Your network is the ultimate source of the freedom produced by smart manufacturing. A well designed and installed network can level even the most onerous challenges, but a poor design can be a source of inconsistent results and worse, intermittent outages. Most people are familiar with “office” Ethernet. This section will explore what you need to know about an “industrial” Ethernet installed for performance and help you build your network for smart manufacturing.
Assessing your Network Requirements
If you are already using Ethernet for industrial applications then it may be a good idea to assess what you have. It has been all too easy for “office” Ethernet components to drift onto the factory floor. Moreover, it’s not uncommon to find exceptionally poor practice, as most uniformed installers are not aware of the critical nature of the network. Finding a patch panel literally duct-taped to a column will make anyone a believer in a network assessment.
A reliable EtherNet/IP installation requires some basic understanding and planning. This is especially true for control applications. Also, the industrial environment itself requires additional criteria for vibration, abrasion, contaminants and noise.
There are three primary considerations when planning a new network:
- Components (media) selection, which may vary pending performance requirements
- Performance, which will vary on the level of application
- Assurance and security
Though outside the scope of this paper, network security requires design considerations at machine/cell, plant floor, and enterprise levels. Cisco provides an excellent overview in their Converged Plantwide Ethernet (CPwE) Guide, which is referred to later in this document. Per the CISCO CPwE guide:
Connecting the industrial automation and control system (IACS) network to the enterprise network exposes the security risks of the Internet and enterprise network to the IACS application. Mitigating these risks may be more difficult and more critical than in the enterprise network because of the higher requirement for availability in an IACS and the sensitivity of these systems to different disruptions. Of the three security properties of confidentiality, integrity, and availability, IACS applications are primarily concerned with availability and integrity.
EtherNet/IP Media Components
The following is a very basic overview of Ethernet media, and it does not delve into fiber optics. More information can be found on the ODVA web site, www.odva.org.
Copper cabling for Ethernet is rated in categories. Most installations will be Cat5e or better, 4 pair cable. Cat6 is typically required for speeds of 1G or higher.
Another cable consideration is the jacket itself. It must be able to withstand the temperature (extremes and fluctuations) as well as the contaminants of the environment (such as oils).
Shielded cable (ScTP) may be required for environments with higher noise levels or when required by performance. Another option is to consider fiber optic media in lieu of copper.
There are two basic types of industrial, IP65/IP67 sealed connectors suitable for use for EtherNet/IP applications. These connectors are designed for appropriate balance, to minimize cross talk, and limit susceptibility to vibration.
The first is encapsulated 8-way modular connector, which looks similar to a standard Ethernet connector. The second is a 4-Pole M12 “D” round connector. Both are designed to provide sealing to IP65/67 in accordance with IEC60529. Another variant of these connectors is a bulk head feed through, which is used to pass a cable connection through a surface, such as an enclosure door. Either connector style is available in plastic and metal housing construction. The ODVA provides a guideline to assist with selection.
M12 Connector Specification Sample, ODVA
As a general rule, hubs are for offices and are generally discouraged in control applications. They are not secure and do not help in managing data collisions. Collisions cause transmission retries or dropped frames and increase jitter in control systems. That’s a bad thing.
Hub technology has been replaced by high-speed switches that “can simultaneously move frames between pairs of ports at full wire speed”. That means each port on a switch can manage traffic at full rated speed while also minimizing collisions. That’s a good thing.
A switch segments a network into many parallel dedicated lines to produce a contention-free architecture. Switches that have the ability to provide full duplex, Internet Group Management Protocol (IGMP) snooping and port mirroring are beneficial in troubleshooting.
 ODVA Ethernet Design Guide
As stated in Part 2, network determinism is always a concern with control level networks. As your network grows, system wide determinism becomes a concern. According to CISCO,
“The network’s main impact on a system’s determinism is based on the following network performance characteristics:
- Latency—The average amount of time a message takes to be transmitted and processed from originating node to destination node
- Jitter—The amount of variance in the latency.”
Fortunately there is a large body of work we can lean on when considering network performance. As discussed in Part 2, network requirements vary depending on the application. Information level sharing does not require the speed of PLC I/O for example. CISCO has taken another step in more clearly defining basic speed requirements, as shown below:
The network design, including the selection and placement of switches, will be dependent on the cycle time required. Other parameters that must be considered include:
- Network media selected, as previously discussed
- The connection topology used
- Number of Ethernet nodes (end-devices) on the network
- Number of multicast addresses in use (roughly the number of I/O connections)
- Number of switches required
- Network bandwidth utilization
CISCO has produced a guideline that they refer to as the Converged Plantwide Ethernet, or CPwE. According to CISCO, “the purpose of the CPwE architecture, a set of manufacturing focused reference architectures, is to help accelerate the successful deployment of standard networking technologies and convergence of manufacturing and enterprise/business networks.” 
Put another way, it is a tested, standard approach to building a killer control network that also needs to support enterprise (IT) requirements. The CPwE model provides for a defined space between your plant floor Ethernet and your Enterprise Ethernet connected to the web. CISCO refers to this as the demilitarized zone, or DMZ. CPwE can therefore be used to ensure your system design meets both real time and security performance requirements.
In our next post we will discuss our final recommendations for building a reliable network for the future of manufacturing.