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How can ring topology help control networks?
December 3, 2007

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.

One simple, but very powerful, topological concept is that of genus, which is a way of classifying extended objects by the number of cuts possible before destroying their connectedness (in other words, splitting them into more than one object). Objects of the same genus are said to be topologically equivalent.

All genus-0 objects are topologically equivalent to a sphere. That is, ovoids, cubes, cylinders, human body shapes, and any other object that can’t be cut completely through without splitting it in two.

All genus-1 objects are topologically equivalent to toroids. Cutting once completely through the side of a doughnut, for example, still leaves it as one object—but reduces it to a genus-0 object. A second cut splits it into two objects. (When you bite into a doughnut, each side of your mouth makes a cut, leaving you with two objects: the part in your mouth and the part in your hand.)

A two-handled cup is genus-2. Cutting through each handle once reduces it to a genus-0 object and a third cut makes it into two objects.

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.

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).

Note that here I’m using the word “hub” to mean the device in the star’s center. There is a device called an “Ethernet hub,” which simply repeats any message it receives to all nodes connected to it. These have generally been superceded for duty in the centers of star networks by routers and switches, which have more intelligence to actively manage traffic.

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 is a genus-0 shape. 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.

Ring-network topology requires two broken links to create an isolated subnet.

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. A ring is a genus-1 shape: it takes two cuts (point failures) to isolate parts of the network. 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.

Control Engineering offers more information on ring topologies.

For Ethernet-related products, check the Control Engineering SupplierSearch Buyer’s Guide.

Posted by Charlie Masi on December 3, 2007 | Comments (0)