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Parallel Redundancy Protocol

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Parallel Redundancy Protocol (PRP) is a network protocol that provides seamless data communication in face of the failure of any single network component. It is specified as international standard of the International Electrotechnical Commission as IEC 62439-3 Clause 4.

PRP can be applied to most Industrial Ethernet applications since it is independent of the protocols, provides seamless failover and integrates nodes with single attachment.

Principle

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

  • Principle of the Parallel Redundancy Protocol
    Principle of the Parallel Redundancy Protocol

The two LANs are completely separated and are assumed to be fail-independent. A source node (DANP) 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 (DANP), in the fault-free case, with a certain time skew. The destination node accepts the first frame of a pair and discards the second. 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.

To simplify the detection of duplicates, the frames are identified by their source address and a sequence number that is incremented for each frame sent according to the PRP protocol. The sequence number and the path identifier and an Ethertype are appended just before the Ethernet checksum in a 6-octet trailer. This trailer is ignored by all nodes that are unaware of the PRP protocol, and therefore these singly attached nodes (SAN) can operate in the same network.

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. Therefore, PRP is a layer 2 redundancy, which allows higher layer network protocols to operate without modification. A PRP node needs only one IP address.

Clock synchronization

Since IEC 62439-3:2016, PRP supports a clock synchronization that achieves an accuracy of 1 μs after 15 network elements as profile of IEEE Std 1588 precision time protocol
Clocks can be doubly attached according to PRP, but since the correction is different according to the path, the duplicate discard method of PRP is not applicable.
When a master clock sends a Sync frame, two copies are sent, but not at exactly the same time, therefore not even the original Syncs are the same.
A slave receives two Sync messages at different times and applies the Best Master Clock algorithm, and when the two Sync come from the same grandmaster, the clock quality is used as a tie-breaker.

This method works for Layer 2 and Layer 3 PTP, and with the peer-to-peer and with the end-to-end delay measurement. IEC 62439-3 defines these two profiles as:

  • L3E2E (Layer 3, end-to-end) that addresses the requirements of ODVA
  • L2P2P (Layer 2, peer-to-peer) that addresses the requirements of power utility.

Legacy versions

The original standard IEC 62439-3:2010 incremented the sequence number of the frames on a per-connection basis. This gave a good error detection coverage but difficulted the transition from PRP to the High-availability Seamless Redundancy (HSR) protocol, which uses a ring topology instead of parallel networks. The revised standard IEC 62439-3:2012 aligned PRP with High-availability Seamless Redundancy (HSR) using the same duplicate discard algorithm. This allowed building transparent connection boxes and nodes that can operate both as PRP and HSR. This version is the one now adopted for the power utility communication.
The old PRP 2010 standard is sometimes referred to as PRP-0, and PRP 2012 as PRP.[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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