Mass Transit Systems

MRP: 50+. Switches based on ring topology. Recovery time is almost independent of the number of switches in the ring

RSTP: up to 40 Switches for any type of topology. Because RSTP works in a hop-by-hop principle, recovery time will almost linearly increase with the number of switches in the ring.

There is no best or worst case recovery time for HSR, since there is no recovery time at all. The network recovery time from no fault to a single fault in a ring will always be zero. Also, the repair operation from one fault to no fault is also with zero switchover time.

HSR, as MRP or RSTP in ring configuration, can only sustain one fault in the ring network. This is due to the physical topology, not due to the redundancy protocol. Rings that are coupled via Quad Boxes do not share the same redundancy domain. Therefore, each individual ring can sustain a single fault.

Both HSR and PRP are specified in the International Standard IEC 62439-3. HSR and PRP are therefore standardized and not proprietary technologies.

While HSR and PRP are superior to MRP or RSTP in terms of reconfiguration performance, there are also drawbacks to the technology:

• PRP: Requires a complete network infrastructure for both networks, which usually means double the costs for network equipment.
• PRP and HSR: For both PRP and HSR, hardware support through FPGAs or ASICs is required, leading to higher device costs.
• HSR: Compared to an MRP ring or RSTP in ring configuration, HSR carries only 50% of the traffic (available bandwidth is halved due to doubled frame transmission).

Where seamless redundancy is not explicitly needed, the use of MRP (with SubRings) or RSTP technology may be more cost-effective than HSR/PRP. But where the application requirements justify the additional costs, PRP/HSR can be utilized.

There are several answers to this question. It is true that the technology is standardized, but there are several key factors why a customer should buy a Hirschmann HSR/PRP device:

• Hirschmann was instrumental in the development of the PRP/HSR standard. With Hirschmann products, a customer can be absolutely sure that the device will satisfy every requirement HSR/PRP was designed to accommodate and that, thanks to our detailed insight knowledge, the implementation is of high performance and absolutely 100% standard conformant.
• A high performance redundancy is almost useless without proper monitoring and fault detection. Hirschmann HSR/PRP switch products will be integrated into our state-of-the-art SCADA tool Industrial HiVision, which adds up to a combined redundancy and network solution that no competitor can match.

HSR and PRP were conceived for use in IEC 61850 substation automation, where network reconfiguration times cannot be tolerated, especially on the process bus with sampled values traffic. However, PRP/HSR can also be used in factory automation, especially as redundancy solutions for motion control applications.

In short, PRP/HSR can be used anywhere when only very low to zero network recovery times can be tolerated. This is especially true in time synchronized networks, e.g. with IEEE 1588v2. HSR in particular with its ring structure and cut-through switching, can also provide very low end-to-end latency on ring networks.

The total number of HSR devices in one ring should be limited to 50. This is mainly to reduce the latency in the ring. For very time-critical applications it may be necessary to limit the number of devices even to a smaller number. Another limitation for the number of devices in a ring can be the size of the duplicate detection table inside the device. This is dependent on the implementation.