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.

Several months ago we asked whether you have moved wireless projects off the back burner yet. The reason we asked is because new advances in technology and standards mean it’s probably time to take a fresh look at industrial wireless.

One of the most common concerns about wireless for wide-ranging mission critical applications has always been – and still is – reliability. Will it work in your noisy environment? Will it be robust enough to ensure your data makes it to its destination? Can it ever provide you the assurance you need that it is stable enough?

These are all good questions. Up until now, there have been many techniques and planning guides written to help address those concerns. However, there hasn’t been an integrated, tried and true solution to really hit the mark I’m sure you’ve been striving for – zero failover, zero data loss.

Recent advances in technology and standards have changed this. These advances have made industrial wireless applications much more stable, reliable, fast, secure and a lot easier to deploy. This is in part thanks to the use of an updated and improved protocol called Parallel Redundancy Protocol (PRP).

In this Part 1 of a two-part series on redundancy techniques for reliable industrial Wireless Local Area Networks (WLANs), I will explain why PRP technology makes wireless worth another look.

Before Parallel Redundancy Protocol

Traditionally, wireless LANs have been used in industry when:

  • Cable is too heavy for the application.
  • Cable will not perform under the wear and tear of the application.
  • Cable is impossible to use because the application involves mobile machines or vehicles.

Parallel Redundancy Protocol

However, the reliability and quality of service of wireless connections used to be problematic when:

  • The application had strict requirements with regard to consistency and latency.
  • Security-critical applications were run over a wireless connection.
  • A high level of dependability was required despite adverse conditions.

For example, video systems that perform important tasks, such as cell security or the monitoring of the interior of trains, often proved problematic due to their sensitivity. Other examples that often draw concern are using WLANs to control production workflows or collect quality data in a regulated environment.

All of these examples are sensitive to interruptions, delays and loss of data packets. These network interruptions can quickly lead to serious problems, such as stopping vehicles or halting production, both of which lead to significant downtime costs.

Parallel Redundancy Protocol – Creating Redundancy by Doubling Packets

In wired industrial Ethernet networks, redundancy techniques have long been established to ensure that the networks continue to operate smoothly even if individual connections fail.

These often involve using PRP in accordance with IEC 62439 standards. The result is seamless redundancy with delay-free and loss-free switching.

To achieve this, a “Redundant box” or “Red box” duplicates and transmits data packets concurrently across two different network paths. Before the duplicated packets are delivered beyond these network paths, the parallel streams are then merged and duplicate packets are removed.

If a single path fails, packets from the other path will be used. The application relying on this network can therefore continue to work without failure, despite serious disruptions in the network (Figures 1 and 2 show PRP in operation).

Parallel Redundancy Protocol

Figure 1. PRP in an reliable network: two redundant paths are used simultaneously. Packets duplicated at point 5; duplicates are discarded at point 1.

Parallel Redundancy Protocol

Figure 2. PRP in a high reliability network: a failure has occurred in Network A and packet 3 does not arrive, however packets from the second network path are used without any resulting switchover times. 

Parallel Redundancy Protocol - High Impact for Industrial Wireless Applications

Great, so you are probably already familiar with PRP because it has been used for years in standard, wired networks. However, the intriguing part comes with the use of PRP in wireless environments – where its impact is even more significant than in wired scenarios.

This is because parallel redundancy not only provides zero-failover; it can also be used to compensate for the inherent small-scale disruptions (e.g., interference) that can occur with wireless connections.

When PRP transmits packets simultaneously on two different wireless transmission paths (Figure 3), the effects of individual path packet losses can be eliminated.

Uncorrelated packet losses are not seen by applications using PRP because a transmission fault or a receive error only occurs if both paths fail simultaneously for the exact same packet.

Parallel Redundancy Protocol

Figure 3. PRP over two WLAN transmission paths: the redundant transmission compensates for packet losses and counterbalances load and interference-related transit time differences.

Beyond this, jitter and latency also see drastic improvements. When PRP is used to transmit packets simultaneously on two different transmission paths, the effects of interference or delays are nearly eliminated. As seen in Figure 3, the delay of packet 5 on Network B will never been seen by the network because the fastest packet will always be used at the point of elimination.

Although the mechanisms used by PRP are the same in both wireless and wired scenarios (packet duplication and elimination), the effect achieved is more dramatic for wireless. The advantages include:

  1. PRP remarkably increasing reliability by compensating for individual packet losses from temporary disturbances, such as interference caused by other radio systems.
  2. PRP decreasing latency, since the faster of the two duplicated packets is always forwarded.
  3. PRP reducing transit time fluctuations (jitter), since as with 2, long delays are reduced with fluctuations only appearing if both packets arrive late.

New call-to-actionIndustrial WLAN Applications that Benefit from Parallel Redundancy Protocol

I hope this has you thinking – “What can I do with this?” “Is it really that much more reliable?” “Zero failover, decreased latency, and improved transit times?” Good, because the answer is resounding “YES.” Industrial wireless, and particularly industrial wireless with PRP, is no longer your grandmother’s wireless, and really worth another look!

Next, we’ll discuss the addition of diversity, what this means, and describe new applications for high availability WLANs in your industrial environment.

Integrated Parallel Redundancy Protocol Extends Industrial Wireless Applications

Above, 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:

  1. It drastically decreases the potential for data loss because it is a zero-failover design –renegotiation and signal recovery times are non-existent
  2. It improves transmission time by decreasing latency.
  3. 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.

Parallel Redundancy Protocol

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 Parallel Redundancy Protocol 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.

Parallel Redundancy Protocol

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.

Parallel Redundancy Protocol

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?

Parallel Redundancy Protocol

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 Parallel Redundancy Protocol 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.

Parallel Redundancy Protocol

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 part of this blog, 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.

Parallel Redundancy Protocol Makes Industrial Wireless Applications Reliable

I hope this blog 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

Related Links