Determinism is an important factor in industrial connectivity design. Applications for industrial control and automation require data to be sent and received at specific time intervals. These applications also mandate guaranteed data delivery with low, bounded latency. A loss of data (e.g., in the case of a mission-critical jet propulsion system) could cause dangerous consequences.
IEEE 802.1 Ethernet, a widely deployed enterprise networking standard, was not designed to meet these deterministic requirements of industrial applications. To achieve determinism, proprietary enhancements to Ethernet (such as EtherCAT, PROFINET, or SERCOS III) are still used in industrial connectors, cables, and controllers. The result has been fragmented industrial networks. Due to the lack of security and interoperability, segmented networks are a bottleneck to integrate industrial control networks with Industrial IoT (IIoT) and Industry 4.0 applications.
As an evolution to the IEEE 802.1 Ethernet standard, Time-Sensitive Networking (TSN) addresses these problems head-on. In addition to the benefits of standard Ethernet, TSN can provide determinism with bounded low-latency and jitter. This provides the roadmap to converge enterprise and factory connectivity. It’s also a compelling reason for industrial companies to embrace IIoT.
In the past few years, IEEE’s TSN task group added a series of extensions to existing 802.1 standards in order to enable packet transfer while adhering to the strict latency and throughput requirements for time-sensitive traffic over the same interconnected enterprise network. Latency and throughput are configurable to suit specific use cases.
The standards set the guidelines to split the time between time-sensitive and best-effort traffic. Eight VLAN priorities are defined to discern the various traffic types. Every end-to-end packet flow has one of the eight VLAN priorities assigned. The highest VLAN priority is typically assigned to time-sensitive traffic.
In vehicles, for example, safety (e.g., lane departure warnings) and engine timing related data require guaranteed latency. For navigation and infotainment traffic, the requirements are much less stringent. TSN is useful in such use cases as it can combine all the flows over the same Ethernet cable without compromising timing requirements. This reduces the cost, weight, and labor to install multiple cables in vehicles for various traffic types.
Figure 1 shows the suite of standards enhanced for TSN. TSN's key capability standards for time synchronization, scheduled delivery, and software-defined configuration are highlighted in the diagram.
Figure 1: IEEE 802.1 suite of standards enhanced for Time-Sensitive Networks (Source: Practical Industrial Internet of Things Security)
IEEE 802.1AS specifies the time synchronization in TSN, which helps to establish a common concept of time between communicating devices. The IEEE 1588 Precise Time Protocol (PTP) standard is used to distribute an accurate timing reference between devices and switches in the network. IEEE 1588ASrev provides the IEEE 1588 profile for TSN. This standard also allows for synchronizing time by using an external reference—such as a GPS. Pilot tests for TSNs reported time synchronization with less than 100-nanosecond accuracy.
IEEE 802.1Qbv defines a time-aware shaper that helps to prioritize traffic in TSN infrastructures. A time-aware shaper segregates data exchange into fixed length, repeating time cycles. Peers agree on TSN communication to divide these cycles into time slots. Each time slot can be assigned to one or more of the eight VLAN priorities.
TSN defines three traffic types:
IEEE 802.1Qcc defines the TSN system configuration. TSN uses a software-defined networking concept for the automated setup and configuration of devices and network equipment. Peers agree in advance on TSN configurations for timing, scheduling, and QoS metrics. These configurations are then provisioned across the various TSN endpoints and switches. Automation eases provisioning and management as well as improves the reliability and scalability of the networks.
TSN is mainly relevant to industrial control and automation products in levels 0, 1, and 2 in the Purdue control hierarchy. Thus TSN enhancements apply to a wide variety of controllers, I/O devices, sensors, and actuators that currently use standard Ethernet or one of its proprietary variants. Industrial cables and connectors that are usually more rugged might also need to evolve with TSN.
TSN is currently at an early adoption phase. To maximize its benefits, ideally, TSN would need new Ethernet switches and TSN capable endpoints. But TSN is also designed for backward compatibility. Any Ethernet device should work normally in a TSN network. Protocol translators and gateways supporting TSN would be necessary for brownfield scenarios.
TSN enhancements to standard Ethernet are at layer 2 of the TCP/IP stack. As such, higher-layer communication standards remain unaffected by these enhancements. Many framework and application layer protocols such as OPC-UA are also adopting TSN.
Key industrial, embedded, and automotive vendors are coming together to define TSN requirements for various industry use-cases, and to extend TSN concepts to wireless networks.
Time-sensitive networking is a promising evolution to standard Ethernet as it extends the benefits of IIoT to tight-looped industrial control domains. As an emerging technology, TSN is still undergoing a lot of pilot testing in various incubation testbeds. However, it may not be too far when TSN-compliant products become the norm.
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Sravani Bhattacharjee has been a Data Communications technologist for over 20 years. She is the author of “Practical Industrial IoT Security,” the first released book on Industrial IoT security. As a technology leader at Cisco till 2014, Sravani led the architectural planning and product roadmap of several Enterprise Cloud/Datacenter solutions. As the principal of Irecamedia.com, Sravani currently collaborates with Industrial IoT innovators to drive awareness and business decisions by producing a variety of editorial and technical marketing content. Sravani has a Master's degree in Electronics Engineering. She is a member of the IEEE IoT Chapter, a writer, and a speaker.