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Industrial Ethernet Book Issue 100 / 18
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TSN: A unified data highway for real-time systems

OPC UA over Time Sensitive Networking is being backed by leading automation and information suppliers as a unified communication solution between industrial controllers and the cloud. Based on open standards, the goal is to enable industry to use devices from different vendors that are fully interoperable.

THE IEEE AUDIO VIDEO BRIDGING (AVB) group was founded in 2004, targeting a standard to transmit audio and video data over the omnipresent Ethernet, taking professional demands into consideration. Their work culminated into the IEEE 802.1Q standard. Subsequently, the group expanded its focus on supporting the demanding, time-critical control tasks - for instance, such as those occurring in the automation technology; in 2012, the group was renamed to "Time Sensitive Networks" (TSN) group.

The term TSN itself does not represent a standard, but a modular family of new or expanded IEEE standards, which can be flexibly combined depending on the specific application requirements. The big advantage of this new family is its seamless integration into all existing and future IEEE 802 series standards related to the Ethernet.

Open approach to the TSN network organisation.

Motivation towards TSN

Due to the recent activities around IIoT and Industry 4.0, TSN has gained increasingly more attention. This is obvious because IIoT and Industry 4.0 build on maximum transparency towards integration of production resources into the Enterprise-IT. The solutions to date, deviating from the IEEE standards to achieve real-time behaviour, obstruct this transparency at field level. TSN makes a valuable contribution towards standardisation of the networks which make today′s special gateway solutions between the Office-IT and the fieldbus level worlds superfluous.

The reason for the breach between the Office-IT and the field level worlds depicted in Figure 1 is the lack of necessary real-time capability at the field level, closely interacting with the controlled processes, which presupposes a deterministic, i.e. predictable, data transmission.

The ISO layer 2 of traditional Ethernet is based upon the "best effort" principle, i.e. it forwards the data as efficiently as possible taking the overall network throughput into account. How long a particular data packet travels, which path it takes and whether it reaches its destination at all is entirely indeterminate. In case of doubt the ISO layer 3, for example TCP/IP, simply retransmits the packet. Such behaviour is totally unacceptable for a real-time application.

Network structure from the enterprise to the field level.

Real-time systems today

Established Ethernet-based real-time systems for automation technology usually are based on the latest state-of-the-art Ethernet technology available at the time of their birth adding the missing real-time characteristics.

Depending on the particular application focus, the development targets differed drastically here, ending up into quite diverse solutions.

The multi-axis control of a robot requires very fast cycle times (<100us) in spatially small network segments. In the production line, on the other hand, cycle times in the millisecond range are sufficient, but in turn it has a larger dimension. A process control system has again fewer timing requirements, but it stretches over very long distances.

Technical aspects are accompanied by commercial considerations. The more a technical solution deviates from the available standards, the higher are the costs of its development, implementation, and maintenance. For reasons of cost, further developments of the proprietary standards hardly take place.

An obvious example of this is the transmission speed. The IEEE 802.3ab (1Gb) standard was already adopted in 1999. At present, all leading Ethernet-based automation systems still operate at 100Mb maximum. The expansion to 1Gb is technically feasible but it would demand the revision of the corresponding specifications as well as the specific hardware.

A similar situation could arise at the lower end of the scale. Since 2016, the "IEEE 802.3 10 MB/s Single Twisted Pair Ethernet Study Group" is examining the feasibility of a new standard for 10Mb full-duplex transmission over a single wire pair. Distances up to 1km and phantom power are planned. This technique has the potential to replace the traditional 4-20mA current loop without having to replace the many kilometre long cabling of the entire systems. Should this transmission standard become available one day, it would immediately integrate itself seamlessly into TSN-based networks.

Technical characteristics

TSN adopts many ground-breaking details of existing automation solutions and funnels them together in new, general IEEE standards - and appropriately supplemented by new functions such as "frame pre-emption" where necessary. TSN is the convergence of today′s isolated solutions.

The most important TSN family standards currently implemented including existing AVB extensions for Ethernet switching (IEEE 802.1Q) are:

Stream reservation plus credit-based shaper (802.1Q) (AVB): Definition of data streams and their maximum bandwidth, to split the available transmission capacity between competing data streams in a controlled manner. Does not guarantee any maximum time delay and a minimum bandwidth is guaranteed only indirectly.

VLANs and priority (802.1Q) (AVB): Logical separation and prioritisation of competing data streams to divide the allocation of network resources and to forward high-priority data preferentially. Enables hard real-time just for a single, highest-priority data stream.

Timing and Synchronisation (IEEE 802.1AS): Timing synchronisation of all network nodes with nanoseconds precision range. Indispensable for synchronous end nodes and time-controlled forwarding within the switches.

Enhancements for Scheduled Traffic (IEEE802.1Qbv): Guaranteed deterministic forwarding of the data packets within the network by assigning fixed, exclusive time slots. Eliminates interference with other network traffic; the data always passes through the network within the specified timetable. Guard interval before each time slot leaves only packets on to the wire which can be transmitted entirely within the previous time slot.

Interspersing Express Traffic / Frame Pre-emption (IEEE 802.3br / IEEE 802.1Qbu): Interruption of data packets on the line to send higher priority data immediately. Interrupted data packet transmission is subsequently resumed from the interruption point.

Without "scheduled traffic": Timely forwarding of prioritised data.

With "scheduled traffic": Bandwidth increment, since guard intervals before the time slots can be considerably smaller. Very efficient for high-performance systems with narrow time slots.

Since the semiconductor manufacturers usually adopt international standards of general interest in their products, the availability of compatible hardware will be assured.

TSN allows convergence to layer 2.

The route

Today′s TSN technology provides only the first part of the transport infrastructure - so to speak the (data) highways, through which the end applications exchange information. But it takes more than the road to reach the destination: vehicles, traffic regulations, route planners, traffic guidance systems, traffic monitoring and much more.

In the network context, it means that the available standards provide only mechanisms that transport real-time data packets from the source to the sink according to the network configuration. But what does this data mean?

Who defines the paths along which packets are routed through the network? Who determines the overall schedule? Who configures the end node devices? Potential answers to these questions are currently being discussed intensively. For the industrial automation sector, two models are visible.

An obvious solution is by directly exchanging the established, diverse Layer 2 specifications by TSN. All further components are reused as far as possible. This minimises the development costs and can be implemented promptly. Synergies result from the use of standardised communication and corresponding hardware and software components. The value chain and the business models remain unchanged.

From the IIOT / Industry 4.0 perspectives, however, this solution is unsatisfactory since it does not simplify the data exchange and interoperability between individual systems. For this reason, organisations such as AVNU, IIC, OPC, and IEEE are pursuing the goal of an open standard architecture for industrial real-time systems that covers all important aspects and offers broad vendor-independent interoperability ranging from field devices up to the cloud application beyond today′s border between operational- and Office-IT.

The diagram on page 41 shows an example of an independent network management. End nodes notify their communication request to the central CUC unit which implements it in interaction with the CNC unit and reports the result back to the requesting node. The CNC unit manages the network resources and configures them accordingly.

Transparency and interoperability pave the way for entirely new business models but also demand the consideration of existing strategies such as network security in many areas. In the past, industrial control systems were isolated standalone solutions. Access to them was strictly limited. This dogma is no longer valid. Systems are accessible worldwide through remote maintenance access and big-data applications. The maintenance is done by external staff. Installed devices could be counterfeited executing suspicious firmware.

In response to these new challenges, many isolated solutions have emerged as in case of real-time communication. Similarly, multiple development and certification expenses as well as the boundaries between the individual security domains are detrimental. Any break in the security architecture increases the attack surface for hackers. The introduction of the TSN technology offers an excellent opportunity to implement in parallel a seamless, uniform security concept from the sensor to the cloud.

The future

Thanks to its significant advantages, TSN will assert itself as an Ethernet-based standard solution for industrial communication networks. First niche products are already available and additional TSN-enabled products are announced for this year. A broad availability of mature TSN standard solutions can be expected by the end of this decade.

TSN will not conquer the automation world overnight. Investments in existing systems and plants have to be amortised over years and decades. There will be a transitional period in which existing systems will be operated, maintained and adapted and, in parallel, new TSN-based systems will be created.

The additional effort involved in this transitional phase can be minimised by the use of multiprotocol chips such as the R-IN device family from Renesas. Based on these devices, future-proof automation components supporting both, the established standards as well as the new TSN technology, can be developed.

Arno Stock works for Renesas Electronics Europe.

Source: Industrial Ethernet Book Issue 100 / 18
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