About those key requirements that make industrial wireless sensor networks special!

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

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

The advent of low-power processors, smart wireless networks and low-power sensors, and “big data analytics” has sparked intense interest in the Industrial Internet of Things (IoT). In short, combining these technologies allows a multitude of sensors to be placed anywhere: not just where communications and power infrastructure is located, but anywhere where vital information about how, where, or what an object is behaving can be gathered. The practice of equipping machines, pumps, pipes, train cars, etc. with sensors is nothing new in industry. A wide range of specialized sensors and networks are deployed in a variety of industrial environments, from refineries to production lines. In the past, such operational technology (OT) systems operated as separate networks, maintaining high network reliability and security standards that consumer technologies simply could not meet. Applicable technologies are screened against these high criteria to ultimately select the technologies best suited for mission-critical IIoT applications. In particular, how these sensors are networked determines whether they can be deployed safely, reliably, and cost-effectively in the harsh environments of industrial applications. This article explores some of the key requirements that differentiate industrial wireless sensor networks (WSNs).

About those key requirements that make industrial wireless sensor networks special!
Figure 1. Sensors are everywhere.Low-power wireless sensor nodes powered by an energy harvesting system, such as this wireless temperature sensor from ABB, powered by harvested thermal energy, can be placed in place to obtain more industrial environmental data

Reliability and safety are the most important

For consumer applications, cost is often the most important consideration, while industrial applications typically prioritize reliability and safety. According to ON World’s survey of global industrial WSN users, reliability and security were the two most important issues they cited. The profitability of a company, the quality and efficiency with which workers produce goods, and the safety of workers often depend on these networks. This is why reliability and security are essential for industrial wireless sensor networks.

A general principle for improving network reliability is to provide redundancy, and to set up failover mechanisms for possible problems so that the system can resume operation without losing data. In wireless sensor networks, there are two ways to utilize redundancy. The first is the concept of spatial redundancy, where each wireless node can communicate with at least two other nodes, and a 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 automatically switches to another path to send data, so the mesh network is similar to point-to-point. Compared with the network, it has higher reliability. The second type of redundancy can be achieved using multiple channels available in the RF spectrum. The concept of channel hopping refers to the fact that pairs of nodes can use a different channel each time they transmit data, so in the ever-changing and harsh RF environment faced by industrial applications, a temporary problem with 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-frequency hopping (single channel) systems. Several wireless mesh networking standards use this dual-redundant time-slot channel-hopping (TSCH) technique, including IEC62591 (WirelessHART) and the upcoming IETF 6TiSCH standard. These mesh networking standards utilize wireless frequencies in the globally available license-exempt 2.4GHz spectrum and stem from work done by the Analog Devices SmartMesh team, which pioneered the application of the TSCH protocol to low power consumption, starting with SmartMesh products in 2002 , on resource-constrained devices.

While TSCH is an essential element in ensuring data reliability in harsh RF environments, the way the mesh network is set up and maintained is also critical for years of continuous, trouble-free operation. Industrial wireless networks often have to operate for many years and face many different RF challenges and data transfer requirements throughout their lives. Therefore, to have the same reliability as a wired network, it must also be equipped with intelligent network management software that dynamically optimizes the network topology, continuously monitors link quality, and can respond to 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 WSN are:

confidentiality: Data transmitted over the network cannot be read by anyone other than the intended recipient.
integrity: Confirm that any received information is exactly as sent, without any additions, deletions or modifications.
authenticity: asserts that information from a given source actually does come from that source. Authenticity also protects information from being recorded and played back if time is included as part of a verification scheme.

Key security technologies that must be incorporated into WSNs to achieve the above objectives include: strong encryption algorithms (eg AES128) and reliable key and key management, cryptographic-grade random number generators to prevent retransmission attacks, message-per-message Integrity Checks (MICs), and Access Control Lists (ACLs) that explicitly allow or prohibit access to specific devices. These advanced wireless security technologies can be easily integrated into many of the devices used in WSNs today, but not all WSN products and protocols incorporate all security technologies. Note that the connection of a secure WSN to a non-secure gateway is another weak point, and end-to-end security must be considered in the system design.

Industrial IoT is not installed by wireless experts

Mature industries are mostly adding IIoT products and services to traditional products, and these customers have both old and new equipment in their deployment environments. The intelligence embodied in an industrial WSN must make IIoT products easy to use, enabling a seamless transition and making it easy for existing field workers to use new IIoT products. The network should be able to self-build quickly so installers can deliver a stable network; self-heal to avoid service interruptions when connections are weak or lost; self-service reporting and diagnostics when service is interrupted; general deployment complete After that, little or no maintenance is required, thus avoiding the high cost of on-site maintenance. The success or failure of many applications depends in part on deployment in hard-to-reach or dangerous areas, so IoT devices must run on batteries, typically lasting more than five years.

In addition, since the widespread adoption of IIoT by end users is often company-wide, systems 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.

About those key requirements that make industrial wireless sensor networks special!
Figure 2. Network Visibility – View important information related to wireless network health through network management software, such as this SNAP-ON software utility from Emerson Process Management

Sensors are everywhere

For IIoT applications, accurate placement of sensors or control points is critical. Wireless communication is expected to be possible with wireless technology, but it would be prohibitively expensive to deploy and impractical to power wireless nodes by plugging in or recharging every few hours or months. For example, equipping rotating equipment with sensors to monitor the health of the equipment is not possible with a wired connection, but the customer can perform predictive maintenance on critical equipment by monitoring equipment in operation to avoid unnecessary and costly downtime.

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

time issue

Industrial monitoring and control networks are business-critical. This network underpins systems that affect the basic cost of producing goods, and whose data timeliness is critical. The Deterministic TSCH WSN system has been field tested in a variety of monitoring and control applications over the past 10 years. Such time-slotted systems, such as WirelessHART, provide time-stamped, time-limited data transmission. In this type of network, nodes that need more opportunities to transmit data will automatically configure more time slots, and by configuring multiple time slots on consecutive paths in the network, low-latency transmission can be achieved in such networks. This data transfer coordination capability also greatly improves the ability to deploy dense networks that frequently transfer data. Without a timetable, a chaotic flood of wireless traffic can crash non-TSCH wireless networks.

In addition, each data packet in the TSCH network contains accurate time stamp information, indicating when the data packet was sent, and each node can obtain a network-wide uniform time to coordinate control signals in the WSN node network when needed. Time-stamped data is provided to correctly sequence data even if it is received out of sequence, and in industrial applications where information from multiple sensors must be reconciled, time-stamped data is helpful in diagnosing the exact cause and effect.

About those key requirements that make industrial wireless sensor networks special!
Figure 3. Driving change – Software analytics, such as Brains.App software from IntelliSense.io, use data from industrial wireless sensor networks to simplify factory operations, optimize output and improve safety

Visibility into network operations is key

Industrial networks need to run continuously for many years, yet no matter how robust a network is, problems can still occur. Even if a network performs well when installed, the quality of the network over its operational lifetime can be affected by various environmental factors. Proper early warning of these types of problems is important in any industrial network, and being able to diagnose and resolve problems quickly is key to high-quality service. When it comes to providing visibility into network management metrics, not all wireless sensor networks are created equal. However, at least the management system of the industrial wireless network should provide visibility into:

• Wireless link quality as measured by signal strength (RSSI)
• End-to-end packet delivery success rate.
• Mesh quality, highlighting nodes that do not have enough alternate paths to maintain network reliability.
• Node status and battery life (where applicable).

In optimized industrial applications with intelligent networking, this problem can be addressed by automatically resending data on alternate paths, while continuously updating the network topology to maximize connectivity (see Figure 2).

Intelligent “everything” should have an intelligent network

People are concerned about improving the intelligence level of “everything”, but the “intelligence” in industrial IoT applications is not only reflected in this. An IIoT network should employ intelligent end nodes and provide network and security management capabilities to demonstrate the technical advantages deployed by enterprise IT and operational technology departments. The network should be highly configurable to meet specific application needs. For example, to meet the low power consumption requirements of extending battery life, it should have the ability to know the available power of the network by itself and adopt intelligent routing to optimize the power consumption of the whole network to the maximum extent. In addition, the network should automatically adapt to changes in the RF environment, and when such changes occur, it may be beneficial to be able to dynamically change the topology. Analog Devices’ SmartMesh Network Manager not only enables network security, management, and routing optimization, but also allows users to reconfigure nodes over the wireless network when needed, providing a feature upgrade path to accommodate future changes in customers’ needs.

IoT is largely an industrial phenomenon that drives business forward with excellent ROI. In these business-critical applications, industrial wireless sensor networks must meet high standards for intelligent, secure, and reliable wireless operation, supporting years of continuous operation. These stringent requirements can be achieved through existing and emerging wireless mesh networking standards that will become key IIoT building blocks that can help industrial customers transform their business and services in the IIoT era (see Figure 3).

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