Truly Uninterrupted Train-to-Ground Communications

A data communication system (DCS) connects moving trains to wide area networks, allowing communication between onboard applications and ground infrastructure regardless of the position of the train. A DCS must be protected by strong security measures to prevent unauthorized access, but this poses a problem for network engineers. When traveling at high speeds the train may only connect to a particular wayside AP for a few seconds, so handover times must be extremely fast. Handoffs that take more than 100 ms may cause critical data loss, but strong security systems require time to negotiate new connections. This is the challenge networking engineers face in the bid to establish uninterrupted, reliable communications for moving trains.

Application Requirements

High-speed roaming with network security

  • 50-milliseconds-or-faster handoffs for light rail and metro systems
  • WPA2 encryption for data security
  • Various wayside propagation modes
  • Supports widely distributed wayside APs

Minimal channel interference

  • Three non-overlapping channels

Redundant network fail-safes

  • Multiple train-to-ground links

Moxa's Solutions

Pre-authentication with Moxa's Wireless Access Controller

To allow seamless roaming for high-speed trains and other mobile clients, Moxa's Turbo Roaming technology utilizes a central Wireless Access Controller (WAC) that manages WPA2 security for every station on its network. When handing over a mobile 802.11 client from one AP to the next, authentication is initiated and maintained not by the individual APs, but by the central WAC. This eliminates device-level re-authentication during the handover, reducing handoff times to within 50 milliseconds.

Inter-controller roaming for wireless WANs

Managing the network from its location in the railway's operations control center, a WAC-1001 can manage up to 200 access points on a single network. For wide area networks with hundreds or thousands of access points, inter-controller (i.e., inter-WAC) roaming can be used. When a client roams from one controller's zone to another, the prior WAC passes along the security keys to the new controller, which then delivers the established key to the APs on its own network. In this way, faster-than-50-millisecond handoffs are maintained without any need for re-authentication or renegotiation, even after the train has left the originating network far behind.

Three-channel roaming

When engineering wireless devices for extremely fast handovers, a common approach is to design access points to utilize a single frequency channel, thus cutting handoff times by eliminating the need for clients to negotiate a new broadcast frequency for every access point they access. A single-channel setup, however, introduces problems with co-channel interference (CCI), so while roaming handoffs are sped up, dropped packets due to data corruption increase. Turbo Roaming, however, offers the best of both worlds. First, access points are restricted to the use of three distinct, non-adjacent broadcast channels; this reduces the pool of available channels while maintaining distinct frequency isolation between adjacent stations. Next, as they move through the network, Moxa's wireless clients are designed to use our proprietary optimization technology to re-negotiate communication frequencies. In this way, CCI is eliminated with no cost in handoff times.

Various wayside antenna scenarios

Different physical methods of signal propagation require different handoff approaches by the client. A client roaming across leaky feeder networks will need a different transfer strategy than one roaming across a network using only omnidirectional antennas, and will need yet another when roaming across unidirectional antennas, or waveguide devices. Moxa’s universal roaming technology incorporates handoff strategies for any method of signal propagation, allowing our clients to be deployed on any network, even those which mix propagation technologies.

Rugged vibration-proof M12 connectors

Moxa uses rugged M12 connectors on all its wireless products, to guarantee they will stay physically connected despite the constant vibration and heavy shocks that are ever-present in railway environments.

Complete redundancy for every point of failure

Wireless technologies are a requirement for communication-based train controllers (CBTCs). CBTCs must maintain a continuous connection with the train, so secure fallbacks must be prepared against link failures. The most effective preparation is link redundancy. Moxa's full implementation of 802.11g means that our APs come with dual radio modules, as well as RSTP for full Ethernet redundancy, and finally dual DC inputs and power over Ethernet, for multiple points of power redundancy. These fail-safes provide a strong, secure array of fallbacks for link redundancy.

Application Architecture

Seamless Roaming Across Redundant Train-to-Ground Networks

Redundant Wired Backbone

Two independent Ethernet switches manage two independent fiber backbones, giving full redundancy even at the most basic network level.

Redundant Wireless Wayside Network

  • Dual access points at each network station
  • Wayside APs are linked by Ethernet switches

Redundant Onboard Network

  • Two clients, one at each end of the train
  • The active client connects with the network's ground stations (i.e., access points)


Product Offerings

WAC-1001 - Wireless Access Controller

AWK-3121 - Train-to-Ground AP/Bridge/Client

AWK-5222 - Train-to-Ground AP/Bridge/Client

AWK-4121 - Train-to-Ground AP/Bridge/Client

AWK-6222 - Train-to-Ground AP/Bridge/Client