Analog Devices
Greater universality through collaboration
OPC UA forms a common application interface through its address space, while TSN adds real-time capabilities to standard Ethernet and enables gigabit data speeds. Therefore, it makes sense to combine the two technologies through a publish/subscribe (pub/sub) model, but there are other possibilities for industrial communication in the context of Industry 4.0. In this interview, Volker Goller, systems applications engineer in the Deterministic Ethernet Technology Group at Analog Devices, provides some background.
Q: In the OPC UA TSN system, what tasks and functions do OPC UA and TSN undertake?
A: To clarify the role of OPC UA, I would like to quote Stefan Hoppe, Vice President of the OPC Foundation: “OPC UA is not a protocol, but an information model.” What he means is that OPC UA is first and foremost An information model. Of course, it’s also a protocol for connecting clients and servers, but the advantage of OPC UA is the address space, which is what makes OPC UA a universal application interface. OPC UA is very flexible and allows mapping of existing user interfaces (Profiles of Industrial Ethernet protocols) to OPC UA. As a result, almost every profile in the Industrial Ethernet protocol now has a representation in the OPC UA address space, or is being developed to implement its representation. These profiles (I/O, drives, security, etc.) are not yet specified by OPC UA, but are likely to change. In the framework of Industry 4.0, OPC UA is seen as a very promising lingua franca in the future.
In contrast, TSN is an extension of IEEE-802.1 Ethernet with a complete set of new capabilities designed to make Ethernet more deterministic and real-time. It can also be seen as the democratization of real-time communications, as numerous manufacturers are expected to produce TSN-enabled hardware in the future. Real-time capabilities are available through TSN for almost every protocol.
In this context, a pub/sub working group was formed with the aim of specifying a real-time transport protocol for OPC UA with the help of TSN. This will enable OPC UA to be real-time and thus an alternative to the Industrial Ethernet protocol. It will be warmly welcomed above the traditional PLC level, as controllers from different manufacturers will be able to interact with OPC UA in real time. TSN also provides guaranteed network bandwidth for OPC UA, so its robustness will be higher than what is currently achievable.
However, pub/sub is not the only way to make OPC UA real-time capable. The industry is also working to develop an OPC UA model for DDS, a widely used and proven real-time protocol. This will enable the operation of distributed systems with DDS/TSN capability and use OPC UA as the application interface.
How it turns out remains to be seen.
Q: Which tasks and functions will be reserved for legacy industrial Ethernet systems and fieldbuses in the future?
A: Legacy Industrial Ethernet protocols are not going away. In the future, some will still exist in different forms (as profiles or profiles in OPC UA), and some will be based on TSN. Traditional fieldbuses will be replaced by Ethernet.
Q: In an OPC UA TSN system, besides OPC UA TSN at the profile level, what tasks and functions can a traditional Industrial Ethernet system perform?
A: To clarify again, TSN does not automatically implement OPC UA. They are two completely separate technologies. OPC UA can play an important role in controller networks (controller to controller). pub/sub vs. TSN has an advantage here; it remains to be seen whether it will work at the field level as well, since OPC UA is not a small stack, at least not if you want to take advantage of its full benefits.
Q: How do user organizations of legacy Industrial Ethernet systems address the TSN challenges?
A: I would say that all user organizations are responding to the opportunities that TSN presents. TSN is expected to provide more hardware options, especially infrastructure components, and achieve higher speeds, ie 1 Gbps or more. Eventually, we’ll see Profinet® TSN, along with EtherNet/IP® over TSN and OPC UA Pub/Sub.
Q: Can TSN support real-time and reduce cycle time to 31.25μs, maybe even lower in the future?
A: At 100 Mbps, to get cycle times below 250 μs, existing industrial Ethernet protocols would have to significantly modify standard Ethernet. The IEEE is not very friendly to non-standard methods such as the lumped frame protocol on which EtherCAT® or even Sercos is based. These extensions are unlikely to be incorporated into the TSN standard.
In response to your question, TSN will hit the IEEE defined limit of 250µs at 100 Mbps – at least as long as true parallel operation of standard TCP/IP applications has to work. For shorter cycle times, the road to 1 Gbps is open.
Q: How does TSN address or is expected to address security concerns?
A: Security generally uses the Black channel principle. Security is defined on top of the actual communication protocol. However, the reliability of the communication channel is one of the security considerations. TSN is no more unreliable than today’s systems.
Q: The OPC UA protocol can also be transported over traditional Industrial Ethernet systems, such as time slots or tunnels. So why does it even need TSN?
A: TSN adds determinism and real-time to standard Ethernet. In many cases, different protocols coexist in the same cable. TSN supports robust coexistence of real-time and “best effort” TCP/IP in one cable.
Q: What are the advantages of TSN over traditional industrial Ethernet systems?
A: TSN is not a new Industrial Ethernet protocol. It is a unified extension to standard Ethernet, adding real-time capabilities. We’ve articulated the benefits: hardware availability, unified infrastructure, and speed-independent definitions.
Q: What role does cost play here?
A: Scalable standardized hardware and infrastructure promises to reduce costs and unify know-how.
Q: What role does the desire to achieve data rates of 1 Gbps or higher play?
A: 1 Gbps (and above) is a logical progression for today’s networks. Will it replace 100 Mbps? will not completely replace, but
1 Gbps enables new applications and can overcome the performance bottlenecks of today’s data-intensive applications.
TSN is not a new industrial Ethernet protocol, but a unified extension to standard Ethernet that adds real-time capabilities.