Editor's Note: This article was contributed by Julia Santogatta, Belden's director responsible for the wireless initiatives and Dr. Tobias Heer, Belden's Head of Embedded Development.
In Part One of this series we explained how recent advances in technology and standards have revolutionized reliability for industrial wireless applications; they are no longer the Achilles heel of signal transmission. Specifically, Parallel Redundancy Protocol (PRP) as defined by IEC 62439, is helping in this revolution as it greatly improves wireless network performance in three important areas:
- It drastically decreases the potential for data loss because it is a zero-failover design –renegotiation and signal recovery times are non-existent
- It improves transmission time by decreasing latency.
- It reduces jitter throughout the data transmission
The improvements in these areas are dramatic. In practical tests, the perceptible packet loss for an application with PRP was reduced to 0.00021 percent using a PRP connection. That’s a 500-fold reduction in packet loss!
It is quite unbelievable – but true to the level of improvement you could realize in the field. (See the technical article “Doubling Multiplies the Benefit” for more details on improvement measurements.)
Now, let’s build on the gains from standardized PRP and look at two additional developments that make today’s wireless even more practical. These implementation options include frequency diversification and the integration of PRP right into wireless equipment.
Figure 1: Recent improvements in industrial wireless technology make challenging applications, like wireless video surveillance for an oil refinery, practical and highly reliable.
Frequency Diversification and Integrated PRP Further Enhance Industrial Wireless
As you are probably aware, in many countries, wireless providers are able to broadcast on several unlicensed frequencies, which likely include two popular bands – 2.4 GHz and 5 GHz. Each band has different benefits and in turn, different weaknesses.
By adding frequency diversity to your PRP design – broadcasting one signal on 2.4 GHz and one on 5 GHz – you are almost guaranteeing another level of assurance. That’s because what might typically cause unintentional signal degradation on one frequency is highly unlikely to cause interference on the other. By introducing this mix, you further enhance the dependability of data transmission in your design.
Belden then further boosts the practicality and financial return of industrial wireless applications by integrating PRP capability directly into wireless access points. This means that in many network designs there is no longer a need for a separate “Red box” (Redundancy Box) to manage the packet duplication and merging.
Figure 2: Using dual frequencies improves the performance of wireless networks. In addition, integrated PRP reduces network complexity and costs.
Furthermore, PRP isn’t exclusive to just wired or wireless network designs. The fact that it can be used for completely wireless and hybrid wired/wireless designs also greatly increases the range and scope of possible solutions. How might these solutions benefit you?
Let’s look at a few examples of interesting network topologies that you might not have considered before. They may just provide you with the increased level of reliability you’ve been seeking.
Wireless as a High Availability Network
Do you have any applications that you’d love to upgrade or implement, but the amount of infrastructure you would have to install is overwhelming? Maybe an access control/perimeter security or video surveillance system?
Have you dismissed wireless because you would have to ensure the system is always functional? If so, think again.
We recently partnered with a local solution provider to implement just such a system for an oil refinery in the Middle East. The opportunity for noise interference, as well as the vast amount of geography and reflection involved made this a traditionally challenging wireless application.
In addition to the physical characteristics, the application transmits video, which required high bandwidth plus stable and reliable wireless links. Despite this, utilizing PRP on a wireless infrastructure provided the level of performance and system reliability needed.
Remember, PRP provides not only redundant, zero-failover data transmission, but improved network performance through inherent latency and jitter enhancements – making it ideal for demanding data applications.
Fig 3: An oil refinery was able to use wireless for a perimeter security and video surveillance system thanks to the high reliability delivered by wireless equipment utilizing PRP. Image source: Industrial Ethernet Book.
Wireless as a Redundant Backup to Wired Signal Transmission
Let’s go further. Imagine that you have a challenging application that is wired today, but has been known to be troublesome. This might be an overhead crane that is constantly on the move or it might be a piece of equipment near an oven or other high-temperature device, like a smelter.
In either case, the network cable has gotten damaged at least once and caused you and your plant heartache. Have you ever considered implementing wireless as a backup to the primary wired link? If you could save one, maybe two downtime events, would this system pay for itself?
Figure 4: PRP allows both wired and wireless routes to be used as redundant paths,
thereby enabling a variety of network topologies.
This combination of wired and wireless routing is possible with IEC 62439 standardized PRP implemented across both signal paths. The wireless path can be used as a switch-free backup path to the wired connection. While there are a few solutions out there using proprietary redundant data transmission, these are always focused on a single transmission path and, so, such a wired/wireless combination is likely not possible.
In comparison, standards-based PRP allows for more complex scenarios like this one to be cleanly implemented for your benefit. Knowing this, what is stopping you from implementing wireless as a backup?
Mobile Applications with Roaming PRP Devices
How about applications that involve a moving machine or a transportation vehicle, such as a rail car or train? Do you have something that travels along a path you would like to connect with several wireless access points? In this scenario, utilizing PRP provides not only two connections that can be operated at the same time, it provides the capability for always maintaining one of the two connections while moving down the track and transitioning between points.
Figure 5: PRP in a WLAN network comprising several access points and a client.
Packet elimination is performed at a central location with PRP.
As with other applications, the resulting quality of the connection will always be as good or better than the best of the two connections here too. This is regardless of mobility effects, such as bad SNR (signal-to-noise ratio) or fading. Why “better than the best,” you ask?
Following the same rational discussed in the first article in this series, this is because the PRP technology not only automatically chooses the packets of the better link, it also compensates for losses on the “better link” with successfully delivered packets on the “other” link. This allows roaming interruptions and service degradation to be avoided with no switchovers.
Furthermore, what’s important is that the PRP functionality does not have to be limited to the wireless channel. Various WLAN connections running over several access points can connect to the network in different ways. Duplicate packets can then be eliminated at a central point in the network, something that is only possible using a standardized PRP and WLAN-independent method.
PRP Makes Industrial Wireless Applications Reliable
I hope this blog series gets you thinking about how today’s wireless solutions with PRP can benefit your industrial applications. Whether the challenge is:
- Rugged or broad geography.
- High noise interference.
- High availability requirements.
- Moving equipment.
- High temperatures.
Don’t be afraid – be educated. Today’s industrial wireless just might exceed your greatest expectations.
Questions? Thoughts? Please post them below and our wireless engineers will be pleased to elaborate and address them.
Related Technical Article
Control Engineering Europe webpage: Doubling Multiplies the Benefit