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Doubling multiplies the benefit

15 April 2014

Wireless solutions continue to be tainted by susceptibility to interference and lack of reliability. Despite recent technical improvements, wireless transmission technology quickly reaches its limits especially when mission-critical processes have to be controlled or monitored via WLAN connections. The robustness, reliability and availability of wireless connections can be increased dramatically by using standards-based redundancy techniques such as the Parallel Redundancy Protocol (PRP).

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

Wireless LAN has become an enabler for many of today’s communication applications in the industry. WLAN is an excellent solution whenever using cables is to heavy, unreliable (wear and tear), costly or simply impossible due to moving parts and vehicles. In addition, the use of wireless on the production floor gives rise to a completely new approach for planning and executing production processes (Industry 4.0).

The ongoing technical advances and much wider acceptance of wireless solutions in recent years are giving rise to increasingly challenging and highly sophisticated application scenarios. Yet the reliability and quality of service of wireless connections in particular poses a problem for applications with strict requirements with regard to reliability and latency. The technology reaches its limits especially when security-critical applications are run over a wireless connection as a so-called black channel, or a high level of reliability is required despite adverse conditions. 

Examples of such critical applications include video systems that perform important tasks, such as interior monitoring of cable cars and trains or controlling production workflows, as these react sensitively to interruptions, delays and loss of data packets. These network interruptions can quickly lead to serious problems (e.g. transition of system to a vulnerable safe status and therefore standstill) and consequently to high follow-on costs. 

Figure 2. PRP in a mission-critical network: Packets from the second network path are used without any resulting switchover times.
Figure 2. PRP in a mission-critical network: Packets from the second network path are used without any resulting switchover times.

The Parallel Redundancy Protocol – Creating redundancy by doubling packets
In wired industrial Ethernet networks, redundancy techniques have been established to ensure that the network continues to operate smoothly even if individual connections fail. these redundancy techniques can also be used in wireless networks in order to significantly increase the reliability and robustness of the connections. 

The standardized Parallel Redundancy Protocol, PRP in short, in accordance with IEC62439 is used increasingly in wired environments to enable seamless redundancy or loss-free switching without delay in the event of failure in a network path or a device. To achieve this, data packets are duplicated and transmitted in parallel across two different network paths. Before the duplicated packets are delivered beyond these network paths, the parallel streams are 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).

 PRP can also be used in a wireless environment, although the impact manifests itself in a completely different way from in a wired scenario, despite using the same method. This is because parallel redundancy can also be used to compensate for the inherent small-scale disruptions (e.g. interference) in a wireless network. When PRP transmits packets simultaneously on two different wireless transmission paths (Figure 3), the effects of individual packet losses on one path can be eliminated; a transmission fault or receive error on a path only becomes visible if both paths fail simultaneously for the exact same packet. In other words, uncorrelated packet losses are not seen by applications employing PRP techniques.

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. 
While use of the PRP allows a seamless switchover between two networks in a wired scenario, its use in a wireless scenario immediately offers a number of different advantages:

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

a)Compensation for individual packet losses in case of temporary disturbances, such as interference caused by other radio systems, increasing reliability dramatically.
b)Decreased latency, since the faster of the two duplicated packets is always forwarded. 
c)Reduced transit time fluctuations (jitter), since as with b), long delays, caused by an occupied medium or by network layer retransmissions, are reduced since fluctuations only appear if both packets arrive late. 

Benefits in practice
The benefits of deploying standards-based PRP can be demonstrated using a simple example: 
Assuming the loss rate is identical on both paths and is approximately 0.1%, the rate for the overall PRP system would be just 0.0001% (0.0001 x 0.0001 = 0.000001) – an improvement by a factor 1000.

However, this calculation assumes that losses are evenly distributed and are not correlated, that means that these losses do not have a common cause. To achieve this in practice, it is necessary to exclude influencing factors that impact both radio channels equally. Both paths can be operated in different frequency bands or for this purpose, with the result that a competing radio transmission or other environmental influences cannot affect both paths at the same time. Furthermore, other factors that cause correlated losses and reduce the uniformity of the loss distribution must also be minimized. For example, permanent overloading of a connection can cause sequences of packets to be dropped, which drives up the loss rates for this connection and therefore significantly worsens the combined loss rate at the same time. These dramatic improvements can also be achieved in reality. In practical tests, the perceptible packet loss for the application with the PRP was reduced from 0.105% and 0.101% for the individual connections to 0.00021% using a parallel redundant PRP connection – an approximately 500-fold improvement. 

Another positive effect of the use of the PRP is that the network latency and transit time differences, the jitter, decrease significantly in the network. A reduction in the average latency from 3.1 ms or 2.8 ms to 1.7 ms, can be observed in practice in the above example. The jitter value likewise falls from 0.45 ms to 0.23 ms. 

Figure 4. PRP allows both wired and wireless routes to be used as redundant paths, thereby enabling a variety of network topologies. Shown here is a wired path with a wireless backup path.
Figure 4. PRP allows both wired and wireless routes to be used as redundant paths, thereby enabling a variety of network topologies. Shown here is a wired path with a wireless backup path.

The reason for the improvement in these metrics is that the faster of the two packets transmitted across the wireless links is always forwarded with the PRP. Outlier packets with long transmission times, as are typical with WLAN because of the shared medium and the non-deterministic channel access, can be largely eliminated in this way. Consequently PRP improves three of the most important quality indicators of a network: loss rate, jitter and transmission time.
 
Topologies and applications
While PRP represents a significant improvement in the redundant protection of individual transmission paths as outlined above, the fact that it is not limited to wireless transmission paths makes the flexibility of this standardized solution all the more obvious when it comes to complex network structures. Even though proprietary WLAN redundancy solutions also offer enhanced transmission performance, such improvements are always focused on an individual transmission path. PRP on the other hand allows more complex scenarios to be realized with radio and Ethernet connections as well as mobile applications with roaming PRP devices.

Figure 4 illustrates a scenario in which PRP is used over a wired and a wireless transmission path. The wireless transmission path can therefore be used as a switchover-free backup connection for the wired path in the case of applications with difficult constraints (e.g. moving parts or high temperatures). Such a combination is not possible if proprietary WLAN redundancy solutions are used. 

Figure 5 shows the use of PRP in a mobile scenario: a dual-radio client (e.g. on a moving machine or a train) travels along a path with several access points. The client can operate two connections at the same time, which means that the path can be p
Figure 5 shows the use of PRP in a mobile scenario: a dual-radio client (e.g. on a moving machine or a train) travels along a path with several access points. The client can operate two connections at the same time, which means that the path can be p

Figure 5 shows the use of PRP in a mobile scenario: a dual-radio client (e.g. on a moving machine or a train) travels along a path with several access points. The client can operate two connections at the same time, which means that the path can be protected with the PRP. 

 
The client can also establish the redundant connections to different access points along the route and switch from access point to access point with one of the two PRP connections remaining active at all times. Moreover, the resulting quality of the connection will always be as good or better as the best of the two connections, regardless of mobility effects (e.g., bad signal to noise ratio or fading) since the PRP algorithm automatically chooses the packets of the better link. This allows roaming interruptions and service degradation to be avoided with no switchovers. What is important in this scenario again is that the PRP is not limited to the wireless channel, since various WLAN connections run over several access points connected to the network in different ways. Duplicate packets must be eliminated at a central point in the network, something that is only possible using a standardized and WLAN-independent method. 

Belden and Hirschmann offer a complete portfolio of PRP-enabled devices with the switches of the RSP switch line, which allow the previously described solutions to be realized. Furthermore, with its OpenBAT series, Hirschmann offers industrial IEEE 802.11n dual-radio access points and clients for automation, transportation and outdoor use. These products will support PRP in the next OpenBAT firmware release (HiLCOS 8.90), which is available in early Q3 2014.

Conclusion
As a standardized redundancy solution, PRP is ideally suited to dramatically improving reliability and quality of service of wireless connections of their. In addition, PRP allows a variety of network topologies comprising wired and wireless connections to be protected. As a result, loss- and latency-sensitive applications can be operated over wireless connections.
 
Dr. rer. nat. Dipl.-Inf. Tobias Heer, Head of Embedded Software Development Functions at Hirschmann Automation and Control GmbH, Neckartenzlingen (Tel.: +49 (0)7127-14-1280, e-mail: tobias.heer@belden.com) 


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