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Time Sensitive Networking (TSN)

Ethernet is the most popular communication medium in the world to transmit data between devices. Ethernet is prevalent in many industries because of its speed, affordable cost, and versatility.

Ethernet standards have been evolving over the years. In the 1970s, the first versions of Ethernet achieved speeds of only 10 Mbps. Then, Fast Ethernet became available in 1995 with speeds of 100 Mbps. Nowadays, gigabit Ethernet is available. Also, as Ethernet has become ubiquitous in most industries, Ethernet components have become more affordable and easier to implement. Finally, most network protocols support Ethernet as its communication medium. Therefore, Ethernet networks can employ a wide variety of protocols to fulfill their applications.

Even though Ethernet speeds have been improving over the years, there is another key performance factor: Determinism. A deterministic network exchanges data in a precise manner with a defined latency. Since Ethernet communication is based on the best effort principle, data exchange in Ethernet networks lacks determinism. Until now, deterministic data exchange in Ethernet is only possible with proprietary solutions, but Time Sensitive Networking (TSN) aims to change that. TSN is an upcoming new technology, and its focus is making Ethernet deterministic by design.

What is TSN?

TSN refers to a set of IEEE 802 standards that make Ethernet deterministic by default. TSN is an upcoming new technology that sits on Layer 2 of the ISO/OSI Model. It adds definitions to guarantee determinism and throughput in Ethernet networks. The following are some of the IEEE standards that make up TSN:

  • Enhanced synchronization behavior (IEEE 802.1AS)
  • Suspending (preemption) of long frames (IEEE 802.1-2018)
  • Enhancements for scheduled traffic (IEEE 802.1Q-2018)
  • Path control and bandwidth reservation (IEEE 802.1Q-2018)
  • Seamless redundancy (IEEE 802.1CB)
  • Stream reservation (IEEE 802.1Q-2018)

TSN evolved from the industry's use of Audio / Video delivery and the need for more devices and synchronized communications. There are more devices on networks than ever before and more information being shared and analyzed. Therefore it makes sense that Ethernet has to perform better.

TSN for Industrial Automation

Many industries require deterministic Ethernet, and Industrial Automation is one of them. The automation industry has continuously sought solutions to achieve fast, deterministic, and robust communication. Currently, several specialized solutions are available for this purpose, such as PROFINET IRT, Sercos III, and Varan. TSN can help standardize real-time Ethernet across the industry.

Current industrial trends like Industrie 4.0 and the Industrial Internet of Things lead to an increase in network traffic in ever-growing converged networks. Such networks require flexibility and scalability to support small devices as well as big data server systems while ensuring bounded latency for time-critical communication. TSN intends to cover all these requirements. It will provide standardized mechanisms for the concurrent use of deterministic and non-deterministic communication.

TSN in PROFINET Networks

PROFINET over TSN comes along with benefits in regards to convergence, scalability, and flexibility. Also, users will continue to have access to all existing PROFINET features and profiles. TSN applies standardized traffic shaping tools to ensure low latency and deterministic data exchange where necessary.

Existing non-TSN PROFINET devices may be used in cooperation with new TSN-based PROFINET V2.4 hardware, although they might not benefit from TSN. Increased robustness is given by the underlying TSN mechanisms which protect real-time critical data against best-effort traffic. Looking at the increasing demand for data volume, which has to be transferred in parallel to time-critical data through the network in future Industry 4.0 scenarios, protecting real-time critical data is an essential feature.

TSN

PROFINET over TSN

PROFINET over TSN is designed for controller to device communication. It can handle up to 1024 devices and achieve cycle times of 31.25 μs. OPC UA may also make use of current and future TSN standards. PI, however, is committed to applying OPC UA for controller to controller or controller/device to cloud communication. The diagram on the right illustrates an example of the complementary and harmonized interplay of PROFINET over TSN and IP based protocols like OPC UA. Within process automation, factory automation, and motion control, PROFINET over TSN is used by PLCs, remote sensors, remote I/O, motion controllers, and drives as well as by computerized numerical controllers.

TSN PN OPC UA

TSN is a promising technology drawing a lot of attention. TSN offers great potential for PROFINET. It aims to combine a wide range of IT networks with the robustness and determinism of automation networks. In short: hard real-time via standard IT networks. This TSN animation video shows you how real-time communication is possible on standard Ethernet and how this fulfills the requirements of Industrie 4.0.

PROFINET over TSN Principles

Definitions

Time-Sensitive Stream: A stream of frames with a given quality of service for timely delivery from a single source, that follows dedicated routes through a network reaching one or multiple destinations. Transmission latencies of stream-based transmissions have to be bounded. Communication relations consisting of back and forth transmissions are set up with two separate streams.

TSN Domain: A quantity of commonly managed industrial automation devices. A set of stations (end-stations or bridges), their ports, and the attached individual LANs that transmit time-sensitive streams using TSN standards. It is an administrative decision to group these devices. Devices that do not support TSN standards cannot be part of a TSN domain. The traffic of non-TSN devices, however, may pass through TSN domains.

1.- Streams: Routes Through the Network

When two nodes in the network are looking to communicate via PROFINET over TSN, streams are defined. Streams describe the path through the network from “Talker” to “Listener.” The diagram on the right shows a graphical example of a stream. The streams must be registered in all switches on the route. A stream characterizes by a destination MAC address and a VLAN tag for priority.

Defined stream between two nodes
Defined stream between two nodes

2.- Time Synchronization: Precise Clocks

Time synchronization is critical to achieving determinism. All devices in the TSN domain are synchronized on a common time basis. A "Working clock" is the control time reference for scheduled traffic and isochronous applications within TSN domains. Also, a "Universal clock" serves as a system and plant-wide reference for tasks such as logging of events in chronographic sequence.

For high accuracy, delays on the line and in the switches must be measured
For high accuracy, delays on the line and in the switches must be measured

3.- Scheduled Traffic: Communication According to Timetable

PROFINET IRT schedules all isochronous traffic. It creates a defined time table with all the transmission and forwarding times between nodes. IRT mechanisms work, but Scheduling can become very sophisticated in converged networks. As a result, PROFINET over TSN doesn't employ Scheduling. Instead, it utilizes a Time Aware Shaper concept.

TAS: Time Aware Shaper

The diagram on the right illustrates the Time Aware Shaper concept. The time schedule creates a repetitive pattern of time slots (network cycle time) for the transmission of frames. To ensure availability, high priority frames have a continuously active queue. Also, protective guard bands prevent frames from entering the time slot of the next cycle.

The Time Aware Shaper creates a repetitive pattern of time slots for the transmission of frames
The Time Aware Shaper creates a repetitive pattern of time slots for the transmission of frames

4.- Frame Preemption: RT Interrupts TCP/IP

Frame preemption plays a key role together with protective guard bands. All real time frames sort themselves before best effort, TCP/IP frames. As shown in the diagram, best-effort traffic never delays real time data exchange. Also, TCP/IP frames are split and reassembled to maximize the use of bandwidth.

TCP/IP frames are split and reassembled to maximize the use of bandwidth
TCP/IP frames are split and reassembled to maximize the use of bandwidth

5.- Seamless Media Redundancy

Redundancy ensures the continuation of transmission in case of device or link failure. Network designers may employ redundant streams to create redundant network paths between devices. Redundant streams are in place to ensure time-critical communication continues seamlessly even after device or link failure.

Two defined streams for
Two defined streams for

Network Management Engine

The configuration of TSN domains will follow a Plug & Play concept and will utilize the familiar PROFINET engineering tools. The TSN domain characteristics (stream paths, VLANs, etc.) are set up and continuously maintained by a Network Management Engine (NME). The NME will be part of TSN capable PROFINET Controllers. It will take care of topology acquisition, path planning, and network configuration. First, the user sets up a specific policy-based network configuration in the engineering tool. Then, the NME uses these rules to create and to shape the TSN domain continuously.