For a long time, IEEE Ethernet was not an alternative to the current diversity of industrial fieldbuses due to its lack of determinism. However, the lack of interoperability with each other and especially with IT, or interoperability that is only possible with significant overhead, is an increasing obstacle to innovation: whether IIOT, software-defined manufacturing, cross-company digital data infrastructures such as Gaia-X and Manufacturing-X or the virtualization of control technology, they all require end-to-end connectivity, which can only be achieved with convergent real-time networks.

Figure 1: The hierarchy of TSN use cases.

© ISW Stuttgart

Traditional IT technologies, which are being expanded to include deterministic capabilities, are seen as key technologies for this. For Ethernet, the most relevant and fundamental technology in this context, this evolution began with the founding of the Time-Sensitive Networking Task Group. New sub-standards extend the quality-of-service mechanisms of Ethernet with functions for real-time communication and thus enable deterministic guarantees. TSN is therefore the next evolutionary stage of Ethernet and not a new technology in itself. Conversely, TSN therefore plays the same role for applications as Ethernet, which must always be supplemented with additional solutions on the upper communication layers - TSN is therefore not a fieldbus, but can be a part of it. The relevance of TSN for the digitalization of production technology increases with the degree of integration into the overall system. Figure 1 shows a four-level hierarchy of use cases that are increasingly utilizing TSN and thus increasingly exploiting its potential:

• Closed application: TSN is used as a communication technology in a closed system. The main advantage is the use of a standard technology.
• Best-effort connectivity: Data is communicated from the field to IT without technological disruption and direct access to end devices is possible. The same connection is used for real-time communication at field level as well as for general communication. Many innovative approaches such as big data, visualizations or AI-based systems can be implemented in this way.
• Deterministic convergence: OT and IT become a common solution space for real-time applications. Deterministic communication is necessary within the field level as well as with IT. The system architectures merge and approaches such as virtual control systems can be implemented.
• Technological convergence: The wired TSN network is seamlessly integrated with other real-time capable communication technologies, such as 5G, deterministic Wi-Fi or DetNet.

Current developments

Figure 2: The current topics of TSN standardization.

© ISW Stuttgart

The contributions in the context of TSN to the IEEE Ethernet standards in 802.1 can be divided into four groups: Time synchronization, guaranteed latencies, availability and configuration.

Time synchron ization is the basis of all deterministic functions. After a long development phase, time synchronization with IEEE 802.1AS version 2020 can be considered relatively stable. New developments focus on YANG models, hot standby and support for links in half-duplex mode.

Guaranteed, low latencies are made possible in the context of TSN by various traffic shapers. The time-aware shaper (IEEE 802.1Qbv), credit-based shaper (IEEE 802.1Qav) and frame preemption (IEEE 802.1Qbu) are particularly well-known and standardized. After a quieter phase, the current focus here is on topics relating to the efficient use of shapers and the standardization of cut-through. In particular, the standardization of cut-through, a technology that is commonplace in many industries but has never been standardized, is a significant step for use in an industrial context.

In the context of TSN,availability is primarily achieved through the standards for input-side filtering (IEEE 802.1Qci) and redundancy mechanisms (IEEE 802.1CB). The current focus here is also on hot standby.

The configuration and management of the TSN infrastructure are essential in order to be able to use TSN efficiently, but unfortunately they are also the two aspects with the most unresolved issues. Configuration protocols and interfaces between the management entities (IEEE 802.1Qdd and IEEE 802.Qdj) are still being worked on very actively and their publication is within reach. New activities have been added for various YANG models, which are necessary for configuration.

Profiles for TSN

On September 27 and 28, 2023, the 8th International TSN/A Conference will take place in Ludwigsburg near Stuttgart. You can find the program here: tsnaconference.de

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The new standardized functions are the basis of deterministic communication, which can be used in various combinations and configurations. In order to achieve a minimum level of interoperability within the application domains, profiles are defined in addition to the actual functional standards, which at least enable the profile-compliant development of hardware, but in some cases should also allow interoperability of parallel applications.

In the industrial context, the dual-logo standard 'IEC/IEEE 60802 TSN Profile for Industrial Automation' is of the greatest relevance; this has been the subject of intensive discussion for many years and is constantly being developed further.

The status of the current drafts gives hope that the standard will be published soon. Furthermore, a profile for the automotive sector is being developed with the IEEE 802.1DG standard and a profile for aviation with IEEE 802.1DP.

Protocols of the higher layers

TSN covers layers 1 and 2 of the OSI model. Specific applications therefore always require additional protocol solutions for the upper layers. All protocols known from the IT environment, such as TCP/IP, as well as specific solutions for OT can be considered here. The extent to which deterministic guarantees can be passed down to the application level depends on a number of factors. Two fundamentally different approaches are used to implement TSN-based real-time applications:

• TSN as a technological building block: TSN is used in a (partially) closed ecosystem
• TSN network as an abstracted resource: An application-independent network is assumed, which can be used by means of resource management.

Both variants are pursued in an industrial context: Profinet with TSN as a new conformance class and CC-Link IE TSN use TSN as a technological building block. Both retain established ecosystems and offer convergence at least for best-effort communication. There are already a large number of standards, an increasing number of products and the first real implementations for both variants.

With a stronger background in IT and the approach of clearly separating communication layers, OPC UA FX pursues an approach that sees TSN networks as a resource in which resources can be allocated depending on the application. OPC UA FX attempts to create a standardized solution up to the application layer for a large number of key players in the automation industry. Up to now, the focus has primarily been on non-deterministic aspects, for example in controller-to-controller communication. However, the first results for real-time communication between controller and field level are also expected in the near future. The PubSub communication of OPC UA serves as the basis for this.

Infrastructure and end devices

TSN not only affects the network infrastructure, but also the hardware and software architecture of end devices. The requirements vary greatly depending on the application.

After many years characterized by prototypes, TSN switches are increasingly coming onto the market. Several manufacturers already offer devices in various forms in terms of supported standards, operating conditions, ports and other parameters.

On the endpoint side, the focus is on hardware-software solutions, which typically consist of a combination of processor and ASICs or IP cores for reconfigurable hardware. This applies equally to highly integrated embedded systems and PC-based systems, where the hardware is typically part of a network card. Software is increasingly available, both in the form of closed and open source solutions. Linux supports TSN mechanisms via the Traffic Control System, for example for the time-controlled transmission of streams (Qdisc ETF) and the time-based transmission of traffic classes (Qdisc TAPRIO). Several open solutions for time synchronization are also available, for example LinuxPTP and stacks on the upper layers, such as open62541 for OPC UA. In addition to Linux, TSN is also integrated into FreeRTOS and various proprietary real-time operating systems.

From vision to implementation

The author: Florian Frick is group leader for real-time communication and control hardware at ISW Stuttgart.

© University of Stuttgart

TSN is accepted across all industries as an enabler for convergent real-time communication. Progress in standardization, available solutions and understanding of how to use the technology is resulting in the first real, productive implementations. Further significant progress is expected in the coming year: on the one hand in the standardization of the configuration, the profiles and also in the context of OPC UA FX. On the other hand, many infrastructure and endpoint components will become available and form the basis for future solutions. It is therefore crucial for companies in the relevant sectors to get to grips with the technology, identify potential and minimize risks.