Today we'll talk about ring networks in industrial Ethernet technology.
Ring network technology
With the rapid rise of industrial fieldbus technology in the 1990s, wiring for machinery and equipment was greatly simplified through communication connections. Early industrial equipment typically formed linear topologies, with all status, control, and diagnostic data transmitted along a single cable, making network design and connections relatively simple. However, with continuous technological advancements, PLC controllers now fully support Industrial Ethernet, and the addition of more and more industrial network devices has made networks increasingly complex. In industrial settings, if a node in a linear topology network fails, subsequent devices will be completely disconnected, causing communication interruptions. To address the high reliability requirements of industrial control networks, Industrial Ethernet ring network technology emerged.
The advantages of ring network technology are as follows:
• It can quickly detect network errors and automatically reconfigure the network.
It can be implemented and used in both small and large networks.
• Clear and simple network wiring.
• The factory can expand its operations during operation.
Standardized protocols ensure compatibility with equipment from different manufacturers.
Why does Industrial Ethernet require ring network technology? Or, to put it another way, why can't Ethernet networks be directly connected in a ring?
This is because when Ethernet networks are directly connected in a loop, broadcast data packets are copied extensively within the broadcast domain. This causes the network to be flooded with broadcast data, consuming a large amount of link bandwidth and preventing normal data from being transmitted effectively. It can also cause MAC address table oscillations in the switch, resulting in switch resources being occupied and leading to system crashes. This is what we commonly refer to as a "network storm."
Therefore, in Ethernet loop topology, one port must be blocked by the ring network protocol, and all data is sent and received through the other port. In fact, it still maintains a linear state. Once the link for forwarding data fails, the blocked port will be opened to ensure data transmission.
The time from when a ring network fails to when a backup port opens and communication resumes is generally referred to as the ring network reconfiguration time. The reconfiguration time depends on the network size and the ring network protocol.
Ring network technology is not Siemens proprietary. Given the wide variety of industrial Ethernet devices, how do you choose the right ring network protocol?
In fact, we can conclude that industrial networks have high requirements for real-time performance and reliability. Once a ring network failure occurs, communication must be restored as quickly as possible through a backup path in order to minimize the impact on production.
Therefore, whether the chosen ring network protocol and equipment are suitable for the current application depends entirely on whether the ring network reconfiguration time can meet the process requirements. Let's look at which ring network protocols are available for us to choose from.
No.1
MRP (Media Redundancy)
This ring network protocol, specifically designed for PROFINET networks, is a network redundancy protocol compliant with the IEC 61158-5-10 standard and suitable for industrial automation applications. The maximum reconfiguration time is 200ms, but the actual reconfiguration time depends on the network size and is generally considered to be in the tens of milliseconds range. Because of its relatively fast reconfiguration time, it is very suitable for industrial network applications.
There are many devices that support MRP. In addition to switches, many CPUs and devices that support PROFINET also support MRP, so the choice of network topology is very flexible.
The latest firmware version of the SCALANCE XM400 switch also supports MRP multi-ring networks.
No.2
HRP (High-Speed Redundancy Ring)
HRP is a proprietary ring network protocol for Siemens switches, with a reconfiguration time of 300ms at the largest network size. HRP is primarily designed for the backbone network of switches, and its biggest feature is that it can be combined with inter-ring hot standby to achieve link redundancy between multiple ring networks.
No. 3
STP/RSTP
STP (Spanning Tree Protocol, IEEE 802.1D) has a network outage recovery time of 30-60 seconds. RSTP (Fast Spanning Tree Protocol, IEEE 802.1w), as an upgrade to STP, reduces the network outage recovery time to 1-2 seconds. While the Spanning Tree Protocol offers flexible network structures, it suffers from slow recovery speed.
The characteristics of these two types of protocols are that they are both open ring network protocols, which are supported by most ring network devices. They also have good compatibility between products from different manufacturers and are flexible in deployment (plug them in however you want). However, the biggest problem is that they are too slow to reconfigure, making them unsuitable for industrial environments. They are typical ring network protocols suitable for office networks.
No. 4
Third-party private ring network protocol
Many third-party network equipment manufacturers have proprietary ring network protocols. Whether they are suitable for current industrial application environments depends on their ring network reconfiguration time. For example, assuming the reconfiguration time of a third party is 100ms, if the maximum allowable latency in our industrial environment is 128ms (such as the watchdog time of PROFINET devices), then the reconfiguration time will not cause communication drops, and it can be used. However, if the maximum allowable latency in our industrial environment is 64ms, it is obvious that reconfiguration will cause network drops, which will affect production, and it is clearly unsuitable.
No. 5
PRP (Parallel Redundancy Protocol)
HSR (High Availability Seamless Ring Network Protocol)
Both the IEC 62439-3 PRP parallel redundancy protocol and the IEC 62439-3 HSR protocol require dedicated equipment for networking.
Unlike the previously introduced ring network protocol where data is transmitted only on one link, PRP copies frames for transmission in two independent networks (LAN A, LAN B). HSR also copies data in both directions. The first frame to arrive is forwarded to the application, and the second frame to arrive is discarded.
PRP does not require intermediate devices to support PRP, and frames from devices connected to it will not be copied and forwarded by RNA switches.
All devices on the HSR ring need to support the HSR protocol.
Both protocols eliminate network reconfiguration time and achieve 0ms switching because data is replicated in both directions. In the event of a failure, the absence of network reconfiguration guarantees reliable, latency-free data transmission, making them suitable for redundant applications requiring high availability and zero reconfiguration time.
Conclusion
In summary, when designing network topologies, engineers need to consider using ring topologies—a fault-tolerant network structure—for production stability. Ring network technology can utilize redundant physical connection paths between network nodes to handle the interruption or loss of any branch of the network. This provides additional security in terms of machine availability and supports fault tolerance to meet the stable requirements of core equipment operation, process automation, and even energy supply in the factory.
