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TSN: What’s all the fuss about?

02 December 2019

You may have heard the term time-sensitive networking (TSN) being bandied around. Volker E. Goller explains why it is being developed and what advantages it will bring to industrial communications. 

For industrial communication applications guaranteed latency is vital. This is why classic Ethernet is unsuitable. This, along with its other shortcomings in the industrial environment, resulted in automation experts developing their own Ethernet extensions. The result is a wide variety of different ‘flavours’ of industrial Ethernet. 

Protocols using Ethernet as a transport medium for a fieldbus claim complete control over the Ethernet medium for themselves. Classic TCP/IP communications are only possible in piggyback style via the fieldbus (EtherCAT  and POWERLINK) or through a channel assigned by the fieldbus (Sercos). Bandwidth control remains firmly in the hands of the fieldbus.

There are also protocols which guarantee bandwidth reservation through a time slicing procedure on the Ethernet. PROFINET IRT, for example, enables hard deterministic real-time data transmission on the same cable on which soft real-time or background traffic is operated. A precise timing model for the transmission paths is necessary for planning of the time slices.

Finally, there are protocols based on sharing of the Ethernet cable. These protocols use quality-of-service (QoS) and are at home in factory and process automation applications. PROFINET RT and EtherNet/IP, for example. These protocols are limited to the range of soft real time (cycle time = 1 ms).

For these standards, special hardware support and, thus, special ASICs are needed. Because PROFINET RT and EtherNet/IP are also based on the embedded 2-port switch with cut-through, they are not exempt. However, flexible hardware-based multiprotocol solutions, such as Analog Devices’ fido5000, help solve the problem in an elegant manner.

Enter TSN
With TSN, extensions for standard Ethernet, in accordance with IEEE 802.1, that break free of past limitations have successfully been developed. There is now a standardised layer 2 in the ISO 7-layer model with upward compatibility to the previous Ethernet and hard real-time capability. With 802.1AS-rev, TSN also defines an interoperable, uniform method for synchronising distributed clocks in the network. Because best effort communication always takes place with TSN, the common use of a cable is possible for hard real-time applications, as well as all other applications (web server, SSH, etc.). TSN is not dissimilar to PROFINET IRT in that regard, and it offers comparable performance.

What is new with TSN is the need for more extensive network configuration. Centralised or decentralised configuration is possible. Both types of configuration are currently being discussed and implemented. Interoperability between the two configuration mechanisms is a future development goal.

All well and good, but what are the practical advantages of TSN? The most common answer is that with a larger market, less expensive network interfaces also appear on the market. After all, TSN will also be found in building automation and the automotive industry in the future. Indeed, the market for embedded TSN solutions is expected to be significantly bigger than the current market for all industrial Ethernet solutions put together.

The greatest technical advantage of TSN over previous industrial Ethernet methods is its scalability. Unlike current industrial networks, TSN was not defined for a specific transmission rate. TSN can be used for 100 Mbps just as for 1 Gbps, 10 Mbps, or 5 Gbps.

It also enables topologies to be better optimised because now adapted data rates can be selected for various segments. Whether it’s Gbps, 100 Mbps, or 10 Mbps, a unified layer 2 – IEEE802.1/TS – is used.

A uniform network infrastructure also helps personnel tasked with setting up and maintaining the network because, thanks to TSN, solutions can now be used in sectors other than automation: building, process, and factory automation and energy distribution alike.

This brings us to the next advantage, the training factor. TSN is already a topic at many universities, mostly in the research stage. However, technical and vocational colleges are already showing an interest. We can safely say that TSN will become basic knowledge for engineers, technicians, and skilled workers in the future so retraining for different fieldbuses will no longer be necessary.

Safe transition
In nearly all TSN-related working groups there is a recurring theme – how to safeguard the transition to TSN and the supply to existing installations, such as brownfield applications?

On all sides, emphasis is being placed on making it possible for customers to smoothly transition to TSN. It can already be said that the existing industrial Ethernet protocols are not just going to vanish overnight. On the contrary, anyone using PROFINET, EtherNet/IP, EtherCAT, or a similarly widespread industrial Ethernet protocol today can safely assume that he or she will also be able to operate networks with these protocols - and receive support and replacement parts - in 10 years’ time.

All industrial Ethernet organisations provide models that describe how existing plants can cooperate with new TSN-based devices. The interface to the existing industrial network is made by a gateway (Sercos), with a coupler (EtherCAT), or without any special hardware (PROFINET RT). Especially PROFINET and EtherNet/IP plan to make their complete protocols available right on TSN as layer 2. This makes stepwise transition to TSN possible.

In summary, TSN will be found everywhere in new installations as well as in the form of islands or segments introduced incrementally into existing installations. However, with TSN, there will be new players in the industrial Ethernet field. OPC UA, which, with the new transport protocol PUB/SUB, in conjunction with TSN, is already viewed as a competitor to the classic protocols. This means that in the future both the classic industrial Ethernet solutions, as well as TSN and the new players, will need to be supported.

Volker E. Goller is a systems applications engineer with Analog Devices 

More about TSN 
TSN is an Ethernet standard developed to enable deterministic communication on standard Ethernet. Put simply, it is the IEEE 802.1Q defined standard technology to provide deterministic messaging on standard Ethernet. The technology is centrally managed and is able to deliver a guarantee of delivery and minimised jitter, using time scheduling for real-time applications that require determinism. 

The forwarding decisions made by TSN bridges use the Ethernet header contents, not the IP address. Payloads of the Ethernet frames can be anything, and are not limited to Internet Protocol. This means that TSN can be used in any environment and can carry the payload of any industrial application. 

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