In a typical telephone system, a subscriber's telephone is connected to a central office switching system by a subscriber loop consisting of two metallic conductors (referred to as "tip" and "ring" respectively). The subscriber loops are organized into cables which are routed to the premises of subscribers and potential subscribers. The subscriber loops terminate on a main distribution frame that services the particular region.
The subscriber loops are connected to terminals on one side of the main distribution frame, and the input lines to the central office switching system are connected to terminals on the other side. The main distribution frame makes the assignments between input lines to the central office switching system and the subscriber loops. In order to connect a new telephone subscriber onto the telephone company's central office switching system, a connection must be made from an input line of the central office switch to the desired subscriber loop. This connection will be referred to as a "call path." Presently, most of the call path connections are made manually. It would be more efficient and less costly to make the connections automatically from a remote location. Some main distribution frames can do this by utilizing switch matrices. A typical switch matrix contains hundreds of relay modules which are themselves smaller switch matrices. Each relay module contains many independently operable crosspoint switches. Each crosspoint has a double-pole, single-throw switch which connects both the tip and ring conductors.
The crosspoint switches in the switch matrix allow pairs of signal lines to be connected. The operations of the switch matrix must be organized so that when the crosspoint switches make the connections to form the call path, other lines in the switch matrix are not affected. Each line into the switch matrix must be capable of being connected to any of the lines out of the switch matrix (provided the line in question is not already connected to another input line). In general, a switch matrix is capable of connecting any of N input lines to any of M output lines without disturbing existing connections. A relay module with n input lines and m output lines has nxm crosspoints.
A call path from an input line to a subscriber loop may take many different paths through the switch matrix. Each call path involves a number of relay modules and crosspoint switches within the relay modules. If a crosspoint connection fails, by the crosspoint switch not closing or opening when desired, the call path will not be operable. A crosspoint switch may fail by being stuck open, stuck closed, or having the tip or ring contact alone being stuck open or closed. A typical failure is such that one contact is defective and the other operational; this is called a "half-crosspoint."
A problem in utilizing switch matrices in telecommunications applications is to find a defective crosspoint switch without disturbing other existing connections. The bad switch must be identified among many good switches and, because of the many possible paths through the switch matrix which a call path may take, the defective crosspoint switch must also be locatable independent of the length of the call path.
The detection, identification, and location of defective crosspoint switches should be done without significantly increasing the complexity of an already complex system. That is, the addition of more wires, instruments, and components for defect detection would complicate the existing system and add greatly to costs.
Further, efficient telephone operation requires that accurate detection and identification of the defective crosspoint be made from a remote location and be capable of automation, for instance, by computer control.
Since the crosspoint switches in the call paths may be rather close together in the sense that the impedance of the electrical conductors connecting these switches is small compared to the impedance of the total path through the cross-connect switch, the crosspoint detection and identification system must be capable of resolution sufficient to distinguish a defective crosspoint switch that is very close to a good crosspoint switch.
If a connection is made from the switch matrix to transmission long lines from outlying regions, there are effects from the long lines which may complicate the interpretation of signals indicating a defective crosspoint switch. An effective detection and identification system must address this problem.
The detection of faults in transmission cables has been done in the telecommunications industry by timing reflections of transmitted pulses by the fault back to the pulse source. This approach is most suitable for relatively uniform conductors which are hundreds to thousands of feet long. The timed reflection method is not suitable for switch matrix defect detection and identification because the reflections may occur because of any change in impedance along the conductor path, for instance, connectors, terminal blocks, and so on, which are not defects. Further, the accuracy and resolution of this method are not adequate for the relatively short paths found in switch matrices.
Broadly, it is an object of the present invention to increase telephone service efficiency by providing a method for detecting, identifying, and locating defective crosspoint switches in switch matrices.
It is a further object of the present invention to provide detector, implementing circuit, and procedure means for detecting, identifying, and locating defective crosspoint switches in switch matrices.
It is yet a further object of the present invention to provide a method and means for detecting, identifying, and locating defective crosspoint switches in a switch matrix without the need to modify existing switch matrices.
It is still a further object of the present invention to provide a crosspoint defect detection method and means which is adaptable to remote and automatic control.
It is another object of the present invention to provide a crosspoint defect detection method and means which is capable of defect resolution such that defective crosspoints which are relatively close to operable crosspoints are detectable and identifiable.
It is yet another object of the present invention to provide a crosspoint defect detection method and means which is capable of detecting, identifying, and locating defective crosspoints when a connection is made to a long-line conductor.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.