Last year, we celebrated our 10th anniversary by republishing some of the more 'timeless' content we'd produced since 2002. They were well-received, so we decided to do it again from time to time. Here's one from Summer 2006 that reminds us, as Ethernet was emerging, how switches looked to be a key to the performance and reliability that industrial users would need.
In Ethernet networks, the three main hardware components besides cable and connectors are hubs, routers, and switches. How and why you select and implement each of them will determine the functionality, reliability, security, and flexibility of your industrial network. Let's look at these devices, starting with the simplest of the three.
Hubs in the Network Universe
A hub acts much like those three-way plugs you might use in your home's electrical outlets. Hubs simply connect one pathway to another. This capability can actually complicate things, reports the engineering staff at FLG Networking Services in Overland Park, Kan. FLG provides network design and security services to industries that include polymer processing, paper goods, and printing. "When you put multiple devices on a cable without some kind of traffic control, you get collisions," says Fred Granville, FLG's principal. "Hubs assume every device is on the same IP subnet." In short, a hub doesn't differentiate between the data paths it's connecting; it just connects them.
"We generally don't recommend hubs, but they still have a place," advises Dan Parker, project applications engineer at Curry Controls in Lakeland, Fla. "For fieldbuses, the hub provides an essential function for impedance matching and multiple connections. And, where a customer requires a simple data connection, we use simple, low-cost hubs to provide it." Curry provides design, engineering, integration, installation, and service for industrial and municipal process control and radio telemetry systems.
So, hubs are useful when you don't need to differentiate between the data paths you're connecting, and just want low speed and no processing in that connection. "We develop our own control systems and use Ethernet in special ways that are internal to our system, so we use hubs to monitor traffic," adds Ernesto Colon, vice president of Turbine Diagnostic Services in Odessa, Fla, a field service company servicing power-generation equipment, manufacturing turbine, and balance-of-plant (BOP) controls for various industries. "We also use hubs due to cost and lower latencies."
In short, because hubs have limited capabilities, the heavy lifting in networking is left to the routers and switches.
Routers Provide Direction
Granville explains that routers are critical for networking. "A router provides a connection from one IP subnet to another, allowing you to talk between IP networks and subnets," he says "Routers interconnect local area networks (LANs) and virtual LAN (VLAN) segments in a switched environment. However, a router doesn't propagate broadcasts. Switches do."
Routers also solve some otherwise intractable security problems. "One customer wanted a webcam, but didn't want to open its network to an outside connection," says R. Andrew Bowman, PE, vice president for engineering at SiteSecure in Sanford, Fla. "We plugged a router into their network. The router became the device with the IP address." SiteSecure provides security and safety solutions, including design, installation and startup.
Parker also has seen many misapplications of routers. "Usually, these result from the great divide between the plant-floor, industrial types and the information technology types," he explains. "The IT folks want every device on the network to be DNS-enabled, so they have control over the device. The plant-floor folks want every device to have a fixed address, so it can be communicated with simply and easily on a regular, repeatable basis, as with, for example, PLC-to-PLC communications."
Parker believes the problem goes beyond turf battles. "Routers often operate as DNS servers," he continues. "In the typical scenario, the plant goes into operation and everything is fine until there's a power failure. Then, as the power-up order changes, each device is re-assigned an address. This drives the control system crazy, and production time is lost while it's all sorted out."