What is HSR?

• 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

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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