Ring Topology for Control Networks

Ring networks have been around since IBM engineer Olof Söderblom conceived of the token ring network in 1981. It is one of six recognized network topologies, the others being bus, star, daisy-chain, tree, and mesh. Hybrid networks combine subnets with different combinations of these six topologies.

The term topology refers to the mathematical study of connectedness, and so obviously applies to networks. It also, however, applies to many other situations. It was originally conceived by Leonhard Euler in his 1736 paper on the famous Seven Bridges of Königsberg, in which he showed that it was impossible to walk a route through the city that would cross each of its seven bridges only once.

Most networks today use star topology in which data terminal equipment at nodes pass messages directly to hubs, which then forward them to wherever they are supposed to go. A point failure in any link splits the network into isolated subnets.

Star issues

The most common network topology in use is the Ethernet star. There, every network node has one connection – to a central hub (usually a router or switch). Star-network hubs connect to other hubs as well as to nodes. Each link is bidirectional, meaning that signals can travel both ways. In fact, a large fraction of the traffic typically involves “handshaking” signals that don’t really carry messages, but are used to ensure that messages get where they’re supposed to go.

The problem with star topology is that it any break in any link splits it into two unconnected subnets.

If the network in question is a factory intranet carrying, say, test data to a server area network for archiving, such a disruption is annoying, but not critical. On the other hand, if the network is carrying time-sensitive control signals, it’s a whole other kettle of fish! It’s even more serious if the network is carrying safety-critical signals. Having your safety network disrupted because somebody bumped a cable tray with a forklift is simply not acceptable.

Robust rings

System integrators working to tie islands of automation together into shop-floor-wide automated systems have started turning to ring topology for a more robust network. Ring-network topology requires two broken links to create an isolated subnet. That means you square the probability of failure (squaring a number smaller than one makes it even smaller) by requiring two failures to disrupt the network instead of only one.

Several companies interested in promoting control networks and networked safety technology have quietly introduced robust ring networks for mission and safety critical applications. Among them are Siemens, Rockwell Automation, and Mitsubishi, but there are others as well. If you are considering network-based control communication, and especially if you are considering networked safety, talk to your automation provider about ring topology.