How to implement Time Sensitive Networking to ensure deterministic communication
Table 2: EtherCAT, PROFINET and TSN have similar features, but implement them in different ways. Image source: Texas Instruments
Table 1: TSN relies on numerous sub-standards to provide deterministic performance, redundancy, and other features in a modular fashion. Image: Texas Instruments
exceed the performance of the other protocols. Support for deterministic traffic in EtherCAT is
IEC/IEEE 60802 will go beyond the IEEE 802 standards and is being developed in recognition of the fact that users and vendors of interoperable bridged time- sensitive networks for industrial automation need guidelines for the selection and the use of TSN related standards and features in order to effectively deploy converged networks that simultaneously support operations technology traffic and other traffic. The release of the IEC/IEEE 60802 TSN Profile for Industrial Automation could prove to be a source of confusion, at least initially, since various field buses are often referred to as the ‘industrial Ethernet’. TSN and field buses The use of TSN and field buses is not an either-or proposition. They are compatible, often used together and all employ concepts related to time synchronization. Yet field buses like PROFINET, EtherNet/
IP, and EtherCAT, implement synchronization in different ways. PROFINET uses the precision time control protocol (PTCP). EtherCAT uses distributed clocks that employ dedicated and associated registers for synchronization.
limited to special types of data packets. Using EtherCAT and TSN in combination can improve flexibility. For example, around synchronization, TSN adds multi- master capabilities. All three protocols provide redundancy in different ways. TSN uses a technique like the parallel redundancy protocol (PRP) and the high-availability seamless redundancy (HSR) protocol as defined in IEC 62439-3 implement zero-loss redundancy (Table 2). TSN does not include an application layer and does not challenge field buses at the application level. For example, interconnecting machines with switches while still using EtherCAT at the machine level can create an industrial Ethernet network that includes TSN functions. A TSN-EtherCAT integrated network does not mix the technologies but defines a seamless integration to use both technologies and realize the best performance aspects of each one.
PROFINET and EtherNet/ IP include the IEEE Ethernet
learning bridge as the underlying switching technology. As a result, these protocols can now adapt the extension of TAS and frame preemption to use standard TSN hardware. EtherNet/IP uses UDP packets for data exchange and is compatible with the TSN switching layer. PROFINET supports a direct layer-2 buffer model for data supported by the programmable real-time unit industrial communications subsystem (PRU- ICSS) TSN solution. TSN is designed to support cycle times at least as low as EtherCAT and PROFINET and other industrial Ethernet protocols.
Related sub-standards include: 802.1CB (frame replication and elimination), 802.1Qca (path control and reservation), 802.1qci (per-stream filtering and policing), and parts of 802.1AS and 802.1AVB (reliability for time synchronization from the timing and synchronization parts of TSN and the IEEE audio bridging standard). There is a group of general sub- standards related to dedicated resources, APIs, and other necessary ‘overhead’ features including higher level planning and configuration and interoperability in heterogeneous networks. Examples of these general sub-standards
include: 802.1Qat (stream reservation protocol), P802.1Acc (TSN configuration), compatibility with YANG (Yet Another Next Generation) data modelling language, and 802.1Qdd (resource allocation protocol). The modular design of TSN enables it to be optimized for specific applications and use cases. Not every feature is needed every time. For example, 802.1AS, timing and synchronization are especially important in all factory automation uses of TSN while redundancy may be required by only a subset of automation use cases.
How does IEC/IEEE 60802 relate to TSN? At the time of this writing, the IEC/ IEEE 60802, Draft 1.4, TSN Profile for Industrial Automation is out for comment and is expected to be approved sometime in 2023. This IEC SC65C/WG18 and IEEE 802 project will define TSN profiles for industrial automation. This joint effort will include profile select features, options, configurations, defaults, protocols, and procedures of bridges, end stations, and LANs to build industrial automation networks. Like the existing IEEE 802 TSN standards, 60802 will be flexible and modular and address a range of networking scenarios.
When upgraded to Gigabit Ethernet, TSN is expected to
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