HSR — High-availability Seamless Redundancy

What is HSR?
HSR — High-availability Seamless Redundancy
HSR (High-availability Seamless Redundancy) is a redundancy protocol for Ethernet networks requiring short reaction times and high availability, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, HSR reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
- HSR is a network redundancy protocol for Ethernet networks standardized by IEC 62439–3:2016 (clause 5)
- Together with PRP, it has been adopted in the IEC 61850 standard for substation automation
- It is most commonly implemented in a ring topology
- It is used in critical communication networks that request high availability and short switchover times
- It it is particularly fast, making it suitable for applications where the recovery time of commonly used protocols such as the Rapid Spanning Tree Protocol (RSTP) is too long
- It is independent of the application-protocol and can be used by most Industrial Ethernet protocols in the IEC 61784 suite
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Advantages and Disadvantages of HSR
Advantages
- It provides seamless failover (zero time recovery) against failure of any network component
- It is independent of & invisible to the application-protocol and can be used by most Industrial Ethernet protocols
- It can be connected without a single point of failure
- The same nodes can be built to be used in both PRP and HSR networks, since both rely on the same duplicate identification mechanism
Disadvantages
- Latency — During the transmission of a frame in a HSR ring, each link introduces additional transmission and propagation delays, while each node causes further processing and queuing delays. This can be tackled by minimizing the number of hops.
- Traffic — The transmission of duplicated copies, which are generated at the source and circulate inside the network in opposite directions, creates additional traffic. Algorithms like QR, VRing can be used to address this issue.
- Bandwidth - more bandwidth is consumed due to the duplication of the frames
When to Use HSR
HSR is specially suited for cost-efficient networks with a ring-like topology, requiring high-speed switchover times close to zero.
How it works
The most commonly used HSR topology is a ring network based on one physical LAN or two interconnected LANs. The source node duplicates the frames and sends them to the destination node using two different paths. This means one frame can arrive several times, but only the first one is accepted/processed. If one of the paths is broken, due to link or node failure, the frames are still able to reach their destination.
HSR Redundancy Networks Explained
The terminals/end-nodes are called DANH (Doubly attached node using HSR). They must carry two Ethernet ports to connect to the HSR ring and may carry additional ports for maintenance purposes. They send two duplicates of each frame, one to each of the directions in the ring and only accept the first copy when receiving a frame, discarding the second. Such nodes can be sensors, IEDs (Intelligent Electronic Devices), merging units and cameras amongst others.
RedBoxes (Redundancy Box) are entities to connect non-DANH nodes or network segments to the HSR ring. They must have at least three external Ethernet ports, two to connect to the HSR network and one for the integration of the non-HSR nodes. Following the principle of a DANH, RedBoxes create and transfer duplicates of each frame they receive from the outside of the HSR network and send the two duplicates to the ring. When they receive a frame, they only forward the first copy and remove the second.

PRP vs HSR — Which one is better?
All in all, the choice between HSR and PRP should depend on the specific project, as the project will determine which drawbacks and advantages weigh more or less. As a rule, PRP network redundancy is more expensive, but also more flexible than HSR.
PRP redundancy usually doubles the cost of a single non-redundant network, which makes the implementation more expensive than many other redundancy protocols. A High-availability Seamless Redundancy (HSR) network can also provide the same level of redundancy and will cost less unless you’re planning on keeping replacement infrastructure anyways.
However, PRP networks provide greater flexibility in terms of network topology and allow to connect (PRP-) “unaware” nodes without a RedBox (Redundancy Box), which would be required by HSR. Yet, these singly attached nodes can only communicate with devices of the same LAN, not taking full advantage from the redundancy.
Other minor disadvantages of HSR are potential technical limitations in relation to traffic, latency and bandwidth. Still, these problems are not very frequent and can be counteracted with different methods.
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Network Communication
- HSR — High-availability Seamless Redundancy
- PRP — Parallel Redundancy Protocol
- Ethernet
- MAC — Medium Access Control
- VLAN — Virtual Local Area Network
- IEC 62439–3
- Network Redundancy
HSR (High-availability Seamless Redundancy) is a redundancy protocol for Ethernet networks requiring short reaction times and high availability, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, HSR reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
PRP (Parallel Redundancy Protocol) is a redundancy protocol for Ethernet based networks requiring high availability and a short switchover time, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, PRP reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
Ethernet is a family of wired computer networking technologies commonly used in local area networks (LAN) and also wide area networks (WAN).
Over time, Ethernet has largely replaced competing wired LAN technologies by providing higher bit rates, a greater number of nodes, and longer link distances and decent backward compatibility.
The Medium Access Control (MAC) sublayer provides flow control and multiplexing for the transmission medium to control the hardware that interacts with the wired, optic and also wireless transmission media in the IEEE 802 LAN/MAN data link layer.
The MAC is accompanied by the LLC sublayer, which provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc.)
A Virtual Local Area Network (VLAN) is a is a subnetwork which can group together collections of devices that are connected to separate physical LANs.
VLANs allow network administrators to partition a single switched network in order to keep network applications separate despite being connected to the same physical network, without requiring new cabling or major changes in the current network infrastructure.
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Network redundancy is a method to ensure network availability, providing failover when a device or network path fails or becomes unavailable.
Redundancy is usually achieved by installing additional or alternative network devices, communication media or equipment within the network infrastructure

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HSR & iGrid
Redundancy has been a key element in many iGrid projects, which is why all of our devices support various redundant schemes and networks, including PRP, HSR and RSTP.
iRTU/iGW‑S#01 devices can acts as HSR DANH and RedBox (Redundancy Box).

iRTU – With I/Os for Direct Data Acquisition
Compact and scalable bay controller which can act as IEC 61850 client or server, featuring configurable I/O boards for direct data acquisition, high-precision timestamping and an optional Ethernet switch for additional Ethernet ports.

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Powerful and reliable substation gateway, able to run either in standalone or redundant modes, with an embedded Ethernet switch (4 ports) and IEC 61850 client and server capabilities.
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