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equipment. There are many advantages when
compared with TBTC, such as a shorter departure
interval, less quantity of hardware, simple
maintenance, more flexible, better security of the
system and a better transmission mode. CBTC train
control system will become a mainstream
technology in the future.
Therefore China should speed up on the research
of train control system, and develop train control
technology with its own intellectual property.
Although China has started the research of CBTC
train control system technology, it is still in infancy
period. Currently, WLAN based on the standard of
IEEE802.11 has become the most widely used data
communication method in urban rail transit
(Grappone and Hubbs, 2007). ATP (automatic train
protection system), which is responsible for the safe
operation of trains, is the core component of CBTC
equipment. Therefore, the study of urban rail transit
ATP system based on WLAN is the main mission in
the research of CBTC. The establishment on the
model of urban rail transit ATP based on WLAN is a
paramount and urgent research work. It is significant
for ensuring the security and stability of CBTC
system. Meanwhile it can help to reduce the project
cost of CBTC systems.
This paper will analyze on giving a model of the
core algorithm formula on the speed-distance curve
of the ATP system based on WLAN, and use
modular design method to establish the model of
ATP system and system maintenance and upgrading
will benefit from it. To test the reliability of the
model, the platform of Labview is selected and an
integrated simulation system of ATP is established
in this research. Then simulates this program and
discusses the feasibility and accuracy of the
proposed solution.
The paper follows the steps by question
prompting, theoretical research, practical research,
solution proposing and model establishing to check
the solution. First, the paper points out the necessity
and background of security problems in application
of ATP system, and elaborates practical significance
of this study. Second, the paper defines the basic
theories, targets and principles of establishing
models on applying WLAN to CBTC, and analyzes
the advantages and disadvantages of current ATP
systems, so that to find out the advantages of ATP
system applied to CBTC based on WLAN. Then,
applies WLAN technology based on IEEE 802.11 to
the train control systems, builds the structure of the
entire wireless communication system, and focuses
on modeling the ATP speed-distance curve in
mathematics and dynamics. Next, based on the
established model, simulates the ATP system in
CBTC based on WLAN and discusses the accuracy
and feasibility of the proposed solution. Finally,
summarizes the work on composing the paper.
2 RESEARCH FOUNDATION
AND RELATED THEORIES
2.1 Problems Existed in WLAN
WLAN obeys an open channel standard. Therefore,
it is hard to prevent attackers from eavesdropping
deliberately, tampering and forwarding maliciously.
Information of WLAN transmits in the air through
radio waves. So attackers can receive, interfere,
tamper, forward or even forge information as long as
they have the same receiving equipment while the
sender can’t detect them. It’s easier for attackers to
camouflage to be legal users because users do not
have to connect with the net actually. In addition,
when the radio wave transmits in the air, the signal
attenuates which leads to the missing of information
for various reasons, such as barrier or shield. Radio
coverage area is limited. Wave energy will become
lower and lower when it spreads in the air. Besides,
there may be some obstacles in the communication
environment. When transmitting through the
obstacles, the information may lose, causing data
integrity problems.
When applied to urban rail transit CBTC, the
WLAN’s characteristics mentioned above will cause
some specific security problems. When CBTC
communicates by WLAN, as the environment is
more complex, the speed of the train is fast and there
are all kinds of interference such as signal reflection
in the subway tunnel, shielding and signal
temporarily blind areas, electromagnetic, signals,
human destruction, etc
[5]
. Equipment processing
capacity, network load capacity problems,
communication equipment or media failure, may
lead to insecurity data communication. Unauthorized
users’ accessing, eavesdropping, simulating base
stations and other malicious behaviours are what we
need to consider.
To solve these problems, there have been some
related researches at home and abroad, for example,
obtain the best AP density to ensure relatively
redundancy by modelling and calculating to solve
the problem of data loss. To carry out effective
authentication, Message sequence number, Time
stamp (Cai, 2000), Time-out, Source and destination
identifiers, Feedback message, and Safety code
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