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TSN IN AUTOMATION: WHERE ARE WE CURRENTLY AT?
These days, anyone working in industrial communications is bound to come up against the topic of time-sensitive networking (TSN). TSN will definitely come; the only thing that still has to be clarified is when and in what form it will come. However, even today the advantages of it for industrial communications are not always clear. Volker E. Goller of Analog Devices, explains...
E
thernet was introduced to offices in the early 1980s and quickly became
very popular due to its high throughput of (at that time) a sensational 10Mbps. However, this Ethernet was not practical for real-time applications because it used a common medium known as a party line. Collisions occurring at high utilisation rates caused problems in office settings. In the next step of its development,
collisions were eliminated through the introduction of switched networks. Additionally, with quality of service (QoS), Ethernet datagram prioritisation was introduced. For industrial applications, guaranteed
latency is particularly important. Despite QoS, standard Ethernet as used in offices can only guarantee latencies up to a certain point, especially with high network utilisation. There are several reasons for this, with the main ones being the store-and- forward strategy commonly used in commercial multiport switches and the fact that it is impossible to reserve bandwidth. ‘Store and forward’ means that a switch receives a complete datagram before
forwarding it. This has advantages in terms of processing in the switch, but it also brings with it potential problems that can negatively impact latency and reliability: • When going through a switch, a datagram is delayed by an amount depending on its length. If switches are cascaded, the effect is magnified. • Because a switch does not have an infinite storage capacity, it can reject datagrams if the network is experiencing overutilisation (too much traffic); this means that datagrams, even those given higher priority, can simply be lost. • Long datagrams can block a port for
relatively long times. Switch cascading posed a challenge in
industrial environments right from the start. Apart from the star topology used in the IT field, line, ring, and tree topologies are frequently used in automation. These adapted topologies significantly reduce Ethernet installation wiring requirements and costs. Hence, in industry, two-port switches employing a cut-through strategy are integrated into field devices. ‘Cut-through’ means that datagrams are forwarded before being completely received.
Figure 1. Ethernet frame: Data fields relevant to TSN data flow identification are shown in green
Figure 2. Topologies
It must be guaranteed that there is
always enough bandwidth (and buffer space) available for high priority datagrams. Standard Ethernet hasn’t been able to provide that yet.
ONE SIZE FITS IT ALL: INDUSTRIAL ETHERNET UP TO NOW Because classic Ethernet did not have sufficient capabilities for bandwidth reservation, automation experts began developing their own Ethernet extensions in 2000. However, the paths they took differed greatly. Differentiation is made between the following approaches: • Protocols using Ethernet as a transport medium for a fieldbus. These protocols claim complete control over the Ethernet medium for themselves. Classic TCP/IP communications are only possible in piggyback style via the fieldbus (EtherCAT and POWERLINK) or through a channel assigned by the fieldbus (Sercos). Bandwidth control is firmly in the hands of the fieldbus. • Protocols that guarantee bandwidth
reservation through a time slicing procedure on the Ethernet. PROFINET IRT should be mentioned here. IRT enables hard deterministic real-time data transmission on the same cable on which soft real-time or background traffic is operated. A precise timing model for the transmission paths is necessary for planning of the time slices. • Protocols based on sharing of the Ethernet cable. These protocols use QoS and are at home in factory and process automation applications. PROFINET RT and EtherNet/IP are noteworthy examples. These protocols are limited to the range of soft real-time (cycle time ≥ 1ms).
8 MAY 2018 | AUTOMATION /AUTOMATION
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