What is TSN, how does it work & why is it important?

TSN Network Infrastructure

TSN is an extension of standard Ethernet that regulates the data communication in Layer 2 – Data Link of the OSI reference model. Its main aim is to make standard Ethernet deterministic but it also provides the mechanisms to allow multiple types of traffic to share the same network, providing the basis of convergence.

OSI Model

TSN technology lies at Layer 2 of the OSI 7 layer model, defined by ISO/IEC 7498.

TSN allows engineers and technicians to precisely understand the exact time it will take for traffic to travel across a network, and also the nature of any delays (called “latency”) and variation in travel time (called “jitter”) that the traffic will experience. Latency and jitter were some of the main reasons why it took time for Ethernet to enter the industrial space. In the IT world, there is a far higher tolerance for latency and jitter, beyond what could be accepted in many industrial processes. Since originally Ethernet could not guarantee when events would happen (a lack of determinism), it could not be used reliably in many machine applications where this lack of determinism could lead to poor quality or even machine damage.

Various open protocols, such as CC-Link IE, were adopted to address these issues, and they will still provide valuable functionality for Industry 4.0 applications when combined with TSN.

While these protocols provided a way to make Ethernet deterministic for industrial applications and hence enabled what is now called industrial Ethernet, they still did not offer much of a solution for convergence. The current trend towards TSN will finally address this missing piece.

However, it is important to not forget that TSN is only a “pipe” for getting data from one place to another in a deterministic way. It does not address higher level application functions such as safety or motion control. Hence these protocols will still be required to deliver these functions.

The importance of bandwidth

A further consideration in TSN networks is bandwidth. In addition to determinism, the standards behind TSN allow industrial Ethernet to utilize this typically fixed commodity with greater efficiency.

The prioritization features of TSN allocate the necessary bandwidth required to allow all traffic to flow on the network without less critical data interfering with that of higher priority. In the past, many industrial Ethernet technologies ran using 100Mbit bandwidth. While TSN will allow this bandwidth to be used in the most effective manner, the increase in data generated by Industry 4.0 is driving the trend towards gigabit bandwidth. TSN is well placed to benefit from this trend. Even though it can improve the use of bandwidth, it is clear that a wider “pipe” will infer fewer compromises between types of traffic and hence the performance of even less critical data transmissions can be improved.

This trend also addresses the way that some systems may have multiple networks to cope with occasional traffic peaks. The move to gigabit will ensure that a single network can meet increased bandwidth requirements, with TSN providing the scope for making sure it is used in the most efficient way as traffic volume increases.

This means that we will have the scope to build future systems where networks have the capacity to combine multiple types of industrial Ethernet protocols along with conventional TCP/IP traffic on a single network, therefore reducing costs and improving productivity and transparency.

IEEE 802.1 specifications

TSN is defined by the IEEE 802.1 Ethernet specifications, which describe how the technology can provide deterministic performance and consequently convergence by implementing time synchronization and traffic prioritization, amongst other functions.

TSN provides the required mechanisms for all types of network traffic to coexist on the same network and hence finally provide the convergence necessary to enable the transparency required by Industry 4.0.

TSN standards

Network Traffic Queues diagram

IEEE 802.1Qbv allows transmission time slots for network traffic queues to be defined. This controls when each traffic type has access to the network. In this example, four time slots are divided between eight queues. Within each slot, the higher numbered queue takes priority.

IEEE 802.1AS – Timing and Synchronization for Time-Sensitive Applications: Time synchronization provides the basis for determinism, as it ensures that all devices on a network share the same sense of time. For example, if it is 10:00 AM, then all the devices on the network know this and their operations are synchronized to the same clock. Therefore, this makes it possible to minimize the likelihood of time drifts that may lead to delays and variation in data transfer (latency and jitter), thus supporting the timely and predictable transfer of critical data traffic.

IEEE 802.1Qbv – Enhancements for Scheduled Traffic: Once a shared sense of time is in place across a network, IEEE 802.1 Qbv defines “time-aware shapers”. These define specific time “slots” that are assigned to different types of network traffic, which are prioritized according to the type of traffic. For example, the traffic related to an emergency stop being pushed would take priority over video frames from a machine vision system.

Different types of traffic can all travel across the network in a predictable way, further supporting deterministic communications, and this method supports convergence, while maximizing the use of network bandwidth.

Evolution of TSN standards

As with any technology, the IEEE 802.1 standards that define TSN are continuously evolving, with existing standards being refined while new ones emerge. Even as the standards evolve, the technology is mature enough to be implemented in projects. An evolving technology can be regarded as more valuable – as it continues to change, it continues to address current demands, hence is less likely to become obsolete. Therefore, this evolution is positive.

The counterpoint to this evolution is that the Ethernet standards have a track record of backwards compatibility. Ethernet technology has been around for about forty years and yet in many cases, earlier devices can often be used with more recent devices. TSN is expected to also follow this trend. Therefore, companies who need to get projects done now can include TSN in them now, confident that they are unlikely to face obsolescence in a few years. There is no need to wait for some undetermined future point when TSN will be “ready”, as this time is not likely to arrive.

This confidence will be further reinforced by the IEC/IEEE 60802 project on the use of TSN in industrial automation, whose role is to define standardized profiles for use of the technology in a variety of use cases.

As a result, forward-looking businesses implementing this innovative technology will be able to benefit from a migration path that addresses current needs while providing a way to support future requirements. Thus, the adoption of TSN now can offer a system that helps companies to optimize their current systems and operations while offering the scope for future enhancements.