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Time Sensitive Networking (TSN) A Basis for Industry 4.0

  Knowledge

The Fourth Industrial Revolution is defined by how information is fed into or acquired from production processes. Real-time support plays a critical role in this. Time Sensitive Networking (TSN) provides standardized methods for providing such support.

In industrial production, tools, equipment and machinery needs to be adapted perfectly to one another to prevent damage to the workpiece or machinery. This requires real-time data communication, which means that all devices involved must have an identical time base and it must be guaranteed that responses are received by the recipient within the specified time. Technologies with Industrial Ethernet support such as Ethercat or Profinet guarantee such response times. As each bus system is optimized for certain applications, several standards have become established.

In non-industrial environments at the upper levels of the automation pyramid, on the other hand, Ethernet is broadly established due to its robustness and reliability. But Ethernet cannot meet the real-time requirements of industrial processes.

For implementation in an Industry 4.0 context, it is necessary to merge the two network environments to create seamless, autonomous systems. This is where the idea of "Time-Sensitive Networking" or TSN comes into play. It runs concurrently with conventional communication technologies and enables real-time communication even in heterogeneous environments - in other words, where different bus systems and Ethernet are in use.

Requirements for Real-Time

One of the most fundamental requirements for real-time systems is ultra-high-precision clocks in each end device to be synchronized to allow each data packet to be timestamped. This is the purpose of Precision Time Protocol PTP-1588. Packets in conventional TCP/IP or UDP protocols are not timestamped, but with (g)PTP and 802.1Qbv-2015 they can still be used for real-time communication.

Based on the timestamp, each data packet is also assigned a time window (scheduling and traffic shaping) and priorities (selection of communication paths, reservations and error tolerances). TSN provides eight priorities that stipulate the maximum response time for the data packet. Time-critical communication is only possible using these three factors.

An Ethernet frame, which is a record containing data such as the destination and source address, control information, etc. measures 1500 bytes when considering just the payload (without a header, trailer and safe time (time between two data packets). When you add the data link information to this, the total data packet comes to 1538 bytes with a 12-byte safe time (9.59ns). So, at 100Mbit/s, such a packet would need 1.23µs. This means that TSN can be used to achieve precisions measurable in µs. With purely hardware-based solutions, time packets measurable in nanoseconds are possible.

The part of the TSN mechanisms responsible for real-time support is located in the second layer - the data link layer - of the seven-layer OSI model. As the functions of this layer are standardized under TSN, different protocols can use the same network infrastructure. The second layer is divided into two sub-layers in which the real-time support protocols are implemented and via which the TSN data can be transmitted: the MAC (Media Access Control) and LLC (Logic Link Control) layer.

TSN offers entirely new opportunities in many fields, but two in particular benefit especially from these standards: industrial automation and automotive engineering.

TSN in Automotive

A growing number of driver assistance systems are essentially a precursor autonomous driving and require not only higher data rates in the vehicle but also deterministic communication - this means that data needs to reach the recipient in a precise pre-determined time - with low latency and failsafe mechanisms.

This can be achieved with TSN regardless of the communication technology used thus far. As the cables are by definition very short in a vehicle, real-time support is fairly easy to achieve here. With Gigabit Ethernet, TSN can achieve even lower latency times and even less jitter.

TSN in Industrial Automation

In industrial automation there is currently a diverse structure of different fieldbuses that all need to be migrated to TSN, which is why it will take longer here for the standards to become established than in the automotive sector. There is a strong motivation to use TSN though, as it ensures that significantly less communication hardware is required and that eliminates the need for the masses of protocols in use.

From a current perspective, one of the long-term objectives would be to use TSN down to the sensor/actuator level, as the costs for communication hardware such as MAC/PHY and microcontrollers are still too high.

Solutions for TSN

Toshiba's Neutrino family is recommended for implementing TSN at manageable cost. The Ethernet-AVB (Audio/Video Bridging)/TSN bridge solution is based on the TC9562 network interface controller. It supports the standards IEEE 802.1as for time synchronization, IEEE 802.1Qav (Ethernet-AVB) and IEEE 802.1Qbv for traffic shaping (the definition of rules for processing and forwarding network packets), IEEE 802.1Qbu and IEEE 802.3br for frame pre-emption, which is a more efficient method of transmitting non-time-critical data.

If the TC9562 is connected to an application process or another host SoC, the host device can transmit data (e.g. audio, video and control data) over 10/100/1000Mbit/s Ethernet. On the host, it has a PCIe interface with 5GT/s. The module features an integrated ARM Cortex-M3 with 187MHz clock speed, fast RAM and, at its heart, an AVB and TSN-capable MAC that enables real-time transmission and Quality of Service. As this is mostly taken care of by the on-chip controller, the extra workload for the host is minimal. All that's otherwise needed is a suitable PHY. A reference board along with extensive software is provided for development, including Linux drivers.

An interesting alternative is Intel's I225, as it has a combined MAC and PHY and includes the IEEE-1588 feature (timestamp generation) in the hardware. Aside from the real-time clock (1588) implemented in the I225, all real-time protocols required for TSN must be implemented by skilled developers in the second layer and processed by the host processor.

 

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Figure 2: How existing machinery and devices can be used to develop a TSN network
Figure 2: How existing machinery and devices can be used to develop a TSN network