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Adam Rickards | Application EngineerUNDERSTANDING INDUSTRIAL NETWORK REDUNDANCY
I’ve often been asked what is the “best redundancy protocol” for Industrial Networks and usually, this question is posed by someone who wants an easy answer so that they can move forward with the comfort of knowing they are going to achieve the best outcome.
However, I think that this question itself poses an interesting thought process about how we should choose the redundancy protocols for our Industrial Networks or a better question would be how do we design the network so as to meet the requirements of the Application(s) inside the Industrial Network.
In the simplest terms, the way to go about this is to understand the applications communication requirements and in the case of a typical Programmable Logic Controller (PLC) this is a cyclical process on a fixed timer with a number of faults (or retries) being allowed before the device stops the process to ensure the integrity of the process or safety of the individuals who may be involved in the process.
Knowing this information, we would then choose an appropriate redundancy protocol to achieve the goal of recovering the network before the PLC stops the process so that in the event of a small network issue the process continues to operate.
Each standardised redundancy protocol available has its own benefits over the others and we can briefly touch base on the common standardised redundancy and the suitable applications to assist in choosing the right one.
RSTP or Rapid Spanning-Tree Protocol offers a typical recovery time of 0-20ms per node/switch in the redundancy and allows for any type of architecture you want which has the advantage in some unique situations (such as redundancy in underground mining where power for some areas will come from another area) of providing additional redundancy paths for multiple points of failure. The disadvantage of this protocol is that you will sometimes get a much higher recovery time if the Root Bridge (master switch) fails which leads to it being hard to predict and therefore you cannot guarantee the process will not be hindered.
MRP or Media Redundancy Protocol is Ring based protocol offering a single point of failure with a guaranteed worst-case recovery time of 500ms or 200m (configurable). The architecture used is a loop/ring so it may not cater for every case, but the predictability makes its use very easy in an Industrial network.
HSR or High-speed Seamless Redundancy is a Ring-based protocol offering a single point of failure with a 0ms recovery time guaranteed. Note that specialised hardware is required to support HSR which can occur additional costs to the application.
PRP or Parallel Redundancy Protocol is a dual bus architecture offering a single point of failure with a 0ms recovery time guaranteed. There is a duplication of some hardware required as well as specialised units that support PRP so quite a large additional cost can be involved.
DLR or Device Level Ring is a Ring based protocol offering a single point of failure with a guaranteed worst-case recovery time of as little as 3ms. The architecture used is a loop/ring so it may not cater for every case and the hardware has additional cost over typical MRP hardware so this can be a factor in your decision process as well.
After a review of the requirements of the applications and some knowledge on the standardised options available it’s just a matter of ensuring that whichever process is selected for the redundancy is compatible and will meet the requirements of the application.
ABOUT THE AUTHORAdam Rickards | Control Logic Application EngineerControl Logic Application Engineer, Adam Rickards, is a passionate technology and communications professional with over 15 years’ experience in industrial networking with industry experience in design, implementation and investigation into complex faults in existing networks.