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Dynamic Ring Coupling for Inter-Consist Network Redundancy

When Amtrak’s Empire Builder railway line departs Chicago for Seattle and Portland, multiple units travel together as a single train for most of the journey. However, when the train reaches Spokane (the junction point), the multiple-unit consists that make up the train split into two shorter trains, with one continuing on to Seattle and the other Portland. The ability to split trains mid-journey offers railway operators a more efficient and flexible way to utilize resources and schedule long-distance passenger services for multiple markets. Moreover, the benefits afforded by multiple-unit (MU) trains are not limited to long-distance inter-city railway lines.

Reorienting consists from separate trains mid-journey also allows train operators for both inter-city and metropolitan mass rapid transit rail systems to save energy and more efficiently allocate resources. For instance, most metro systems do not always need to be running at full capacity. Consequently, the ability to reconfigure the consists to include fewer cars during off-peak hours but also add more cars during peak travel hours reduces energy and operational costs.

Since the train consists that comprise a multiple-unit train may need to be rearranged mid-journey, ensuring redundancy for multiple-unit Ethernet consist networks can be especially challenging. First, manually configuring network settings when train consists are reoriented requires sufficient knowledge of Ethernet protocols. If railway operators do not already know how to do this, then additional time and labor costs must be factored in. Moreover, during reorientation (i.e., adding or removing consists from a train), each consist may encounter security issues if the network system needs to restart due to a topology change. This disruption in onboard services, such as passenger information systems and passenger Wi-Fi access, is also likely to upset many discerning passengers and hurt overall customer satisfaction.

Existing Redundancy Methods

A number of solutions can be leveraged to provide railway automation networks and multiple-unit Ethernet Consist Networks (ECN) with redundancy. The most common of these are traditional ring coupling, Rapid Spanning Tree Protocol (RSTP), and Dynamic Ring Coupling. Although each of these has its own drawbacks and benefits, Dynamic Ring Coupling offers a better solution for the unique redundancy requirements of multiple-unit ECNs.

  Ring Coupling RSTP Dynamic Ring Coupling
Nodes Unlimited 40 Ethernet switches/train Unlimited
Inter-Consist Recovery < 1 s 3 s < 1 s
Intra-Consist Recovery 20 ms 3 s 20 ms
Automatic Configuration No Yes Yes
Consist Network Interference No Yes No

Conventional Ring Coupling:
Ring coupling is the traditional method used to ensure redundancy for multiple-unit ECNs. There are two steps to the process:

  1. The Ethernet switches in a single consist are connected to form a redundant ring. Redundancy is achieved by identifying one link in the ring as a backup path, with this backup path’s traffic blocked to prevent packets from looping endlessly around the ring. When one of the active paths in the ring fails, the backup path is activated automatically, thereby allowing packets to reach all of the switches in the ring.

  2. When two or more consists are combined to form a larger train, the individual redundant rings for adjacent consists are “coupled” together to form a larger redundant network. In this case, the train operator must access the Ethernet switch’s configuration utility and manually configure which link is designated as the “primary path” and which link is designated as the “backup path.”

The drawback is that each time the train consists change, the train operator will need to reconfigure the coupling topology.

Rapid Spanning Tree Protocol:
Rapid Spanning Tree Protocol, or RSTP for short, is an open standard that many Ethernet switch manufacturers have implemented on their managed switch products. RSTP supports automatic configuration for onboard Ethernet switches, which is certainly a plus, but the number of nodes on an inter-consist network using RSTP redundancy is generally limited to 40 Ethernet switches per train. Moreover, when RSTP reconnects inter-consist networks mid-journey, all network transmissions will be blocked for about 3 seconds. Although a few seconds of network delay may be acceptable in some enterprise networks, shutting down passenger information systems, CCTV surveillance systems, onboard Wi-Fi access, and multi-media entertainment services on long-distance journeys even for several seconds will likely upset today’s railway passengers.

Dynamic Ring Coupling:
If the Ethernet switches on a train could determine automatically which ports to set as redundant ports and which ports to set as a normal ports then the problems with traditional ring coupling and RSTP outlined above can be overcome. This is exactly the advantage provided by Dynamic Ring Coupling (DRC). With DRC, two port pairs in each ring will automatically identify which pair should be set to redundant mode, without requiring any configuration from the operator. DRC allows train operators to rapidly connect and reconnect train consists, streamlining operational efficiency and minimizing configuration errors.

With Dynamic Ring Coupling, two ports in each ring will automatically identify whether they should be active or inactive without operator intervention.

When two Moxa Ethernet switches are connected as ring coupling switches, dynamic ring coupling identifies which port should be set as the inactive, blocked port, without any assistance from the operator. This allows the train to enjoy the fast redundancy of Moxa's advanced ring redundancy technology, while still being flexible enough to re-deploy again and again.

DRC automatically configures the network's redundant ring in under 1 second, free from human error and interference from intra-consist network devices used for CCTV surveillance, Wi-Fi access, and passenger information systems

Redundancy that Works for Rail

High network availability, reliability, and efficiency are key objectives railway operators need to consider when deploying multiple-unit Ethernet consist networks for long-distance trains. Besides offering the speedy, automatic configuration for onboard Ethernet switches when consists are rearranged mid-journey, the Dynamic Ring Coupling technology provided on Moxa’s EN 50155 managed Ethernet switches guarantees inter-consist network recovery in under 1 second, and even faster intra-consist recovery in 20 milliseconds. And unlike RSTP, Dynamic Ring Coupling also does not disrupt intra-consist networks during reorientation. As a result, all the Ethernet network services today’s long-distance railway passengers and operators have come to expect—CCTV surveillance, Wi-Fi access, passenger information systems, public announcement systems, and more—can be seamlessly enjoyed even when coupling switches are rearranged mid-journey.

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