High-availability Seamless Redundancy

High-availability Seamless Redundancy (HSR) is a redundancy protocol for Ethernet (standardized as IEEE 802.3) networks.

Properties

HSR provides zero recovery time in case of failure of one component. It is suited for applications that demand high availability and very short switch over time.[1] Such applications are protection for electrical substation automation and controllers for synchronized drives, for instance in printing machines. For such applications, the recovery time of commonly used protocols like the Rapid Spanning Tree Protocol (RSTP) is not acceptable.

HSR was standardized by the International Electrotechnical Commission, Geneva, as IEC 62439-3 Clause 5. It is one of the redundancy protocols selected for substation automation in the IEC 61850 standard. HSR is application-protocol independent and can be used by most Industrial Ethernet implementations that use the IEC 61784 suite.

HSR is typically used in a ring topology, however redundant connections to other networks are possible (such as a mesh topology). Especially the connection to Parallel Redundancy Protocol (PRP) has to be mentioned. Since HSR and PRP use the same duplicate identification mechanism based on MAC source address and sequence number PRP and HSR can be connected redundantly.

Operation

The nodes (devices) in an HSR network are attached by two Ethernet ports. A source node sends the same frame over both ports. A destination should receive, in the fault-free state, two identical frames within a certain time skew, forward the first frame to the application and discard the second frame when (and if) it comes. A sequence number is used to recognize such duplicates.

In contrast to PRP (IEC 62439-3- Clause 4), with which it shares the operating principle, HSR nodes are arranged into a ring, which allows the network to operate without dedicated switches, since every node is able to forward frames from port to port. HSR originally meant "High-availability Seamless Ring", but HSR is not limited to a simple ring topology. Redundant connections to other HSR rings and to PRP networks are possible.

Since the forwarding delay of every node in a HSR ring adds to the total network latency, it is important that frames are forwarded quickly. In practice, special hardware support is required to bring down the per-hop latency to a reasonable value, often using cut-through switching. Another property of a HSR ring is that only about half of the network bandwidth is available to applications (compared to RSTP). This is because all frames are sent twice over the same network, even when there is no failure. However, since also the network infrastructure is also doubled in closed ring topologies the nominal network bandwidth is available. E.g. in a 100 MBit/s Ethernet ring 100 MBit/s are available (but not 200 MBit/s).

See also

References

  1. ↑ Hubert Kirrmann (August 17, 2010). "HSR – High Availability Seamless Redundancy: Fault-tolerance in Ethernet networks IEC 62439-3". Archived from the original on August 19, 2011. Retrieved August 17, 2013.

Further reading

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