Multiplex communications – Diagnostic testing
Reexamination Certificate
1998-08-24
2001-10-09
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Diagnostic testing
C370S251000, C379S029010
Reexamination Certificate
active
06301227
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to telecommunication test systems and methods, and more particularly to telecommunication tests and methods that allow transmission systems, such as Subscriber Line Multiplexer Systems to effectively respond to automatic test procedures, such as the MLT (Mechanized Loop Test).
Transmission systems have been used to provide cost-effective connections between a central office and subscribers' telephones. A Subscriber Line Multiplexer system is a type of transmission system that uses digitally multiplexed channels to connect two or more telephone subscribers to a central office. This system uses the ISDN U-interface technology, or xDSL technologies, such as HDSL (High Data Rate Subscriber Line), ADSL (Asymmetric Digital Subscriber Line) and their variations, to multiplex several voice and/or data channels over a single copper pair, in order to provide additional virtual telephone or data lines to one or more subscribers over the existing cable facilities.
When the personnel in the central office of a telephone company receives a complaint from a subscriber about telephone problems, the subscriber's line is remotely tested from the central office to verify the accuracy of the complaint and to locate defective network components. Thus, a repairing team may be dispatched to the appropriate location. This provides an efficient and economic solution for the telephone company in dealing with problems about telephone lines.
Typically, the telephone line tests are automated, and one of the most popular automated tests is known as the MLT procedure performed by an MLT system. In an MLT procedure, the MLT system applies known electrical signals on the subscriber's loop comprising the telephone line and the subscriber's telephone set and makes appropriate measurements. Based on the measurements, the MLT system can calculate the loop impedance, the parasitic impedances, as well as possible foreign voltages (i.e., voltages other than those supplied from the central office) present on the line. The MLT system reports back to the central office the results of the measurements, as well as the probable status of the line. The operator needs to know these results in order to understand the problems and take appropriate actions. For example, if the insulation between the wires of a telephone line is damaged, the impedance between wires may become very low. In this case, the transmission characteristics of the telephone line may become very poor, and the quality of the services could thus be impaired. Also, the presence of a foreign voltage due to, e.g., induction from defective high power electrical ducts, can impair the transmission characteristics of the telephone line and can even make it dangerous to use the telephone.
The MLT procedure is most effective for remote testing telephone lines without having any transmission system, such as a fiber optic transmission system, or a Subscriber Line Multiplexer System connected to the lines. If the subscriber's telephone set is not directly connected to the central office, but instead it is connected through a transmission system, the MLT system does not have direct access to the line. In such a case, when the transmission system detects the presence of an MLT signal, it feeds back to the MLT system the values of three resistors located at the subscriber's loop which represent the “signature” of the current state of the system. These three resistors are respectively connected between the ring and tip lines, the ring line and ground and the tip line and ground.
In order to present the proper signature to the MLT system, the transmission system must perform a series of self tests. If the number of lines serviced by the system is large, the remote equipment can be provided with a rather complex test head for performing elaborate tests of the subscriber's drop (i.e., the copper pair that connects the remote terminal to the subscriber). The test head can report back the results to the central office terminal for presenting the proper signatures. Because large number of lines are serviced, the per-line cost of the test head is relatively small. However, in the case of multiplexer systems with small number of lines, e.g., 2 or 4 lines, the per-line cost of the test head is very high. One solution would be to use a less sophisticated test head with simple self-test circuits. In such case, the system periodically performs (or at a time when the MLT is detected) simple tests which will establish signatures for presenting to the MLT system. However, the number of tests and the accuracy of the results obtained with these simple circuits are insufficient for detecting the problems of the telephone lines. Thus, complex circuits have to be used, which dramatically increases the cost.
Accordingly, there is a need to provide an improved and cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems.
SUMMARY OF THE INVENTION
This invention provides a cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems. By using the invention, the information returned by the multiplexer system is maximized when MLTs are run by the operating personnel for maintenance or failure detection purposes. The invention uses the same pair of wires for transmission and test.
According to one embodiment of the invention, a system for allowing a test procedure to be performed on a communication system is provided. The communication system has a transmission line for connecting to telephone exchange and a subscriber line for connecting to a communication set. The system of the invention comprises a detector, coupled to the transmission line, for detecting test signals on the transmission line; a bypass circuit, coupled between the transmission and subscriber lines, for allowing the test signals to bypass the communication system; a discriminator, coupled to the subscriber line, for discriminating a connection status of the communication set; and a controller, coupled to the detector, the discriminator and the bypass circuit, for activating, in accordance with detection of the test signals and the connection status of the communication set, the bypass circuit so as to allow the test signals to bypass the communication system and connect to the communication set and perform measurements.
According to another embodiment of the invention, the system of invention additionally includes a status circuit, coupled to the controller, for reproducing, upon discriminating a predetermined connection status of the communication set, the predetermined connection status of the communication set for presenting to the test signals. The predetermined connection status includes off-hook and short statuses of the communication set. The status circuit reproduces the predetermined connection status by emulating an impedance of a loop comprising the subscriber line and the communication set. The status circuit emulates a linear impedance if the connection status discriminated by the discriminator indicates a short status and a non-linear impedance if the connection status discriminated indicates an off-hook status.
According to a further embodiment of the invention, the communication system includes a second transmission line for connecting to the telephone exchange and a second subscriber line for connecting to a second communication set. In this communication system, the first transmission line transmits signals to the first subscriber line and the second transmission line transmits signals to the second subscriber line. The controller activates the bypass circuit in accordance with the connection statuses of both the communcation sets.
According to a still further embodiment of the invention, the system of the invention additionally includes a signature generator, coupled to the first and second transmission lines, for generating a signature, indicative o
Antoniu Tudor
Ho Edward
Rafko Michael C.
Hsu Alpus H.
Qureshi Afsar M.
Terayon Communication Systems, Inc.
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