Method for testing subscriber line of private automatic branch exchange using dual-tone multi-frequency signal

An apparatus for testing a subscriber line of a private automatic branch exchange using a Dual-Tone Multi-Frequency signal, comprising of:

• a trunk card connected to a central office line;
• an extension subscriber circuit connected to extension subscribers;
• a time switch forming a speech path between a port of said trunk card and a port of said extension subscriber circuit;
• a control unit controlling said trunk card, said extension subscriber circuit, and said time switch;
• a memory used by said control unit, said memory storing a diagnostic program for execution of an automatic testing operation;
• a Dual-Tone Multi-Frequency transmitter for transmitting a Dual-Tone Multi-Frequency signal to said subscriber line, said control unit controlling said Dual-Tone Multi-Frequency transmitter in response to a line test request from an exchange operator to transmit the Dual-Tone Multi-Frequency signal to said subscriber line;
• and
• a Dual-Tone Multi-Frequency receiver detecting a component of the transmitted Dual-Tone Multi-Frequency signal reflected from the subscriber line due to impedance mismatching, said control unit determining from a strength of the received Dual-Tone Multi-Frequency signal component whether said subscriber line is abnormal.

A method, comprising of:

• controlling a Dual-Tone Multi-Frequency transmitter in response to a line test request from an exchange operator to transmit the Dual-Tone Multi-Frequency signal to a subscriber line;
• receiving a component of the transmitted Dual-Tone Multi-Frequency signal reflected from said subscriber line due to impedance mismatching;
• and
• determining automatically from the received Dual-Tone Multi-Frequency signal component whether said subscriber line is abnormal.