Composable Modeling Method for Generic Test Platform for Cbtc System Based on the Port Object

The Communications-based train control(CBTC) system has gradually become the first choice for signal systems of urban mass transit. However, how to guarantee its safety has become a research hotspot in safety fields. The generic test system with high efficiency has become the main means to verify the function and performance of CBTC system. This paper discusses a composable modeling method for the generic test platform for CBTC system based on the port object. This method defines the port object(PO) model as the basic component for composable modeling, verifies its port behavior and generates its compositional properties. Based on the port description and the test environment description, it builds port sets and environment port cluster, respectively. Then it analyzes and extracts possible crosscutting concerns, and finally generates a variable PO component library. It takes the modeling of block port objects in line simulation of generic test platform for CBTC systems as an example to verify the feasibility of the method. Keywords—composable modeling; test platform; CBTC; port object; line simulation


INTRODUCTION
By allowing trains to operate safely at closer headways, CBTC system can permit more effective utilization of rail transit infrastructure.It has become the preferred standard of urban rail traffic signal system.As a kind of safety critical systems (SCS), however, CBTC system, with the highest safety requirements, is directly responsible for the train operation [1].Once the system fails, it will lead to a great or even a catastrophic loss of lives, property, and environment.
As one of the important means to improve the safety and assure the quality of the system, testing plays an important guiding role in the process of researching and developing the CBTC system.However, the key issue of the implementation of system testing is how to build a simulation and test environment in accordance with the real operational scenario of CBTC system under test(SUT) [2,3].Hence, the more attention paid on establishing the test platform for CBTC system, the higher the requirement.The test platform for CBTC system is developing to the direction of network, intelligence and generalization.The test platform is a virtual environment which is used to verify the correctness and reliability of system design.It generally includes the input, processing, validation, and output of signal data, which can not only meet the needs of SUT for functional verification but also some non-functional verification, such as performance test, pressure test and safety test [4].

II. TEST PLATFORM FOR CBTC SYSTEM
A perfect test platform for CBTC system is the combination of simulation technology and testing technology.The simulation activity builds up the desired external scenario and simulates the external environment for SUT.The key to the following testing process is whether the design of a simulation environment is successful or not.And the testing process is a continuation of the simulation activity and its sequential execution [5,6].A full process of building the test platform for CBTC system is shown in Figure 1.So far, a widely used and generic test platform for CBTC system hasn't been built.Besides the challenge of developing CBTC system, there are also some problems of building the test platform for CBTC system.One is the architecture of CBTC system differs between the vendors, requiring the test platform makes quick adjustment to adapt to the test of different venders; another is some of the CBTC interfaces differ between the vendors, requiring the test platform has variable interfaces and is easy to switch and roam seamlessly from interfaces to interfaces.
By aiming at establishing a generic test platform for CBTC systems developed by multiple vendors, it focuses on how to build a generic test model for CBTC system, and how to design effective test platform architecture for CBTC system in this paper.Although [7], [8] and [9] have discussed the port-based approach to integrated modeling and simulation of SCS, respectively, they consider the SUT more without the property to safety, real-time and was not suitable to build the test system for SCS.This paper further the evolution towards a seamless integration of simulation for SCS test platform with the idea of a PO.comes from the external environment [10,11].Furthermore, the system is designed and modeled merely based on external interface documents and its modeling method should both consider port evolvable and module replaceable, renewable and reusable, which pose a great challenge to the modeling method for the test platform.The concept of Port Object is given below, and a composable modeling method based on Port Object is proposed.

A. Port Object
The Port Object, PO is a kind of novel software abstract that is configurable and replaceable, and the basic unit that generates the system components.As is shown in figure 2, it incorporates the concept of objects and the port automata model used for concurrent processing, and meet the needs of the communication mechanisms in safety-critical areas [12].
Where * is the reflexive transitive closure, and the union operation means the combination of two relations [13].

B. Ports' Behavior Description of PO Model
The ports' behavior elements of PO model is described as below: To ensure security and real-time communication for CBTC test system, it usually needs timeliness and order calibration.Here, it assumes that the PO model in simulation port as a receiver, and then it receives secure data sent by CBTC system periodically.After receiving the secure data, it will carry out a real-time inspection of the time sequence.If it isn't synchronized, then returns the timeliness calibration request and waits to receive the response from timeliness calibration.If synchronized and passed validation, then it shall proceed with receiving secure data [14].The internal port behavior of this PO www.ijacsa.thesai.orgmodel is shown in Figure 3.The branch output expression is derived as :

C. Configuration Constraint of PO Model
The PO configuration constraint is one of the important parts of the PO model which describes the configuration instruction, external environment interfaces, internal interfaces between objects and properties of PO [15].

• Configuration Content
The structure of PO configuration constraints is shown as Figure 4.

1) Configuration Description
The Description of the current configuration constraints, describing the current configuration, such as version, release date and operating environment for PO, mainly facilitates the management of PO, avoids confusion version, as well as the combined behavior or debugging failure due to operating environment errors.Also, configuration description is the main information when components added into the component library.

2) Interface Configuration
Interface configuration comprises the interface configuretion with external environment and between POs.The external interface refers to the interfaces with the CBTC system and testers, which includes the interface type, interaction protocols, interface parameters and Input/Output(I/O) description, etc. Depending on its interface mode, the described interface properties shall be different with different modes.

3) Object property table Object property table describes the relation between interface properties or between properties and values. There
can also be a description of the objects' initial states in the table.
• Configuration Description Based on XML By nesting and referencing the hierarchical relations between the specific elements, the extensive markup language (XML) uses elements and properties to describe data.The XML configuration description is given below based on the structural characteristics of the configuration constraints in PO model [16].The XML configuration template is described in Figure 5.
The environment E of the PO model M shall meet the following conditions：1) E and M is composable; 2) E is non-null; 3)

A. Acquisition of PO model
There are three main sources of demands for the generic test platform for CBTC system: interface document, test environment description and test requirements.The interface document describes all the external interfaces of tested CBTC system, including the interface type, interface parameters and interaction protocols.The test environment description includes all the required external system, infrastructure and environmental constraints.The test requirement mainly describes the needed tests for CBTC system, including the functional test and performance test.The key to composable modeling a generic test platform for CBTC system is doing requirements analysis based on the interface document, test environment description and test requirements, acquiring the PO model and then generating the component library.
The PO model mainly focuses on the port, so does acquiring the PO model.The acquisition process of PO model is described in Figure 6.
Step1: Based on the interface document, the test system is divided into different ports, and generate port sets.
Step2: Based on the test environment description, the environmental resources is it analyzed, then it is added to those covered by the interfaces to the appropriate port set, and the environment port cluster is generated.
Step3: Based on the test requirements, the man-machine interface is extracted and it is added to the appropriate environment port cluster or as an independent environment port.
Step4: After the environment port cluster is analyzed, and the possible crosscutting concerns is extracted, the problem of requirement distraction and requirement entanglement will be resolved.
Step5: Based on the crosscutting concerns, the environment port cluster is divided into different PO.

B. Generate Port Cluster
Description of the test environment comprises all the resource sets needed by the test platform.These resource sets are the state-description objects in PO model.Resources' dependency on the interface comprises.

1) Call, resources call interface to communicate in operation.
2) Interrupt, resources interrupt interface communication in operation due to state changes.
3) Modify, resources (interfaces) modify resources (interfaces) parameters or states in initial start when it is needed.
4) Update, resources update their states due to interaction information.5) Mutually exclusion, this relationship is less likely to appear, which means the interface will not be activated if resources participated in operation at the very beginning.
Define the resource set as ResourceSet, which comprises all the resources of the test system.Define the port set as PortSet, which comprise all the communication interfaces of the test system.Define the port cluster as PortCluster.Resources' dependency on the interface denotes as r.The way of generating port clusters is given below, as shown in Figure 7.

C. Extract Crosscutting Concern
In the built environment port clusters, some resources will appear in different clusters, which are both related to one PO and another, in other words, exist in the intersection of different environment port clusters.
The crosscutting concerns of a resource in two environment port clusters can be written as CC={Re|Re∈PortCluster1∧Re ∈PortCluster2}, where CC is the crosscutt-ing concern, Re is the resource and PortCluster is the environment port clusters.
In an environment port cluster, if there is a resource which has dependency on several ports, then this crosscutting concern can be expressed as: Here an algorithm used seeking all the possible crosscutting concerns is introduced, where CS is the cluster set of all the possible crosscutting concerns, RS is the set of all resources, RRS is the resource relationship set.ExistR(Re, Po, r) is a boolean function, if there is a relationship r between the resource Re and the port cluster Po, then its value is true.If not, its value is false.The way of extracting crosscutting concerns is described in Figure 8.

D. Generate PO Model
Once the environment port cluster is formed and the crosscutting concerns which causes distraction and entanglement are extracted, the PO model can be generated based on environment port cluster and crosscutting concerns.Define the port object model set as POMS and CS is the cluster set of all the possible crosscutting concerns.The way of generating the PO model is described in Figure 9.An example of the PO in the line simulation of test platform for CBTC system is presented, and a detailed modeling process is introduced.Some of the descriptions of the three documents are tabulated in Table 1 for further illustration.
Table 1 only tabulates some of the details about the feature points and appropriate PO models can be built based on these details.The modeling process is shown in Figure 10.  2) After environment description is added, a connection object <port type, port constraint, resource description, dependency> of the 24V relay is generated, where the port type and the port constraints integrate the description of the port set.Resources description includes the section number, the section length and the starting point.Dependency is described as the following.Assume that one section connected with one relay (ignore reacquisition).When the section is occupied and the drive relay is energized, the output is "Occupy", or the section is vacant and the drive relay deenergized, output is "Idle".
3) After test requirements are added, the man-machine interface is added to the environment port cluster.The sector port cluster has two external inputs: the man-machine interface and the train motion simulation PO.Moreover, both of them are POs.The output of the sector port cluster is the environment port of 24V relay.
4) With the further analysis on test requirements, the fault simulation will be appeared in many port clusters.Thus, they are extracted and are named as crosscutting concerns.All of them are POs, their inputs are man-machine port clusters and outputs are other environment port clusters.
5) It is much easier to convert the section port cluster to the section port.Define that the PO's state {0,1} = S where "0" is "Occupied" and "1" is "Idle" and its initial state is 1.Each of the section ports The configuration constraints can be easily generated based on the port constraints, as shown in Figure 11.The test and verification of CBTC system has become one of the important means of ensuring system security.While building the generic test platform for CBTC system is the prerequisite for test and verification.Nowadays, CBTC system having a more complex architecture, more diversified interfaces and more often upgrading, which poses a greater challenge to the modeling and simulation technology.In this paper, some preliminary study on how to build and realize the generic test platform for CBTC system will be done, a composable modeling method for CBTC simulation and test system based on the port object will be suggested, and integral composite-oriented modeling process as well as verify the feasibility of the method will be illustrated In this paper, a meaningful technology method are put forward for resolving the problems in the process of development for test platform of CBTC system, but there are still many works to be further researches, such as another important aspects with the combination of system modeling, namely the assembly based on component technology.In view of the PO model based assembly technology, a best method of assembly needs further discussion and analysis.

1 ); 3 )
where m is the input identification, which means messages; : { } = R r , where r is the output identifica- tion, which means responses; : { } = C c , where c is the Boolean expression.The internal port behavior of the PO has four different situations: Null output: : Sequential output:

Fig. 3 .
Fig. 3.The internal port behavior of PO mode

Fig. 4 .
Fig. 4. The structure of PO configuration constraints

Fig. 5 ..Definition 3 . 4 (
Fig. 5.The XML configuration template D. Composability of PO Model Definition 3.3 (Composability of PO) The two PO models M and N, if the following conditions are met: , , M N M N M N P P P P P P A A I I O O φ φ φ ∩ = ∩ = ∩ = Then M and N are called composable, and are defines as: ( , ) ( ) ( ) = ∩ ∪ ∩ M N M N P P P P M N I O O I shared

Definition 3 . 7 (Definition 3 . 8 (Definition 3 . 9 ((
Legal Environment of PO) If PO models M and N are composable, E is the environment of M and then E is called a legal environment of ( , ) M N , Compatibility of PO) If PO models M and N are non-null and composable, additionally, there is a legal environment ( , )M N , and then M and N are called compatible.Replaceability of PO) N is the PO model, N E is the environment of N in system S , I is the bind action set of N and environment N E , ( , ) C Replaced N S is the replace action of N in system S , and therefore

Fig. 6 .
Fig. 6.Process of acquiring the PO model

Fig. 7 .
Fig. 7.The way of generating port clusters

Fig. 9 .
Fig. 9.The way of generating the PO mode V. INSTANCE ANALYSIS

Fig. 10 .
Fig. 10.The modeling process of PO of line simulation

TABLE I .
PORT DESCRIPTION, ENVIRONMENT DESCRIPTION AND TEST REQUIREMENTS