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NOV-DEC 2018

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36 INTECH NOVEMBER/DECEMBER 2018 WWW.ISA.ORG Time-Sensitive Networking in automation Where are we now? By Volker E. Goller T oday, those working in the industrial communications industry are destined to confront Time-Sensitive Networking (TSN). TSN is coming, and while there are ques- tions about when and in what form it will come, it is unquestionable that the technology will have a profound impact on industrial commu- nications and automation. However, for many, the advantages of TSN are not clear. History Introduced to offices in the early 1980s, Ether- net quickly became popular due to its (at that time) high throughput of 10 Mbps. However, this iteration of Ethernet was not practical for real-time applications, because it used a party line, and collisions occurring at high utilization rates caused problems in office settings. Collisions were eliminated in the next stage of development through the introduction of switched networks. Additionally, quality of service (QoS) brought Ethernet datagram pri- oritization. However, even with QoS, standard Ethernet can only guarantee latencies up to a certain point, especially with high network uti lization—making it unsuitable for industrial applications, which rely on guaranteed latency. For high-priority datagrams for industrial ap plications, there must be guaranteed avail- able bandwidth and buffer space. Standard Ethernet cannot provide that, because of the store-and-forward strategy commonly used in commercial multiport switches and the impos - sibility of reserving bandwidth. Store and forward means that a switch re - ceives a complete datagram before forwarding it. This has advantages in terms of processing in the switch, but also brings potential problems that can negatively affect latency and reliability: l When going through a switch, a datagram is delayed by an amount depending on its length. If switches are cascaded, the effect is magnified. l Because a switch does not have an infinite storage capacity, it can reject datagrams if the network is experiencing overutilization. This means that datagrams—even those given higher priority—can simply be lost. l Long datagrams can block a port for relatively long times.

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