Thursday, August 09, 2007

Running Trains on WiFi

For those having difficulty getting their WiFi to work reliably at home, you may not want to take a trip on a train in Asia. The mechanical signals used to stop trains colliding are being replaced with 802.11b wireless communications (ie: WiFi):

Increasingly, moving block train control systems are being used, operating as communication-based train control (CBTC) systems. Modern CBTC systems require up to 1Mbps (megabit per second) of uninterrupted communication between the trackside automation equipment and fast-moving trains.

Because most rail operators in Asia demand a high local content, it seems appropriate to use international radio standards and commercial off-the-shelf radio components, which can provide the necessary bandwidth. This is generally achieved by using standards and technologies for wireless local area networks (WLAN), and typical CBTC systems are based on the well-known 802.11b standard. ...

From: Wireless technology takes off in Asia, International Railway Journal, July 2007
Railways use very stringent safety standards, so it would be interesting to see how they made the case that WiFi would be reliable enough for controlling trains. It may be that the article is wrong and a WiFi-like systems is being used, perhaps using different dedicated frequencies. As an example of that European railways use a modified form of the GSM phone standard, adapted for railway requirements, called GSM-Railway (GSM-R). This uses separate frequencies from the GSM phone networks and has special features for safety and reliable working. Alternatively the railway might use several different commercial networks (as has been proposed in Australia).

An example of an 802.11 train system is
Alcatel's SelTrac, one version of which uses 802.11. The first use of this was the Las Vegas Monorail. There is a detailed technical paper on the technology used:

Alcatel is pioneering the implementation of an open standards RF communications technology (802.11 Frequency Hopping Spread Spectrum (FHSS)) for trains moving in excess of 120km/h. Whether it’s used for Communication-Based Train Control (CBTC) or Closed Circuit Television (CCTV), 802.11 remains the preferred choice since it’s the only standard that supports mobility and defends against obsolescence. Alcatel adopted 802.11 FHSS technology in 1999 and has performed several trials and demonstrations since then.

From: Open Standards for CBTC and CCTV, Radio-Based Communication, Ed Kuun, date: ????
It is not clear how different the technology Alcatel is from ordinary office and home WiFi.

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