireless Industrial Networks: Deterministically Meeting IPv6 Standards: Maria Rita Palattella1, Pascal Thubert2, Xavier Vilajosana3,4, Thomas Watteyne4,5, Qin Wang4,6, Thomas Engel1 Published in: IEEE Communications Magazine (Vol. 52, No. 12).

3 Protecting Wireless Sensor Networks from Attacks, Kristofer Pister and Jonathan Simon,