A data communication system (DCS) for railway applications provides both ground-based and onboard applications with a solution for exchanging information where and when it is needed, irrespective of the position of the train.
The usual DCS architecture for railway applications is an integrated Ethernet-IP network that includes a wired backbone network, wireless wayside network, and onboard network, with the onboard network handling communications between all communication-based train controller (CBTC) sub-systems. A CBTC must be protected by a robust security system, and requires continuous communication in circumstances where roaming is an unavoidable reality and occurs at very high speeds. For this reason, fast secure roaming and complete redundancy for a DCS are fundamental requirements for achieving smooth CBTC operation.
Major challenges in achieving train-to-ground communications
Perhaps the most crucial aspect of onboard vehicle stations is that they cannot lose any data during handover. For train-to-ground communication links that do not use an improved 802.11 roaming mechanism, the handover time may take up to several seconds. This is not acceptable, particularly since a moving train is only connected to a particular wayside application point (AP) for a few seconds, and handover times that exceed 100 ms could result in significant data loss. Reducing this roaming ‘break time’ to a negligible level is a challenge faced by the rail industry.
Train passengers now expect data connection services with the same level of quality that they experience at home or in the workplace, and sensitive data must be reliably authenticated to ensure that the communications network is protected from outside attacks and internal threats. An important question then is this: how can we create a secure high-speed communications link between train and ground to ensure seamless data services for all passengers, keeping in mind that the networking devices will be affected by many tough operating environments?
Key drawbacks of currently available roaming solutions
Until recently, only two general types of roaming solutions have been available on the market. The first approach is client-based roaming. In this case, each handoff must perform authentication, re-association and security key exchange processes, which consumes a lot of time and can result in roaming times ranging from a few hundred milliseconds up to several seconds. Roaming times of even one second are intolerable for railway environments since the vehicle stations are highly mobile and need to switch frequently from one AP to another. The second approach involves using expensive complex controllers. In this case, all of the data must be transmitted through the controllers when a vehicle station needs to roam to a new AP and corresponding thin APs with only routing capability consume a huge amount of bandwidth. The undesirable outcome is that the costly heavy duty controllers result in handoff times greater than 100 ms.
Secure sub-50 ms Turbo Roaming
For some time now, railway operators have been on the lookout for products that guarantee near-zero hand-off latency between train and ground. Keeping the roaming ‘break-time’ under 50 ms at high speeds ensures uninterrupted connections for transmitting critical data, such as real-time voice and video streams. The CBTC is performed by the non-vital Automatic Train Operation (ATO) components under the supervision of the vital Automatic Train Protection (ATP) components. Therefore, security is critical for DCS to authenticate all communication between subsystems.
Enable pre-authentication using a wireless controller
To overcome slow handoffs on high-speed railways, Moxa’s rail-specific Turbo Roaming technology uses WAC wireless access controllers that provide centralised security management. When roaming to the next available AP, the client can be pre-authenticated by the WAC, which eliminates the need for authentication and reduces the handoff time to 50 ms. Train speeds of 120 km/h and more are supported to guarantee stable roaming handover for light rail and metro vehicles. With a pro-active encryption and authentication mechanism, sub-50 ms Turbo Roaming supports a high level of security features, including WPA, WPA2, and 802.11i, which ensure secure end-to-end communications.
Excelling in many wayside antenna scenarios
To ensure effective handoff for both vital and non-vital applications, Moxa offers a universal roaming method suitable for a variety of wayside antenna scenarios. Roaming is governed by the following 3-step logic:
1. Threshold 1: when AP1 is below this threshold, and
2. Threshold 2: when AP2 is above this threshold, and
3. Roaming offset: when AP2’s signal is stronger than AP1 with at least this RSSI offset, roaming is triggered.
By tweaking the various parameters, network operators can configure a roaming logic that works for their particular antennas and AP layout.
Links with complete redundancy
A communication-based train controller (CBTC) is a train automatic control system based on a DCS architecture. Because of recent advances in wireless transmissions, CBTCs now rely heavily on WLANs to ensure constant train-to-ground data connections. The best way to avoid a link failure on a WLAN is through redundancy, which ensures that clients remain ‘always on’ and allows critical network links to continue to transmit data. Moxa’s dual-RF products can be configured to operate as either redundant APs or redundant clients, with Turbo Roaming added to enable redundant roaming. APs and clients are then able to communicate with each other as long as at least one of the two links remains connected. WLAN products support Ethernet redundancy using RSTP and power redundancy using dual DC inputs and PoE. These multi-redundant features guarantee that the DCS system can provide uninterrupted robust seamless mobility, and ensure that safe and secure communication will always be available to meet the demands of video, voice, and other bandwidth-demanding applications, such as maintenance tasks and passenger information systems.
Conclusion – building more successful train DCS systems
Three major factors contribute to the success of DCS deployments:
* Seamless high-speed roaming.
* Universal roaming for different antenna scenarios.
* Complete redundant links.
Many successful train-to-ground projects have been completed using railway-specific sub-50 ms Turbo Roaming and multiple redundant technologies to ensure reliable DCS connectivity.
| Tel: | +27 11 781 0777 |
| Email: | [email protected] |
| www: | www.rjconnect.co.za |
| Articles: | More information and articles about RJ Connect |
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version