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Industrial Ethernet Book 105

Technology Results of time synchronization using IEEE 802.1AS in a line of 50 devices. Every 10th device has been measured. The standard deviation of the PPS precision is well below 50 ns under laboratory conditions. UA TSN is a combination of the two methods introduced above. The frame transmission delay with values for Pub/Sub – thanks to frame aggregation and an efficient frame format – becomes: τ = 8(51 + max(33, n × (x + 3))) C The total minimum cycle time becomes: Γ = δ + l + τ It can be noted that the achievable cycle time compared to today’s solutions over various parameter combinations is much lower, roughly by a factor of 18. Compared to hypothetical devices with Gigabit circuitry based on otherwise unchanged mechanisms of today’s fieldbus technology, the factor is close to two. Industrial traffic types Companies developing new OPC UA TSN systems have a variety of TSN standards from which to select the right features for their application. This often involves attempting to match the behavior of the legacy technology as closely as possible. Extrapolated to the industrial automation market at large, what this tells us is that, in order to be broadly adoptable, a solution must support all currently used industrial traffic types simultaneously. Today’s technologies implement a variety of traffic types. Most of them allow for a distinction between periodic and non-periodic traffic, which in turn differ in the nuances of their actual properties – ranging from hard real-time traffic with distinct sending, transmission and receiving times per cycle; to periodic traffic with or without time synchronization; to non-periodic traffic stemming from a multitude of sources, of which TCP/IP is an increasingly significant example. In some cases, network control, diagnostic information and user control messages have distinct priorities. We have evaluated these and arrived at a superset, and the variety of traffic types being communicated through industrial communication systems. A converged network needs to support all those types, even if not used in a particular application. The selection of the shaping mechanisms used for implementation needs to be globally standardized The main feature of TSN is the possibility of coexistence of different traffic types, while maintaining the timing properties of real-time traffic. Some existing real-time technologies (EtherNet/IP, Profinet) use traffic planning and QoS to ensure real-time behavior under the condition of well-behaving devices. With TSN as data link layer, those technologies can leverage better bandwidth efficiency, since TSN protects high priority traffic unconditionally. Setup Calculating theoretical performance estimates and defining traffic class requirements are one thing; real-world implementations with hardware and/or software limitations are a different matter entirely. 100-Mbit industrial Ethernet technologies have reached a very high level of maturity, meaning that almost all current devices are capable of delivering full network performance. For Gbit technologies, this is not the case. As mentioned above, Gbit increases performance by approximately a factor of 10 on switched networks. Frame aggregation, optimized headers and ultra-low cut-through latency can bring further improvement by approximately a factor of two. In order to exploit this performance in a real product, many of its components need to be optimized. Many prototype devices have already been implemented and also tested by the authors, for instance in the IIC testbed. Two of those prototypes have been used for evaluation in this article: one based on a single-port industrial PC running Linux, and an embedded one in the form factor of a head station of a modular I/O block featuring two external network ports, also running a Linux OS. Standards and technologies An overview of the protocols and services used by OPC UA TSN shows how they fit into the layers of the ISO/OSI reference model. The following will discuss the requirements and properties of the respective layers. Physical layer The following physical media are the most widely used in industrial networks and therefore offered by most vendors: • Copper-based: Fast Ethernet (100BASE-T/ T1) & Gigabit Ethernet (1000BASE-T/T1) • Fiber-based: Fast Ethernet (100BASE-SX) & Gigabit Ethernet (1000BASE-SX) For process automation, a working group has been founded to develop 10-Mbit single twisted pair Ethernet (10SPE). This media could facilitate the spread of Ethernet to even smaller and more cost-sensitive sensor and actuator devices and Zone 1 hazardous areas. Data link layer The term TSN refers to a family of standards under development by the Time-Sensitive Networking task group of the IEEE 802.115 working group. It is worth noting here that 802.1 standardizes Ethernet switches (they call them “bridges”), and 802.3 standardizes Ethernet endpoints. The following list introduces the standards relevant for industrial communication: IEEE 802.1AS-Rev: A profile of the IEEE 1588-2008 clock synchronization standard was developed and adopted for addressing larger Ethernet systems resulting in IEEE 802.1AS. Unfortunately, the two are not compatible. Within the TSN working group a revision of IEEE 802.1AS (.1AS-Rev) is being developed. This revision addresses the mechanisms for grandmaster redundancy and multiple clock domains (e.g. simultaneous distribution of a working clock as the basis for isochronous transmission and a wall clock for applications such as logging messages). The publication of .1AS-Rev is planned for 2018; we strongly encourage machine, factory, and process automation vendors to implement .1AS (rather than IEEE 1588) for reasons of interoperability and proximity to the final solution. Also, 802.1AS is the default solution promoted by AVnu and the IEEE TSN Task Group. IEEE 802.1Qbv: This is used for isochronous transmissions with real-time guarantees. It specifies the transmission windows in order to guarantee bounded latency and small jitter. Qbv also makes it possible to periodically give egress queues prioritized wire access, so it can also be used to provide bandwidth guarantees. IEEE 802.1Qav: This can be used for 24 industrial ethernet book 4.2018


Industrial Ethernet Book 105
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