Parallel Redundancy Protocol

Parallel Redundancy Protocol (PRP) is a data communication network standardized by the International Electrotechnical Commission as IEC 62439-3 Clause 4. It allows systems to overcome any single network failure without affecting the data transmission.

It can be applied to most Industrial Ethernet applications since it is independent of the protocols and provides seamless failover.

Under PRP, each network node has two Ethernet ports attached to two different local area networks of arbitrary, but similar topology.

Properties

The two LANs are completely separated and are assumed to be fail-independent. A source node sends simultaneously two copies of a frame, one over each port. The two frames travel through their respective LANs until they reach a destination node, in the fault-free case, with a certain time skew. The destination node accepts the first frame of a pair and discards the second, taking advantage of a sequence number in each frame that is incremented for each frame sent.

Therefore, as long as one LAN is operational, the destination always receives one frame. This protocol provides a zero-time recovery and allows to check the redundancy continuously to detect lurking failures.

Non-PRP nodes are either attached to one network only (and therefore can communicate only with other nodes attached to the same network), or are attached through a RedBox, a device that behaves like a doubly attached node.[1]

Node failures are not covered by PRP, but duplicated nodes may be connected via a PRP network.

Each node in PRP has two Ethernet interfaces that use the same MAC address and present the same IP address(es). Therefore, PRP is a layer 2 redundancy, which allows higher network protocols to operate without modification.

Versions

The original standard IEC 62439-3 (2010) was amended to align PRP with the High-availability Seamless Redundancy (HSR) protocol, which uses a ring topology instead of parallel networks. To achieve this, the original PRP was modified at the cost of a loss of compatibility with the PRP 2010 version.

The revised standard IEC 62439-3 (2012) describes both HSR and PRP. Many technical details are now aligned with HSR, which eases the implementation of multi-protocol nodes. In particular, a redundant transition between HSR and PRP networks is now possible.

The old PRP 2010 standard is sometimes referred to as PRP-0 (according to the protocol version), and PRP 2012 as PRP-1.[2]

See also

References

  1. ↑ "Redundancy Box". Zurich University of Applied Sciences. Retrieved 20 August 2014.
  2. ↑ "Standardization of PRP". Zurich University of Applied Sciences. Retrieved 20 August 2014.
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