Lots of Network Choices. Choose Your Design Wisely

Make Informed Decisions About the Data Link and Physical Layers. Here's a List of Other Decisions to Be Made

By Ian Verhappen, Certified Automation Professional

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Engineering is in large part about making choices — informed choices — with sound reasoning and judgment. This is no less true when you design industrial networks, especially if you plan to use Ethernet. I say this about Ethernet because it has become so ubiquitous and relatively robust that I feel many people believe that all you have to do is run cable, plug it in to the right port on the end device or intermediate node, and then the software available in the transport and network layers (TCP, UDP and IP) on the appliances will overcome any design mistakes.

Assuming that software will compensate for design mistakes, you still need to make other choices for the data link and physical layers, including whether your physical layer actually will be physical (copper, fiber) or wireless.

SEE ALSO: Is Ethernet the Best Choice? Find Out in Our Industrial Networking Q2 2013 Issue

A sampling of some of the other decisions to be made include:

1) With the increasing use of IPv6 will your network require supporting this revision and all the enhancements it brings? Is quality of service (QoS) required, so you can assign priorities to different types of messages? If so, QoS, which is part of IPv6, could push you in this direction. What about support for other protocols, such as 6lowPAN and ISA100.11, which both use IPv6?

2) What about node power? Will you have separate power to each device or use Power over Ethernet (PoE)? If PoE, which version? IEEE 802.3af, IEEE 802.3at or a proprietary eight-wire offering?

3) Will you use static or dynamic IP addressing, and how will you manage your addresses to avoid conflict, as well as device replacement?

4) What about the distance between nodes, since copper still is restricted to 100 meters? Beyond that length you generally move to fiber and then must choose between single- or multi-mode fiber, as well as plastic or glass core. If you choose the wireless route, then distance becomes more complex. (See "Wireless Topology: You've Got Choices")

5) If you install a wireless network, will you use a dedicated licensed frequency or a license-free, ISM-based system? Regardless of which you choose, it's a good idea to conduct a spectrum study to determine possible sources of interference, especially in an industrial setting where there are multiple sources of EMI/RFI noise, as well as opportunities for signal reflection.

If you choose to implement an ISM-based solution, then spectrum management becomes more important because there are potentially many signal sources using the same frequencies and channels, including those you're using with IEEE 802.11-based systems for backhaul and IT infrastructure, and IEEE 802.15 systems for field-level networks and Bluetooth devices.

Ethernet represents the lowest four layers of the OSI model. The protocol (typically Layer 7 or "User Layer 8") and security are equally important. Remember that most, but not all, industrial networking protocols use "standard" TCP/UDP- and IP-based Ethernet, so verify that your network equipment supports the protocols you plan to use. Ask the company from whom you're buying the field nodes to suggest potential suppliers. The suggested supplier likely has been tested with the other equipment you're buying, thus removing another potential conflict.

The most basic consideration for cybersecurity, besides the physical security aspect, is to work with your IT department to avoid the potential of duplicate IP addresses and preferably to negotiate for a separate, dedicated range of IP addresses for the process control network(s) you will install. Depending on the size and complexity of the networks, this negotiation can be as simple as having a dedicated subnet mask reserved for industrial network use. An important side benefit of working with the IT department is that making IT aware you have networks means you now a potential ally to help you in the event that something does go wrong.

Devices and networks are becoming more intelligent, so making connections is that much easier because of the solid engineering behind the products. A good, solid industrial network, like anything else, starts with a good foundation, which means a well-thought-out, engineered and designed solution that requires knowledge and making the right choices at the right time.

Each new year brings with it exciting new technologies, each solving a particular problem or creating a new opportunity. However what remains the same is that we will continue to have to make choices to obtain working systems and solutions. That's what engineering is all about. 

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