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The Design Scheme and Realization Method of Telecommunication Cable Fault Observation System Ke Wang Yukuan Ma Jilin University of Technology, Changchun, Jilin, 130025, China. Email : wk@post.jut.edu.cn Fengyie Hu Changchun Institute of posts and Telecommunications ,Changchun, Jilin, 130022, China Contact

Abstract

This paper describes the design scheme and realization method of Telecommunication cable fault observation system by using impulse reflectance. The overall design scheme of TCFOS is put forward and the composing block diagram of System Testing Font Terminal is given. The devising process of hardware circuitry and Management Termination software in STFE are analyzed in detail. Using the Single Chip Processor and by means of telephone net, the data is transmitted and receipted as well as storage. The real-time monitoring of cable is realized. And the problem emerging in system debug is analyzed and the solution method is given. At last, the function and merit as well as feature of system are globally summarized.
Keywords: Telecommunication Cable, Fault, Observation System, Communication Software

1. Introduction

The fault of telecommunication cable mainly includes the short circuit, broken and mix line as well as insulation bad of cable etc. There are many method of testing cable fault being used in now. For example, distribution capacity detection method, pulse reflection detection method, the bridge detection method of direct current, the induction current detection method and detection method of atmospheric pressure etc. In which, the detection method of atmospheric pressure is used in internal gas pressure cable mainly; Induction current detection method applies to the armored cable in tunnel that lies; Line distribution capacity detection method only can be used in the broken state of cable. Because of the much difference of detection method, the detection equipment of cables fault are varied. According to its function, the detection equipment can be divide into mainly some kinds as following:

1. Simple portable detection equipment. The structure of equipment is simple, and function is unitary.
2. The detection equipment which have certain additional function. This kind of equipment has some additional functions, which can carry out simple handling for detection data, and print, storage.
3. Powerful testing system. Some parts such as proscenium detection, data transmission and the handling of backstage supporter form the equipment. It has the function of collecting state data, transmission and processing data.
The paper chose the method of microprocessor control detection with pulse reflection theory. The system can automatically detect the fault of telecommunication cable online and carry out communication with the management terminal of backstage supporter. And that can be used to test the short circuit of cable, the broken and the founder (cable in water).

2. The basic principle of the pulse reflection method For the judgement of fault

2.1 The basic principle of the pulse reflection method
When the cable work as normally, terminal load impedance Z_l and line property impedance Z_c match, that is Z_l = Z_c, shown as Fig.1. The electromagnetism wave online will reach load all right without reflection. As the line fault occur on certain point, the input impedance becomes the terminal load before fault point as Fig.2 shows. That is Z_x ¹ Z_C and Z_x ¹ Z_l. The reflection coefficient r is shown as follow:




Fig 1:

Fig 2:

When electromagnetism wave transmission goes to fault point, part or all electromagnetism waves will return to excite terminal. According to the different reflection of fault nature, the size of coefficient is also different.
As the broken fault with single line or two- line occur, impedance Z_x = ¥.

Fig 3:

1. The reflection coefficient
   
   Now electromagnetism energy of wave will reflect completely, and the phase of reflection pulse and the phase of excite pulse are identical, as Fig.3 shown.
2. As the short circuit or mix line fault occur, impedance Z_x = 0. T
   The reflection coefficient
   
   The total reflection will be produced for now, the phase of reflection pulse and the phase of excite pulse are opposite, as Fig.4 shown.
3. When cable in water, the capacity has increased about one time than original capacity. The impedance of founder point is
   



Fig 4:

Fig 5:

The reflection coefficient


From this we know, the phase of the reflection pulse of starting point and the phase of excite pulse are opposite.
The impedance in founder terminal point from increase to, reflection coefficient is


So, the phase of the reflection pulse of founder terminal point and the phase of excite pulse are identical.
From above-mentioned we can know, we can judge the type of fault according to the phase of reflection pulse.

2.2 Measurement distant and distinguishing ability.
Because of the spread speed of electromagnetism wave in certain kind cable, if spread time and the speed of reflection pulse have been known, the fault point can be found.

Where Lx------ Measurement distant

V ------ Speed of reflection pulse

Dt------ March time


Distinguishing ability can be expressed with the formula as following

Where S------ Distinguishing ability

t------ Pulse width

From above we can know, if reduce pulse width, the distinguishing ability will increase. However, the spread speed of pulse will, transmit loss also will increase.

3. The fault telecommunication cable of overall scheme design.

3.1 Frame of Cable Fault Observation System
The system is consist of two parts, Measurement Termination and Management Termination
Testing Font Terminal is make up by plug board, sampler and Single-chip processor. Modem is used to transmit signals to Management Termination though Local telephone network. Single-chip processor read and processes the data from the sampler. Management Termination is make up by center computer and printer as well as modem. The data is receive and sent to the database, as Fig.6 shown.

Fig 6: Frame of the system

3.2 Design of the system software
The system software includes program of Testing Font Terminal and program of Management Terminal. In the program of Testing Font Terminal, there are Measurement Subprogram and Frame of Communication Subprogram, as Fig.7 and Fig.8 as well as Fig.9 shown.

Fig 7: Frame of main program

Fig 8: Frame of Measurement Subprogram

Fig 9: Frame of Communication Subprogram

The design of management terminal software adopts the window scheme with Visual Basic language. Which is consisting with program of database and program of series communication as well as cable distributes picture etc.

4. The Capability of system

4.1 The basic function of system
a. Automatic fault warning.
Detection system can keep on the online working state of detection telecommunication cable. When telecommunication cable occurs fault, the system can quickly and accurately make sure the location of fault, fault category, concurrent make the fault warning system.
b. Data is defended.
Cable fault Detection system possesses 4 kinds of databases including basic equipment installation database, data installation database, fault point warning database, fault historical database.
c. Cable distribute picture is glanced over and can be printed.
The system can give the cable distribute picture of normal state and the picture of contain fault points and print them.
d. Data is transmitted.
The fault information goes to the management terminal of backstage supporter automatically through telephone net transmission and be processed by center computer.

4.2 Criteria of the system
The biggest count of test front high-speed count circuit is FFFF.
The counting frequency with high speed circuit is 100 MHz.
The minimum test scope is 37Km.
The minimum of measure blind area is about 1.98 m.
The measure capacity is 128 couple wires.
The maximum distinguish capacity is to be 0.989 m.

Reference

1. Xuehong Liang, Design of communication cable broken position instrument, Wire and cable, No.6, 1996, p.45-p.46.
2. Zhengqiu Yang, The observation system model of communication software, Journal of Beijing Post and Telecommunication University, Vol.19, No.4, 1996, p.19-p.22.
3. Silian Zhang, Fault of communication cable, Press of Fudan University, 1995

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