HSR (High-availability Seamless Redundancy, pronounce: HASAR) is a redundancy protocol for Ethernet (IEEE 802.3) networks.
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HSR provides zero recovery time in case of failure of one component. It is suited for applications that demand high availability and very short reaction time.
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 is not acceptable.
HSR has been 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 of the IEC 61784 suite.
HSR is typically used in a ring topology, however redundant connections to other networks are possible (eg. a mesh topology).
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 can be arranged into a ring, which allows 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 its topology is not limited to a single ring, a system of rings with many structures can be built.