Optimized Ethernet Redundancy for Inter-Consist … there are still limitations to what traditional ring coupling can achieve. In conventional ring coupling, the coupling switches

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Optimized Ethernet Redundancy for Inter-Consist

Networks Sean Wang Product Manager

Luke Chang Senior Engineer

Released on July 15, 2014

© 2014 Moxa Inc. All rights reserved. Moxa is a leading manufacturer of industrial networking, computing, and automation solutions. With over 25 years of industry experience, Moxa has connected more than 30 million devices worldwide and has a distribution and service network that reaches customers in more than 70 countries. Moxa delivers lasting business value by empowering industry with reliable networks and sincere service for automation systems. Information about Moxa’s solutions is available at www.moxa.com. You may also contact Moxa by email at [email protected].

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WHITE PAPER Optimized Ethernet Redundancy for Inter-Consist Networks

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Executive Summary

Network redundancy is a critical factor in ensuring passenger safety and journey reliability in onboard railway communications for both intra-consist and inter-consist Ethernet networks. In addition, railway operators are looking for ways to more efficiently allocate resources and serve passenger demands. For example, metropolitan rail services may want to run trains with fewer consists during off-peak hours but longer trains with more consists during rush hour. Railway operators can also serve multiple markets and conserve track assets by connecting multiple consists with different destinations into one train if they will travel along the same route for at least part of the journey.

Moreover, the ability to quickly and reliably preserve and reconfigure network settings when consists are rearranged mid-journey is another key factor affecting operational efficiency, passenger safety, and the provision of seamless onboard services and amenities. To solve these issues, Moxa’s Dynamic Ring Coupling allows railway operators to maintain network redundancy between consists, automatic network setting under 1 second for operational efficiency, and no interference with intra-consist networks for seamless delivery of onboard passenger services.

Overview

Multiple-unit (MU) train control is often used on both long-distance inter-city trains and regional metropolitan rail services that terminate at more than one destination. At some point mid-journey, the makeup of the train can be changed at a junction point by splitting the consists into shorter trains for different destinations. Changing the makeup of the train mid-journey allows railway operators to conserve track resources for trains heading in the same direction, accommodate more passengers, and serve more markets simultaneously.

For example, Amtrak’s Empire Builder railway line departs Chicago for both Seattle and Portland, with multiple units travelling together as a single train for most of the transcontinental journey. However, when the train reaches Spokane (the junction point), the multiple-unit consists split into two shorter trains and continue onto their final destinations separately. Clearly, 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 MU trains are not limited to long-distance inter-city railway lines.

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Amtrak’s Empire Builder railway line uses the same stretch of track for most of the journey, saving operation and maintenance resources. Source: Wikimedia Commons.

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 enables operators to save energy and operation 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 re-oriented requires sufficient knowledge of Ethernet protocols. If railway operators do not already have these costly skills on-hand, then additional time and labor costs will need to 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 adversely affect 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 optimized 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 appears to offer a more optimal solution for the unique redundancy requirements of multiple-unit Ethernet Consist Networks.

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 N Y Y Consist Network Interference N Y N

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Conventional Ring Coupling Ring coupling is traditionally used to ensure redundancy for multiple-unit Ethernet Consist Networks. As the name suggests, ring coupling essentially connects what would otherwise be standalone ring topologies for each consist comprising the multiple-unit train. More specifically, Ethernet redundancy is generally achieved within individual consists by connecting onboard Ethernet switches in a ring topology with a backup path that is rapidly assigned by a proprietary protocol. This backup path’s traffic is virtually blocked, so as not to result in looping for a ring topology. When the normal path encounters a fault in the ring, the backup path activates and forwards all of the traffic as quickly as possible.

Nonetheless, there are still limitations to what traditional ring coupling can achieve. In conventional ring coupling, the coupling switches handle the redundant links between consists and redundant ports link to non-redundant ports. This definition is adequate as long as the network configuration remains static. However, when train consists change, the coupling switches will be rearranged. After a train consist re-couples, the normal port pairs could easily become connected to another normal port pair on the corresponding coupling switch in the adjacent train consist. Moreover, railway engineers will also need to have greater familiarity with Ethernet protocols to reconfigure the Ethernet switches when the makeup of the train changes mid-journey.

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. Besides offering a significantly faster spanning tree convergence when compared to its predecessor STP, RSTP supports automatic configuration for onboard Ethernet switches. Although support for automatic configuration for onboard Ethernet switches overcomes conventional ring coupling’s primary limitation, RSTP is not without its own shortcomings. In particular, the number of nodes on an inter-consist network using RSTP redundancy is generally limited to 40 Ethernet switches per train. Since long-distance railway passengers today increasingly expect non-stop information, Web access, and entertainment services, capping the number of onboard Ethernet switches for inter-consist networks at 40 switches per train could result in either shorter trains or poorer service quality for longer trains. Moreover, when RSTP reconnects inter-consist networks mid-journey, Ethernet switches for the intra-

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consist networks will need to renegotiate communications and block all services 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 These problems can be overcome without compromising the system's network redundancy if the Ethernet switches could determine for themselves which port to set as a redundant port, and which port to set as a normal port. With Dynamic Ring Coupling, two port pairs in each ring will automatically identify which pair should be set to redundant mode without operator intervention. That way, train operators can rapidly connect train consists and leave configuration up to the system. This process would streamline operational efficiency and minimize configuration errors.

With Dynamic Ring Coupling, two ports in each ring will automatically identify whether they

should be active or inactive without operator intervention.

Thankfully, Moxa's DRC redundancy protocol already supports an auto-negotiation technology: Dynamic Ring Coupling. When two Moxa Ethernet switches are connected as ring coupling switches, dynamic ring coupling identifies which port should be set as the inactive and 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 very flexible to re-deploy again and again.

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Consequently, Dynamic Ring Coupling is not only able to handle consist reorientation, but also provide railway operators with efficient and time-saving automatic network settings in under 1 second, free from human error and intra-consist network interference.

Conclusion

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, Dynamic Ring Coupling technology provided on Moxa’s EN 50155 managed Ethernet switches guarantee 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—such as 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.

Visit the Moxa website (www.moxa.com/rail) for product details, or subscribe to our railway newsletter (www.moxa.com/railnews) to stay up-to-date with the latest railway trends and learn about Moxa's newest IP-based solutions for railway applications.

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