Industrial control requires real-time communication with deterministic latency. The used technologies have evolved from serial fieldbuses to industrial Ethernet protocols defined in IEC standards such as EtherCAT. These standards use parts of IEEE Ethernet to leverage some of the economies of scale that Ethernet offers, but the standards add small changes such as cut-through switching that go beyond and partially restrict the use of typical IEEE bridges and endpoints. Contrary to typical consumer or enterprise systems, where average responsiveness or throughput are key performance indicators, in these applications the performance need is bounded by the worst-case latency of interacting with inputs and outputs across the network.

A typical industrial control network topology is shown in Figure 1-1. Ethernet specifies layers 1 and 2 of the local area network (LAN). For standard Ethernet, this allows for stateless unreliable transmission of variable-sized frames from one endpoint to another endpoint and the switching in between. This domain is shown by the area in the dotted rectangle to the left in Figure 1-1. The protocol on top, for example EtherCAT, is highly asymmetric; there is one controller managing a few or even hundreds of devices. Different protocols use slightly different terminology for this asymmetric relationship, and there are varying levels of this asymmetry. This document uses the terms “controller” for the controlling entity and “device” for the devices being controlled.

Figure 1-1 A Typical Industrial Ethernet Network