94 ◾ Advances in Communications-Based Train Control Systems
A data communication system, which is the basis for train control, is one of the key
subsystems for communications-based train control (CBTC). A couple of wireless
communication technologies have been adopted in CBTC, such as Global System
for Mobile Communications-Railway (GSM-R) and wireless local area network
(WLAN). For urban mass transit systems, WLAN is commonly used due to the
available commercial o-the-shelf equipment, open standards, and interoper ability
CBTC systems have stringent requirements for communication availability .
Whereas in commercial wireless networks, less service availability means less rev-
enues and/or poor quality of services , in CBTC systems, it could cause train
derailment, collision, or even catastrophic loss of life or assets . erefore, it
is important to ensure the train–ground communication availability in CBTC
ere are several WLAN-based CBTC systems deployed around the world, such
as Las Vegas Monorail from Alcatel  and Beijing Metro Line 10 fromSiemens .
Most system integrators claim that redundancy is used in their systems. However,
they do not reveal the details about redundancy due to condential considerations.
Moreover, availability analysis is largely ignored in the literature of CBTC systems.
In this chapter, we study the availability issue of WLAN-based data communi-
cation systems in CBTC. e contributions of this chapter are as follows:
1. We propose two WLAN-based data communication systems with redun-
dancy to improve the availability in CBTC systems.
2. e availability of WLAN-based data communication systems is analyzed
using continuous-time Markov chain (CTMC) model , which has been
successfully used in call admission control (CAC) in mobile cellular networks
 and wireless channel modeling , among others. e transmission errors
due to dynamic wireless channel fading and handos that take place when
the train crosses the border of two successive access points (APs)’s coverage
areas are considered as the main causes of system failures.
3. We model the WLAN-based data communication system behavior using
deterministic and stochastic Petri net (DSPN) , which is a high-level
description language for formally specifying complex systems. e DSPN
solution is used to show the soundness of our proposed CTMC model.
DSPN provides an intuitive and ecient way of describing the com-
plex system behavior and facilitates the modeling of system steady-state
4. Using numerical examples, we compare the availability of the two proposed
WLAN-based data communication systems with an existing system that has
no redundancy. e results show that the proposed data communication sys-
tems with redundancy have much higher availability than the existing system.