Martin Rostan, executive director of the EtherCAT Technology Group, was recently in the country to give a presentation at the Connected Industries Conference.
He took time out to chat to SA Instrumentation and Control about an Ethernet layer 2 concept that is beginning to capture interest in the automation community – time-sensitive networking (TSN).
Origins in the audio/video industry
TSN has its roots in a model pursued by the audio/video industry known as audio video bridging (AVB). Essentially, AVB is comprised of extensions to the IEEE802.1 specifications to enable networks such as Ethernet to stream time-synchronised A/V data. The name TSN was adopted when the industrial automation industry, the telecommunication industry and the automotive industry got on board and the AVB task group widened its focus.
Within an Ethernet network, the TSN extensions help differentiate real-time data streams from the non-real-time traffic – like sending a file to a printer – that can also flow through the network. All based on an industry standard approach, it allows for plug-and-play communication between systems from multiple vendors. AVB/TSN was developed to enable synchronised playback of audio and video data on different devices – in the automotive case these could be backseat monitors, amplifiers and speakers – using standardised IEEE 802 Ethernet.
“TSN is the extension of these concepts under the charter of the TSN Task Group, which aims to guarantee deterministic communication via Ethernet through low packet latency, low delay variation and low packet loss,” explains Rostan. “But what one must keep in mind is that TSN cannot be considered another fieldbus because it does not provide an application layer.”
TSN is a toolbox of functionality
TSN is not a single technology for deterministic communication over Ethernet; rather, it is a set of IEEE 802.1 standards that extend the functionality of Ethernet networks to enable the coexistence of real-time (deterministic) and non-real-time data communication on the same network, while supporting fault-tolerance and bandwidth preservation.
“TSN is not a technology per se,” explains Rostan. “Rather it is a toolbox of standards that the TSN Task Group is in the process of developing. Some of these are almost complete and others are still in the early phases of definition.”
In other words, TSN is a suite of different extensions to the IEEE802.1 standard with some elements completely defined and ready for use, while others are still in the early stages of development.
So which features should I choose?
Rostan believes that the answer to this does not lie with any individual control system or network designer, but that these decisions will be taken by technology groups and associations – he cites Profibus & Profinet International as an example – which will choose the set of TSN features that they believe will best support the next generation of their Ethernet-based fieldbus offerings, in this case Profinet.
“What one must always remember,” emphasises Rostan, “is that while TSN adds determinism to standard Ethernet, it does not make the cycle times any faster. To improve the speed, you still need to migrate to a faster Ethernet backbone, 1 GB for instance, and then enable the TSN functionality that you require.”
Advantages and disadvantages
“The TSN standards aim to define a method for the transmission of time-synchronised data over a standard Ethernet network,” says Rostan. “Once this is in place, then it can be built into the next generation of Ethernet chips, which will eliminate the need for most of the proprietary solutions currently required to solve the latency and clock synchronisation problems.”
In practical terms, this means that a drive could now be controlled, using standard (TSN-enabled) Ethernet switches, which was not possible before due to the unpredictable latency and other timing related issues. According to Rostan, sub 500-microsecond cycle times are achievable using these techniques on a 1 GB Ethernet backbone. This is more than adequate for most of today’s factory automation and motion control applications, and TSN will make it achievable using commodity Ethernet chipsets.
Rostan believes that one of the biggest disadvantages at the moment is the lack of maturity of the technology. Even though there is enough of the functionality complete now to demonstrate Ethernet devices with basic TSN functionality, he believes it will be at least another three years before all the current development within the TSN Task Group is complete, and five years before the results are widespread and available in the market. “This is potentially going to hold up the decision-making processes within those fieldbus organisation that are looking to incorporate TSN functionality into their Ethernet-based standards,” he adds.
TSN in manufacturing
TSN is seen by many as the basis of the network infrastructure that will support the IIoT revolution. “To a certain extent this is true,” says Rostan. “But before that becomes a reality there is still a lot of work to be done.
“Firstly, the projects being undertaken by the TSN Task Group are all at different stages of maturity. This means that while there is already enough in place to demonstrate, and maybe even implement, the deterministic capability of the architecture, we are a long way from being in a position to finalise the design of a standardised TSN chipset, let alone the configuration tools that will be required to setup the networks.
“Secondly, we must remember that TSN is an extension of the capabilities of a standard Ethernet network. In a manufacturing context this means that real-time deterministic process control data will be mixed-in with the more traditional data found in manufacturing applications. Even though the technology will be capable of doing this very effectively once the chipsets become commercially affordable, it is unlikely to become a reality until the IT/OT convergence has matured significantly from where it is today.”
Rostan stresses again that since TSN is not in itself a fieldbus, it is never going to take the place of one. He sees the role of TSN as either an extension of the capability of an existing Ethernet-based fieldbus, Profinet for instance, or as a deterministic Ethernet technology for architectures working one layer above a traditional deterministic fieldbus.
He cites an EtherCAT example (he is after all the head of the EtherCAT Technology Group). The scenario is a server in the control room i.e. a clean temperature controlled environment. This computer is running a soft PLC, which is remotely controlling a group of robots on the factory floor through a TSN backbone linking all the EtherCAT segments that form the control layers of the individual robots.
The value added by TSN here is the ability to setup real-time (deterministic) control via the plant network for a group of robots working together on a task – a synchronous network of robots in other words.
The TSN-enabled network does not challenge EtherCAT’s microsecond on-the-fly processing capability. It is the level above that where Rostan sees TSN adding value in the future, which is why the EtherCAT Technology Group is an active participant in the TSN Working Groups today.
What became obvious during the discussion with Rostan is that TSN has the potential to add substantial value to the Ethernet-based networks that will dominate the IIoT era. But, as with the IIoT itself, the TSN functionality will only add value in those projects that have been intelligently conceived, analysed and understood. In the future it may extend the capability of an existing fieldbus network, but it will not replace it – TSN is not another fieldbus.
For more information contact EtherCAT Technology Group, +49 911 540 5620, [email protected], www.ethercat.org
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