Automation of industrial sites is becoming a must due to the time, cost and effort that is saved in the running of the site, as well as the increased safety and productivity that automation brings. However, when automating an industrial site, the communications network is key and having a stable and reliable network is crucial. In this article we will explore how industrial networking has evolved from being just another component handled by the IT department and why it now requires specialist skills to design, implement and maintain an industrial Ethernet network.
In the past, automation was generally done using serial communication to interlink devices. Serial communication is generally quite simple as the communications are not happening over a full network, but rather over simple point-to-point links, or in some cases point-to-multipoint using RS-422/RS-485. These communications are simple in that there are not many configuration options needed.
Serial communication comes with some downsides as well, such as the fact that the communication distances are limited, the actual limits depend on cable, serial protocol used, end equipment, interference and many other factors. Other downsides include a large amount of cabling as well as a lack of redundancy, only end devices with multiple serial ports would be able to support redundancy. Serial communication also requires new cabling to be laid and connected for any new serial links.
Industrial Ethernet
To solve these issues, engineers in charge of automation on industrial sites are showing a tendency to upgrade to Ethernet as the technology of choice for running the automation. Ethernet can solve many of the shortfalls of serial communications and provide many more benefits. However, an Ethernet network does require correct planning, design and implementation in order to cater for the reliable running of critical processes. But often we see that the responsibility of planning, implementing and maintaining the Ethernet network is given to an engineer who, despite having a thorough knowledge of serial communication and automation, is still a newcomer to Ethernet. Because of the complexity of Ethernet, when compared to serial communications, it can be very intimidating to setup the network and for this reason the networks are often very simple and not utilising the advanced technologies and functionality available within Ethernet. Simple networks such as these are often not suitable for the running of site automation and will often require more time and money than they would save.
One of the key points to aim for on a mission-critical network is reliability and uptime, as any failure or downtime in communications can cause shutdowns, damage to equipment, loss of production and even, in extreme cases, injury or loss of life. For this reason network redundancy must always be kept in the forefront when designing and implementing the network. This is one of the biggest differentiators between an IT network and an industrial one. In a corporate environment, network failures can lead to a delay in emails or inability to access the Internet. This is an irritation not a disaster like it would be on a mission-critical network. The focus for corporate networks is more often about increased bandwidth, which means that corporate networks have a much different priority to industrial ones.
Redundancy is essential on industrial networks
There are various redundancy options that can be applied to an industrial network each of which protects against different failures. One of the first to look at during the design stage is cable redundancy, which protects against cable breaks by redirecting the traffic along a different path in the event of a failure. In Ethernet, a physical ring with no redundancy intelligence can cause broadcast storms that fail the entire network if no redundancy protocol is configured. The most common redundancy protocol used these days is RSTP (Rapid Spanning Tree Protocol) which is an open standard and thus most, if not all, Ethernet hardware vendors support it. RSTP, along with most other cable redundancy protocols, works by disallowing network traffic across one of the ‘loop’ connections under normal operating conditions, meaning that they impose a virtual break in the cable and thus avoid traffic loops. However, in the event of an active cable being broken, one of the redundant links will activate in order to keep the network alive and allow all devices to continue communicating.
Here we find another of the big differences between corporate and industrial networks. Whilst a corporate network can afford to wait a few minutes in the event of a cable break, on a mission-critical network this amount of delay is unacceptable. RSTP provides a recovery time in the region of 30 seconds in worst case, however even this can often be unacceptable for industrial systems. Various other redundancy protocols are available from different vendors providing much improved recovery times. For instance RuggedCom’s eRSTP (Enhanced RSTP) which can provide recovery times of only 5 ms per hop. However, it is important to research which redundancy protocol will best fit the requirements without vendor locking to a single manufacturer in the future. (Some vendors' proprietary protocols will provide backwards compatibility with open standards such as RSTP).
Ethernet is constantly evolving, and two of the newer redundancy protocols to look out for are PRP (Parallel Redundancy Protocol) and HSR (High-Availability Seamless Redundancy). The former works by implementing two completely separate LANs and end devices need to have two separate Ethernet ports. Within each LAN one will also run further redundancy for that individual LAN. This means that even in the event of one of the LANs failing, the second LAN will take over the load completely which has the advantage of allowing users to replace or repair hardware and links without affecting live communications.
HSR works by sending two identical data streams in different directions around the network. This means that if there is a failure on one of the communication paths traffic will still make it through the second communication path. Intelligence within the HSR hardware will allow the end devices to discard duplicate information received and thus aggregate the two data streams into a single one with no duplicate packets.
The next redundancy to look at is hardware redundancy, which means protecting against the failure of networking equipment such as routers and switches. Various options are available: equipment with dual-redundant power supplies, carrying enough spares on site to facilitate rapid replacement of faulty equipment, or picking hardware that caters for hot-swappable power supplies and modules. One can also look at providing multiple backbone switches and correctly configured link redundancy, such that if a switch fails a backup path will always be available. When talking about critical routers, one can use a protocol called VRRP (Virtual Router Redundancy Protocol), which pairs two physical routers into a single virtual router on the network. This means that if the main router fails, the backup router will take over with no configuration changes required on the end devices, and, the end devices will not need to re-associate a new MAC address to the router IP.
Conclusion
As we can see, one of the key differences between a corporate commercial network and a mission-critical industrial network is the level of reliability required. Whilst corporate networks are more concerned about high bandwidth, even if the network is sometimes not available, mission-critical networks are much more concerned with uptime and redundancy in the case of failure. Generally, they do not require too much in terms of bandwidth as the protocols used for site automation are generally not very bandwidth intensive. In a future article we will explore the differences between protocol usage and security levels on mission-critical networks.
For more information contact Tim Craven, H3iSquared, +27 (0)11 454 6025, [email protected], www.h3isquared.com
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