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.
Yesterday’s Industrial Wireless Applications
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.
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).
Figure 1. PRP in an reliable network: two redundant paths are used simultaneously. Packets duplicated at point 5; duplicates are discarded at point 1.
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.
PRP Has 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.
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:
- PRP remarkably increasing reliability by compensating for individual packet losses from temporary disturbances, such as interference caused by other radio systems.
- PRP decreasing latency, since the faster of the two duplicated packets is always forwarded.
- PRP reducing transit time fluctuations (jitter), since as with 2, long delays are reduced with fluctuations only appearing if both packets arrive late.
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!
Stay tuned for Part 2 of this series where we’ll discuss the addition of diversity, what this means, and describe new applications for high availability WLANs in your industrial environment.
What is your reaction to the impact of PRP on industrial wireless? Does it convince you to give wireless another look? I look forward to hearing from you.