A hub is not a router is not a switch

Industrial Networking, ControlDesign.com

Keywords: Mark Lamendola, industrial network, hubs, routers and switches

This back-to-basics piece from Mark Lamendola shows how network performance relies heavily on a thorough knowledge of the capabilities and functional differences between hubs, routers, and switches.

By Mark Lamendola, Contributor

IN DATA networks, the three main hardware components besides cable and connectors are hubs, routers, and switches. How you select and implement each will determine the functionality, reliability, security, and flexibility of your industrial network. Let’s look at these devices, starting with the simplest of them.

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, including 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, 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. “Since we develop our own control systems and use Ethernet in special ways that are internal to our system, we use hubs to monitor traffic. We also use hubs due to cost and lower latencies,” adds Ernesto Colon, vice president of Turbine Diagnostic Services Odessa, Fla, a field service company servicing power generation equipment, manufacturing turbine, and balance-of-plant (BOP) controls for various industries.

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 (VLANs) segments in a switched environment. But a router doesn't propagate broadcasts. Switches do."

Routers also solve some otherwise intractable security problems. “A 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 Inc., 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 believes. “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 communication.”

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 each device, as the power-up order now has changed, is re-assigned an address. This drives the control system crazy, and production time is lost while it’s all sorted out.”

	FIGURE 1: COMPLEX MACHINE, COMPLEX NETWORK
	An industrial communications network can be as complex and multi-layered as the production machine system itself, in this case a paper-making system. Souce: Contemporary Controls

Switches Make it Run
While routers connect networks together, switches actually run the network. This is when expertise-driven functions such as product selection, system design, and installation quickly become complicated (See Figure 1).

So, the switch is the real workhorse of industrial networks, as Granville hinted, but it needs to be industrial grade. “Industrial switches have several advantages over their commercial counterparts,” says Roger McFall, control systems project manager at A&E Engineering in Greer, S.C., which provides automation and information services from design through integration. “Compact size with DIN-rail mounting is one advantage. Low port density is another, as many plant applications have a small number of Ethernet devices per panel, but have many panels.”