Automatic train control system with using of fiber optic wireless Local Area Networks

Аuthors

Strobel O.

Esslingen University of Applied Science, 101, Flandernstrasse, Esslingen, D-73732

e-mail: otto.strobel@hs-esslingen.de , weweme00@hs-esslingen.de

Abstract

This article presents the design of optimum WLAN-Settings for maximum throughput or respectively minimum of collisions in a WLAN over fibre system. Although it is shown that the fibre delay might violate some of the timing boundaries of the media access (MAC) protocol, with a careful choice of slot time, these networks can benefit even without the need for modifying the existing protocol. The network topology is based on a WLAN-Environment that is used in a subway in Nanjing (China) to accomplish the Trainguard MT System.

Radio over fibre (RoF) technology enables both, distribution of wireless signals over long distances and reducing costs of antenna sites, since the signal processing can be done at a central point. The 802.11 standards were developed for WLAN’s which operate in relatively small areas, where no significant signal delay occurs. Normally the propagation delay in air is much smaller than 1 µs. In WLAN over fibre systems additional delay time occurs. The propagation time is the sum of an air propagation delay and the delay caused by the propagation through the fibre.

In continuous communication or point-communication conditions, Siemens Automatic Train Control (ATC) system ensures train safety and continuous supervision. The safety of train separation is based on the moving block principle. The ATC sets two terms on data stream. The throughput for the ATC has to be 10 kBit/s for downlink and 1 kBit/s for uplink, and every transmitted frame must not be older than 10 ms. There are two typical frame sizes used by the ATC system. 30% of all frames are 170 Byte in size and 70% are 220 Byte in size. The used WLAN standard is 802.11b, therefore the maximum throughput is 11 MBit/s.

The simulation results have shown that WLAN over fibre can only be realized with the change of slot time depending on the length of fibre. For demonstration that there are too many collisions without the change of the slot time, another simulation has been performed. The result fr om a simulation shows wh ere the slot time has not been changed. Especially the retry-counter reaches often its maximum value, which leads to the fact that frames were discarded. As a result the ATC system fails because now many frames have not been transmitted within a period of 100 ms. In summary, it can be stated that without a change in slot time there are too many collisions, thus leading to a system failure. Therefore the slot time is enlarged by the latency time.

Thus, finally it has been proofed that a common MAC-Layer can be used in a WLAN over fiber scenario.

Keywords:

wireless Local Area Networks, reliability, cost-effectiveness, network topology modeling

References

1. D. Wake. «Trends and prospects for radio over fiber picocells», in Proc. Int. Topical Meeting in Microwave Photonics, Awaji, Japan, Nov. 2002, pp. 21-24.

2. Siemens AG:"Siemens Transportation Systems". http://w3.siemens.dk/home/dk/dk/mobility/Signalsystemer/togkontrol/Documents/Trainguard_MT_CBTC_eng.pdf (last accessed date 01. August 2010).

3. Pablo Brenner. A Technical Tutorial on the IEEE 802.11 Protocol". Stand 18. July 1996. http://www.sss-mag.com/pdf/802_11tut.pdf (last accessed date 1. August 2010)

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